Calculation of the vibration frequencies of ?-quartz: The effect of Hamiltonian and basis set

Is Ortho-Terphenyl a Rigid Glass Former?

Ortho-terphenyl (OTP) has long been used as a model system to study the glass transition due to its apparent simplicity and a widespread assumption that it is a rigid molecule. Here, we employ terahertz time-domain spectroscopy and low-frequency Raman spectroscopy to investigate the rigidity of OTP by direct observation of the low-frequency vibrational dynamics. These terahertz phonons involve complex large-amplitude atomic motions where intramolecular and intermolecular displacements are often mixed. Comparison of experimental results with density functional theory and ab initio molecular dynamics simulations shows that the assumption of rigidity neglects important implications for the glass transition and must be revisited. These results highlight the significance of terahertz modes on elasticity, which will be even more critical in more complex systems such as biomolecules.

Terahertz Signatures of the Methane Replacement Reaction in Hydroquinone Clathrates

We report a comprehensive experimental and computational study into low-frequency vibrational dynamics of hydroquinone clathrate during in situ gas loading, in order to monitor replacement of carbon dioxide with methane in its atomic-level pores. We used terahertz time-domain spectroscopy, because terahertz modes are highly sensitive to the identity and structure of enclathrated guest molecules. Through ab initio simulations, we determined that the replacement reaction is not completed. Instead we observed the formation of a heterogeneous material, with methane molecules occupying approximately one-third of available adsorption sites. While the structure of the methane-hydroquinone clathrate system has been previously determined, our observations suggest the reported symmetry is incorrect due to methane molecules weakly interacting with the framework, resulting in dynamic (as opposed to positional) disorder of guests, unlike the related fully ordered carbon dioxide clathrate. This work puts us on the path to quantitatively tracking gas loading in porous materials using terahertz spectroscopy.

Further Insights into the Chemical Synthesis of F2 and on Drying moist HF

It is well established that solid K2MnF6 reacts with excess SbF5 forming elemental F2. However, if the reaction is carried out in anhydrous HF (aHF) as a solvent, this is surprisingly not the case. Instead, the green Mn(IV) compound K3[(MnIVF)(SbF6)5]F is obtained. The reductive elimination of F2 was not observed under the applied conditions. The compound was characterized by its crystal structure, by Raman spectroscopy, and by quantum-chemical solid-state calculations. It crystallizes in the monoclinic space group P21/c, mP164, with the lattice parameters a = 12.2393(13), b = 12.167(2), c = 20.115(5) Å, β = 110.805(8)°, V = 2800.1(9) Å3, Z = 4 at T = 200 K. As the use of strictly anhydrous HF is crucial in this and other similar reactions, methods for drying moist HF are discussed.

Fabrication of ZnO Scaffolded CdS Nanostructured Photoanodes with Enhanced Photoelectrochemical Water Splitting Activity under Visible Light

CdS, characterized by its comparatively narrow energy band gap (∼2.4 eV), is an appropriate material for prospective use as a photoanode in photoelectrochemical water splitting. Regrettably, it encounters several obstacles for practical and large-scale applications, including issues such as bulk carrier recombination and diminished conductivity. Here, we have tried to address these challenges by fabricating a novel photoelectrode (ZnO/CdS) composed of one-dimensional ZnO nanorods (NRs) decorated with two-dimensional CdS nanosheets (NSs). A facile two-step chemical method comprising electrodeposition along with chemical bath deposition is employed to synthesize the ZnO NRs, CdS NSs, and ZnO/CdS nanostructures. The prepared nanostructures have been investigated by UV-visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy. The fabricated ZnO/CdS nanostructures have shown enhanced photoelectrochemical properties due to the improvement of the semiconductor junction surface area and thereby enhanced visible light absorption. The incorporation of CdS NSs has been further found to promote the rate of the charge separation and transfer process. Subsequently, the fabricated ZnO/CdS photoelectrodes achieved a photocurrent conversion efficiency 3 times higher than that of a planar ZnO NR photoanode and showed excellent performance under visible light irradiation. The highest applied bias photon-to-current conversion efficiency (% ABPE) of about ∼0.63% has been obtained for the sample with thicker CdS NSs on ZnO NRs with a photocurrent density of ∼1.87 mA/cm2 under AM 1.5 G illumination. The newly synthesized nanostructures further demonstrate that the full photovoltaic capacity of nanomaterials is yet to be exhausted.

Theoretical prediction of nanosizing effects and role of additives in the decomposition of Mg(BH4)2

The energetic transition towards renewable resources is one of the biggest challenges of this century. In this context, the role of H2 is of paramount importance as a key source of energy that could substitute traditional fossil fuels. This technology, even if available in several manufactures, still needs to be optimized at all levels (production, storage and distribution) to be integrated on a larger scale. Among materials suitable to store H2, Mg(BH4)2 is particularly interesting due to its high content of H2 in terms of gravimetric density. Nanosizing effects and role of additives in the decomposition of Mg(BH4)2 were studied by density functional theory (DFT) modelling. Both effects were analyzed because of their contribution in promoting the decomposition of the material. In particular, to have a quantitative idea of nanosizing effects, we used thin film 2D models corresponding to different crystallographic surfaces and referred to the following reaction: Mg(BH4)2 → MgB2 + 4H2. When moving from bulk to nanoscale (2D models), a remarkable decrease in the decomposition energy (10-20 kJ mol-1) was predicted depending on the surface and the thin film thickness considered. As regards the role of additives (Ni and Cu), we based our analysis on their effect in perturbing neighboring borohydride groups. We found a clear elongation of some B-H bonds, in particular with the NiF2 additive (about 0.1 Å). We interpreted this behavior as an indicator of the propensity of borohydride towards dissociation. On the basis of this evidence, we also explored a possible reaction pathway of NiF2 and CuF2 on Mg(BH4)2 up to H2 release and pointed out the major catalytic effect of Ni compared to Cu.

Quantification of Crystalline Phases in Hf0.5Zr0.5O2 Thin Films through Complementary Infrared Spectroscopy and Ab Initio Supercell Simulations

In the quest for thinner and more efficient ferroelectric devices, Hf0.5Zr0.5O2 (HZO) has emerged as a potential ultrathin and lead-free ferroelectric material. Indeed, when deposited on a TiN electrode, 1-25 nm thick HZO exhibits excellent ferroelectricity capability, allowing the prospective miniaturization of capacitors and transistor devices. To investigate the origin of ferroelectricity in HZO thin films, we conducted a far-infrared (FIR) spectroscopic study on 5 HZO films with thicknesses ranging from 10 to 52 nm, both within and out of the ferroelectric thickness range where ferroelectric properties are observed. Based on X-ray diffraction, these HZO films are estimated to contain various proportions of monoclinic (m-), tetragonal (t-), and polar orthorhombic (polar o-) phases, while only the 11, 17, and 21 nm thick are expected to include a higher amount of polar o-phase. We coupled the HZO infrared measurements with DFT simulations for these m-, t-, and polar o-crystallographic structures. The approach used was based on the supercell method, which combines all possible Hf/Zr mixed atomic sites in the solid solution. The excellent agreement between measured and simulated spectra allows assigning most bands and provides infrared signatures for the various HZO structures, including the polar orthorhombic form. Beyond pure assignment of bands, the DFT IR spectra averaging using a mix of different compositions (e.g., 70% polar o-phase +30% m-phase) of HZO DFT crystal phases allows quantification of the percentage of different structures inside the different HZO film thicknesses. Regarding the experimental data analysis, we used the spectroscopic data to perform a Kramers-Kronig constrained variational fit to extract the optical functions of the films using a Drude-Lorentz-based model. We found that the ferroelectric films could be described using a set of about 7 oscillators, which results in static dielectric constants in good agreement with theoretical values and previously reported ones for HfO2-doped ferroelectric films.

Ba12 [BN2 ]6.67 H4 : A Disordered Anti-Skutterudite filled with Nitridoborate Anions

Skutterudites are of high interest in current research due to their diversity of structures comprising empty, partially filled and filled variants, mostly based on metallic compounds. We herein present Ba12 [BN2 ]6.67 H4 , forming a non-metallic filled anti-skutterudite. It is accessed in a solid-state ampoule reaction from barium subnitride, boron nitride and barium hydride at 750 °C. Single-crystal X-ray and neutron powder diffraction data allowed to elucidate the structure in the cubic space group Im3 ‾ ${\bar{3}}$ (no. 204). The barium and hydride atoms form a three-dimensional network consisting of corner-sharing HBa6 octahedra and Ba12 icosahedra. Slightly bent [BN2 ]3- units are located in the icosahedra and the voids in-between. 1 H and 11 B magic angle spinning (MAS) NMR experiments and vibrational spectroscopy further support the structure model. Quantum chemical calculations coincide well with experimental results and provide information about the electronic structure of Ba12 [BN2 ]6.67 H4 .

A database of computed Raman spectra of inorganic compounds with accurate hybrid functionals

Raman spectroscopy is widely applied in identifying local structures in materials, but the interpretation of Raman spectra is non-trivial. An accurate computational database of reference spectra calculated with a consistent level of theory can significantly aid in interpreting measured Raman spectra. Here, we present a database of Raman spectra of inorganic compounds calculated with accurate hybrid functionals in density functional theory. Raman spectra were obtained by calculating dynamical matrices and polarizability tensors for structures from the Inorganic Crystal Structure Database. The calculated Raman spectra and other phonon properties (e.g., infrared spectra) are stored in a MongoDB database publicly shared through a web application. We assess the accuracy of our Raman calculations by statistically comparing ~80 calculated spectra with an existing experimental Raman database. To date, the database contains 161 compounds and is continuously growing as we add more materials computed with our automated workflow.

Phase transitions and spectral shifts: a quantum mechanical exploration of vibrational frequency in magnesium ferrite

Spinel ferrites represent an integral subset of magnetic materials, with their inherent properties largely influenced by cation occupancy and spin interaction. In this study, we present an in-depth theoretical exploration of the phase transition landscape of pure magnesium-ferrite, deploying hybrid functionals and a local Gaussian basis set to scrutinize the relaxed lattice structure, relative energy, magnetic properties, electronic characteristics, and vibrational frequencies. Our investigation reveals that the ground state of magnesium-ferrite is an open-shell system with an inverse structure. This is characterized by the complete occupancy of octahedral sites by magnesium atoms, with Iron atoms dispersed between both tetrahedral and octahedral sites. We found a relative energy difference of 0.41 eV between the antiferromagnetic ground configuration and the ferro arrangement within the inverse structure. Furthermore, our research also delved into the impact of spin rearrangement and inversion (X = 0.0, 0.5 and 1) on Raman and infrared spectra. Notably, the lattice distortion from the cubic form, apparent in the optimized structure, resonates in the IR and Raman spectra, resulting in significant splitting. The frequencies calculated in this study align well with experimental values, suggesting that the literature's current assignments warrant reevaluation in light of this new data. The results presented herein can be instrumental in detecting the phase of Mg ferrites from experimental spectra, thereby paving the way for a more profound comprehension of their properties and possible uses.

Accurate density functional theory prediction of low-dimensional yttrium nitride: From 2D hexagonal and square monolayers to 1D zizag single walled nanotubes

Through this contribution, we aim to highlight the structural stability of low dimensional YN structures ranging from the 3D bulk to the 2D square and hexagonal monolayers and their corresponding 1D zigzag single walled nanotubes. For all arrangements, geometry optimization is achieved at the DFT/B3LYP level of theory using a Gaussian basis set. Then, the coupled perturbed Kohn-Sham and Hartree-Fock (CPKS/HF) computational approach is used to simulate Raman and IR spectrum. Rolling, cohesive and relaxation energies, electronic and vibrational contributions to the polarizability and equilibrium lattice parameters are also reported. Insights into their structural stability are provided by combining optimized parameters and vibrational phonon spectra. For the optimized 3D bulks, 2D monolayers and 1D square nanotubes, no imaginary frequency has been recorded in their vibrational spectra which reveals a dynamic stability. Likewise, imaginary frequencies appeared only for relatively large YN (n,0) single walled hexagonal nanotubes (n > 6) indicating that the optimized structures are not a real global minimum and implying a dynamic instability. A scaning mode procedure along the largest imaginary vibrational mode has been adopted to obtain the equilibrium geometry of (22,0) YN hexagonal nanotube. Therefore, it must be emphasized that the obtained potential energy surface presents two minima between a saddle point. These minima corresponds to a stable structures slightly distorted compared to the initial one. The absence of imaginary phonon frequencies in the Raman and IR spectra of the optimized (22,0) YN hexagonal nanotube confirms its structural stability.

A Prato Tour on Carbon Nanotubes: Raman Insights

The functionalisation of carbon nanotubes has been instrumental in broadening its application field, allowing especially its use in biological studies. Although numerous covalent and non-covalent functionalisation methods have been described, the characterisation of the final materials has always been an added challenge. Among the various techniques available, Raman spectroscopy is one of the most widely used to determine the covalent functionalisation of these species. However, Raman spectroscopy is not a quantitative technique, and no studies are reported comparing its performance when the same number of functional groups are added but using completely different reactions. In this work, we have experimentally and theoretically studied the functionalisation of carbon nanotubes using two of the most commonly used reactions: 1,3-dipolar cycloaddition of azomethylene ylides and diazonium-based radical addition. The number of groups introduced onto the tubes by these reactions has been determined by different characterisation techniques. The results of this study support the idea that data obtained by Raman spectra are only helpful for comparing functionalisations produced using the same type of reaction. However, they should be carefully analysed when comparing functionalisations produced using different reaction types.

Combining Nitridoborates, Nitrides and Hydrides-Synthesis and Characterization of the Multianionic Sr6 N[BN2 ]2 H3

Multianionic metal hydrides, which exhibit a wide variety of physical properties and complex structures, have recently attracted growing interest. Here we present Sr6 N[BN2 ]2 H3 , prepared in a solid-state ampoule reaction at 800 °C, as the first combination of nitridoborate, nitride and hydride anions within a single compound. The crystal structure was solved from single-crystal X-ray and neutron powder diffraction data in space group P21 /c (no. 14), revealing a three-dimensional network of undulated layers of nitridoborate units, strontium atoms and hydride together with nitride anions. Magic angle spinning (MAS) NMR and vibrational spectroscopy in combination with quantum chemical calculations further confirm the structure model. Electrochemical measurements suggest the existence of hydride ion conductivity, allowing the hydrides to migrate along the layers.

Gas-Phase vs. Grain-Surface Formation of Interstellar Complex Organic Molecules: A Comprehensive Quantum-Chemical Study

Several organic chemical compounds (the so-called interstellar complex organic molecules, iCOMs) have been identified in the interstellar medium (ISM). Examples of iCOMs are formamide (HCONH2), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), or formic acid (HCOOH). iCOMs can serve as precursors of other organic molecules of enhanced complexity, and hence they are key species in chemical evolution in the ISM. The formation of iCOMs is still a subject of a vivid debate, in which gas-phase or grain-surface syntheses have been postulated. In this study, we investigate the grain-surface-formation pathways for the four above-mentioned iCOMs by transferring their primary gas-phase synthetic routes onto water ice surfaces. Our objective is twofold: (i) to identify potential grain-surface-reaction mechanisms leading to the formation of these iCOMs, and (ii) to decipher either parallelisms or disparities between the gas-phase and the grain-surface reactions. Results obtained indicate that the presence of the icy surface modifies the energetic features of the reactions compared to the gas-phase scenario, by increasing some of the energy barriers. Therefore, the investigated gas-phase mechanisms seem unlikely to occur on the icy grains, highlighting the distinctiveness between the gas-phase and the grain-surface chemistry.

Exploration of the Conformational Scenario for α-, β-, and γ-Cyclodextrins in Dry and Wet Conditions, from Monomers to Crystal Structures: A Quantum-Mechanical Study

Cyclodextrins (CDs) constitute a class of cyclic oligosaccharides that are well recognized and largely applied in the drug delivery field, thanks to their biocompatibility, low cost, and the possibility to be derivatized in order to tune and optimize the complexation/release of the specific drug. The conformational flexibility of these systems is one of their key properties and requires a cost-effective methodology to be studied by combining the accuracy of results with the possibility of exploring a large set of conformations. In the present paper, we have explored the conformational potential energy surface of the monomers and dimers of α-, β-, and γ-cyclodextrins (i.e., 6, 7, and 8 monomeric units, respectively) by means of fast but accurate semiempirical methods, which are then refined by state-of-the-art DFT functionals. Moreover, the crystal structure is considered for a more suitable comparison with the IR spectrum experimentally recorded. Calculations are carried out in the gas phase and in water environments, applying both implicit and explicit treatments. We show that the conformation of the studied molecules changes from the gas phase to the water, even if treated implicitly, thus modifying their complexation capability.

Unraveling the Complex Interplay of Phase Transitions in Spinel Ferrites: A Comprehensive Quantum Mechanical Vibrational Study of ZnFe2O4

The rich phase transition landscape of spinel ferrites and its profound impact on their physical properties have garnered significant interest in recent years. The complex interplay of divalent and trivalent cations distributed across A- and B-sites gives rise to a captivating variety of interactions. In this study, we delve into the structural, electronic, magnetic, and vibrational properties of ZnFe2O4 as a function of the degree of inversion, employing first-principles density functional theory with global and range-separated hybrid functionals and a local basis set. The ground state of ZnFe2O4 is an open-shell system, characterized by Zn atoms occupying tetrahedral sites, Fe atoms residing in octahedral sites, and Fe atom spins exhibiting ligand parallel alignment. In the normal structure, the antiparallel arrangement is less stable than the ferro arrangement by 0.058 eV (673 K) for fully relaxed structures, decreasing to 0.034 eV (395 K) upon incorporating a zero-point vibrations contribution. For normal ferromagnetic ZnFe2O4, we calculated scattering for A1g, Eg, and 3T2g symmetry at 676.6, 367.1, and (189.7, 457.7, 602.3) cm-1, respectively. Additionally, four T1u vibrational frequencies predicted by group theory were obtained at 524.59, 358.48, 312.49, and 192.9 cm-1, demonstrating excellent agreement with the experimental studies. We also explored the influence of spin rearrangement and inversion (X = 0.5 and 1) on Raman and infrared spectra. By analyzing the infrared spectra of isotopic substitutions, we reevaluated the assignments of the four T1u modes in light of available experimental data. Notably, the sensitivity of peak positions and intensities for some Raman modes, particularly A1g and T2g(2), to spin arrangement could provide a convenient experimental tool for detecting phase transitions induced by changes in temperature or external electric fields. This investigation shines a light on the complex interplay of phase transitions in spinel ferrites, paving the way for a deeper understanding of their properties and potential applications.

Investigation on the predictive power of tolerance factor τ for A-site double perovskite oxides

We have used a recently introduced new tolerance factor τ to create a stability map of all possible A-site double perovskite titanates AA'Ti2O6 and niobates AA'Nb2O6. The predictive power of τ is relatively good based on comparisons with available experimental data for A-site double perovskites. We carried out quantum chemical calculations on two hypothetical double perovskite compositions CsScTi2O6 and YRbTi2O6, where τ predicts high probability for their existence. In both cases, we found limits in the predictive power of the new tolerance factor for ion combinations on the A and A' site which are very different in size. A difference in oxidation state may decrease the accuracy, as well. Overall, the A-site double perovskite stability mapping provides a starting point for the discovery of novel A-site double perovskites.

Discrete Mono-, Di-, and Trinuclear Anions [MoOF5]-, [MoVOF5]2-, [MoO2F4]2-, [Mo2O2F9]-, [Mo3O3F13]-, and the Infinite Chain Anion [MoO2F3]- Obtained from Reactions of MoOF4: Synthesis and Analysis of the Structure-Chemical Relations of the Compounds

The herein-reported oxyfluoridometallate salts were synthesized and structurally characterized during the studies of the Lewis acidity of MOF4 (M = Mo, W) with various fluoride ion donors (RbF, CsF, TlF, AgF, SrF2, BaF2, PbF2) in different solvents (aqHF 48%, aHF, BrF3, ClF3). Phase-pure MoOF4 was either synthesized by hydrolysis of MoF6 with SiO2 in anhydrous HF (aHF) or by reactions of BrF3 with MoO2 or MoO3, respectively. The compound was characterized by infrared and Raman spectroscopy, solid-state quantum-chemical calculations, as well as powder and single-crystal X-ray diffraction. MoOF4 reacted with PbF2 in aHF forming Pb[MoOF5]2, while under comparable conditions, WOF4 formed Pb3[WOF5]4F2, containing the [WOF5]- anion. Salts containing such [MoOF5]- anions were also directly obtained from reactions of BrF3, MoO3, and AF2 (A = Sr, Ba), while with AgF, the compound Ag[Mo2O2F9] was observed. ClF3 reacted with MoO3 to form [ClOF2][Mo3O3F13]. Carrying out similar reactions in aqueous HF (aqHF) in autoclaves under hydrofluorothermal conditions leads to O-richer compounds with the composition A[MoO2F4] (A = Sr, Ba). With RbF or Tl2(CO3), the compounds A[MoO2F3] (A = Rb, Tl) were obtained. With CsF reduction to Mo(V) occurred as Cs2[MoVOF5] was formed. We report on similarities and differences within the respective anions and within the crystal structures of these compounds.

Evolutionary Algorithm-Based Crystal Structure Prediction of CuxZnyOz Ternary Oxides

Binary zinc(II) oxide (ZnO) and copper(II) oxide (CuO) are used in a number of applications, including optoelectronic and semiconductor applications. However, no crystal structures have been reported for ternary Cu-Zn-O oxides. In that context, we investigated the structural characteristics and thermodynamics of CuxZnyOz ternary oxides to map their experimental feasibility. We combined evolutionary crystal structure prediction and quantum chemical methods to investigate potential CuxZnyOz ternary oxides. The USPEX algorithm and density functional theory were used to screen over 4000 crystal structures with different stoichiometries. When comparing compositions with non-magnetic CuI ions, magnetic CuII ions, and mixed CuI-CuII compositions, the magnetic Cu2Zn2O4 system is thermodynamically the most favorable. At ambient pressures, the thermodynamically most favorable ternary crystal structure is still 2.8 kJ/mol per atom higher in Gibbs free energy compared to experimentally known binary phases. The results suggest that thermodynamics of the hypothetical CuxZnyOz ternary oxides should also be evaluated at high pressures. The predicted ternary materials are indirect band gap semiconductors.

Synthesis and Stability of Ammonium Selenocyanate [NH4][SeCN] and Its Reactivity toward Ag[SeCN]

[NH₄][SeCN] and Ag[SeCN] were synthesized by salt metathesis starting from the respective chlorides and K[SeCN]. Both products were fully characterized by single-crystal and powder X-ray diffraction, differential scanning calorimetry-thermogravimetric analysis (DSC-TGA), and solid-state IR and Raman spectroscopy. Quantum-mechanical calculations allowed for detailed assignment and interpretation of vibrational spectra. For dissolved [NH₄][SeCN], nuclear magnetic resonance spectra were collected. High-temperature powder X-ray diffraction (HT-PXRD) allowed for the analysis of the thermal behavior of solid [NH₄][SeCN]. Furthermore, the reaction of [NH₄][SeCN] with Ag[SeCN] leads to the formation of the ternary salts [NH₄][Ag(SeCN)₂] and [NH₄]₃[Ag(SeCN)₄]. The structures of the latter were determined from single-crystal X-ray diffraction (SC-XRD) data, and bulk analysis was performed by Rietveld refinement, Raman spectroscopy, and elemental analysis.

Low-Frequency Raman Spectroscopy of Pure and Cocrystallized Mycophenolic Acid

The aqueous solubility of solid-state pharmaceuticals can often be enhanced by cocrystallization with a coformer to create a binary cocrystal with preferred physical properties. Greater understanding of the internal and external forces that dictate molecular structure and intermolecular packing arrangements enables more efficient design of new cocrystals. Low-frequency (sub-200 cm^-1) Raman spectroscopy experiments and solid-state density functional theory simulations have been utilized together to investigate the crystal lattice vibrations of mycophenolic acid, an immunosuppressive drug, in its pure form and as a cocrystal with 2,2'-dipyridylamine. The lattice vibrations primarily consist of large-amplitude translations and rotations of the crystal components, thereby providing insights into the critical intermolecular forces governing cohesion of the molecular solids. The simulations reveal that despite mycophenolic acid having a significantly unfavorable conformation in the cocrystal as compared to the pure solid, the cocrystal exhibits greater thermodynamic stability over a wide temperature range. The energetic penalty due to the conformational strain is more than compensated for by the strong intermolecular forces between the drug and 2,2'-dipyridylamine. Quantifying the balance of internal and external energy factors in cocrystal formation indicates a path forward in the development of future mycophenolic acid cocrystals.

On the Origin of Raman Activity in Anatase TiO2 (Nano)Materials: An Ab Initio Investigation of Surface and Size Effects

Titania-based materials are abundant in technological applications, as well as everyday products; however, many of its structure-property relationships are still unclear. In particular, its surface reactivity on the nanoscale has important consequences for fields such as nanotoxicity or (photo)catalysis. Raman spectroscopy has been used to characterize titania-based (nano)material surfaces, mainly based on empirical peak assignments. In the present work, we address the structural features responsible for the Raman spectra of pure, stoichiometric TiO₂ materials from a theoretical characterization. We determine a computational protocol to obtain accurate Raman response in a series of anatase TiO₂ models, namely, the bulk and three low-index terminations by periodic ab initio approaches. The origin of the Raman peaks is thoroughly analyzed and the structure-Raman mapping is performed to account for structural distortions, laser and temperature effects, surface orientation, and size. We address the appropriateness of previous experimental use of Raman to quantify the presence of distinct TiO₂ terminations, and provide guidelines to exploit the Raman spectrum based on accurate rooted calculations that could be used to characterize a variety of titania systems (e.g., single crystals, commercial catalysts, thin layered materials, facetted nanoparticles, etc.).

Determination of Vibrational Modes of l-Alanine Single Crystals by a Combination of Terahertz Spectroscopy Measurements and Density Functional Calculations

Density-functional theory may be used to predict both the frequency and the dipole moment of the fundamental oscillations of molecular crystals. Suitably polarized photons at those frequencies excite such oscillations. Thus, in principle, terahertz spectroscopy may confirm the calculated fundamental modes of amino acids. However, reports to date have multiple shortcomings: (a) material of uncertain purity and morphology and diluted in a binder material is employed; (b) consequently, vibrations along all crystal axes are excited simultaneously; (c) data are restricted to room temperature, where resonances are broad and the background dominant; and (d) comparison with theory has been unsatisfactory (in part because the theory assumes zero temperature). Here, we overcome all four obstacles, in reporting detailed low-temperature polarized THz spectra of single-crystal l-alanine, assigning vibrational modes using density-functional theory, and comparing the calculated dipole moment vector direction to the electric field polarization of the measured spectra. Our direct and detailed comparison of theory with experiment corrects previous mode assignments for l-alanine, and reveals unreported modes, previously obscured by closely spaced spectral absorptions. The fundamental modes are thereby determined.

Combined DFT-D3 Computational and Experimental Studies on g-C3N4: New Insight into Structure, Optical, and Vibrational Properties

Graphitic carbon nitride (g-C₃N₄) has emerged as one of the most promising solar-light-activated polymeric metal-free semiconductor photocatalysts due to its thermal physicochemical stability but also its characteristics of environmentally friendly and sustainable material. Despite the challenging properties of g-C₃N₄, its photocatalytic performance is still limited by the low surface area, together with the fast charge recombination phenomena. Hence, many efforts have been focused on overcoming these drawbacks by controlling and improving the synthesis methods. With regard to this, many structures including strands of linearly condensed melamine monomers, which are interconnected by hydrogen bonds, or highly condensed systems, have been proposed. Nevertheless, complete and consistent knowledge of the pristine material has not yet been achieved. Thus, to shed light on the nature of polymerised carbon nitride structures, which are obtained from the well-known direct heating of melamine under mild conditions, we combined the results obtained from XRD analysis, SEM and AFM microscopies, and UV-visible and FTIR spectroscopies with the data from the Density Functional Theory method (DFT). An indirect band gap and the vibrational peaks have been calculated without uncertainty, thus highlighting a mixture of highly condensed g-C₃N₄ domains embedded in a less condensed "melon-like" framework.

Efficient calculation of derivatives of integrals in a basis of non-separable Gaussians

A computational procedure is developed for the efficient calculation of derivatives of integrals over non-separable Gaussian-type basis functions, used for the evaluation of gradients of the total energy in quantum-mechanical simulations. The approach, based on symbolic computation with computer algebra systems and automated generation of optimized subroutines, takes full advantage of sparsity and is here applied to first energy derivatives with respect to nuclear displacements and lattice parameters of molecules and materials. The implementation in the Crystal code is presented, and the considerably improved computational efficiency over the previous implementation is illustrated. For this purpose, three different tasks involving the use of analytical forces are considered: (i) geometry optimization; (ii) harmonic frequency calculation; and (iii) elastic tensor calculation. Three test case materials are selected as representatives of different classes: (i) a metallic 2D model of the Cu(111) surface; (ii) a wide-gap semiconductor ZnO crystal, with a wurtzite-type structure; and (iii) a porous metal-organic crystal, namely the ZIF-8 zinc-imidazolate framework. Finally, it is argued that the present symbolic approach is particularly amenable to generalizations, and its potential application to other derivatives is sketched.

Geometrical Stability and Nonlinear Optical Properties of Crystallogen and Pnictogen Fullerene Analogues

The linear and nonlinear optical (NLO) properties of fullerene and fullerene-like structures, including crystallogen and pnictogen elements, are computed quantum mechanically. The tensors of optical polarizability, α, and second hyperpolarizability, γ, for a series of buckyball fullerene analogues, namely, Si₆₀, Ge₆₀, Sn₆₀, Pb₆₀, P₆₀, As₆₀, Sb₆₀, and Bi₆₀, are reported and analyzed. The eight considered nanocages are here classified into four categories: nanocages stabilized in the X₆₀ form, including C₆₀, As₆₀, Sb₆₀, and Bi₆₀; nanocages that are not stabilized in the X₆₀ form but are found to be stable in a distorted buckled b-X₆₀ form, with X = Si and Ge; nanocages stabilized only in an exohedral decorated X₆₀-Y₆₀ form, X = Sn, Y = H or F; and finally nanocages that are not stable in either distorted or decorated form; however, their corresponding tabular nanotubes are found to be stable; such group includes P and Pb elements. A suggested nomenclature for the above-mentioned fullerenes is given for the first time, where many geometrical, energetic, and optical parameters are discussed extensively. These systems are energetically stable. The cohesive energies of Bi₆₀ and Sn₆₀-F₆₀ range from -1.2 to -4.8 eV/atom and can be compared to -2.4 and -3.3 eV/atom from the corresponding 2D bismuthene and stanene monolayers, respectively. While bismuthellene, Bi₆₀, shows vigorous optical responses compared to standard fullerene, the (9, 0) phosphorus nanotube gives not only enhanced polarizability and second hyperpolarizability but also an inducing first hyperpolarizability, β, which was null by symmetry in the case of spherical fullerenes. The proposed models are expected to be promising materials for optoelectronic and NLO applications.

Bromine Pentafluoride BrF5 , the Formation of [BrF6 ]- Salts, and the Stereochemical (In)activity of the Bromine Lone Pairs

BrF₅ can be prepared by treating BrF₃ with fluorine under UV light in the region of 300 to 400 nm at room temperature. It was analyzed by UV-Vis, NMR, IR and Raman spectroscopy. Its crystal structure was redetermined by X-ray diffraction, and its space group was corrected to Pnma. Quantum-chemical calculations were performed for the band assignment of the vibrational spectra. A monoclinic polymorph of BrF₅ was quantum chemically predicted and then observed as its low-temperature modification in space group P2₁ /c by single crystal X-ray diffraction. BrF₅ reacts with the alkali metal fluorides AF (A=K, Rb) to form alkali metal hexafluoridobromates(V), A[BrF₆ ] the crystal structures of which have been determined. Both compounds crystallize in the K[AsF₆ ] structure type (R 3 ‾ ${\bar 3}$ , no. 148, hR24). For the species [BrF₆ ]⁺ , BrF₅ , [BrF₆ ]^- , and [IF₆ ]^- , the chemical bonds and lone pairs on the heavy atoms were investigated by means of intrinsic bond orbital analysis.

Raman spectra and DFT calculations of thiophenol molecules adsorbed on a gold surface

We report the calculation of Raman modes of thiophenol molecules adsorbed on a real gold surface. The calculated Raman spectra strongly depend on the absorption configuration of the molecule on the metallic surface, a feature that should be carefully taken into account in the interpretation of the surface enhanced Raman spectra (SERS). The calculated Raman spectra are compared with experimental SERS measurements, the best accordance being obtained for a tilted configuration of the absorbed molecule. The present study supports the necessary combination of computational approaches with SERS measurements to predict the type of molecular adsorption configurations on metallic surfaces.

Strain-Tuneable Magnetism and Spintronics of Distorted Monovacancies in Graphene

The electronic and spintronic properties of the monovacancies in freestanding and isotopically compressed graphene are investigated using hybrid exchange density functional perturbation theory. When the effects of electronic self-interaction are taken into account, an integer magnetic moment of 2 μ_B is identified for a Jahn-Teller reconstructed V₁(5-9) monovacancy in freestanding graphene. For graphene with stable ripples induced by a compressive strain of 5%, a bond reconstruction produces a V₁(55-66) structure for the monovacancy, which is localized at the saddle points of the ripple. The sizeable local distortion induced by reconstruction modifies both the geometric and electronic properties of rippled graphene and quenches the magnetic moment of the vacancy due to the sp³ hybridization of the central atom. The nonmagnetic V₁(55-66) structure is found to be stable on rippled structures, with the formation energy ∼2.3 eV lower than that of the metastable distorted V₁(5-9) structures localized at sites other than the saddle points. The electronic ground state of distorted V₁(5-9) corresponds to a wide range of fractional magnetic moments (0.50-1.25 μ_B). The computed relative stabilities and the electronic and magnetic properties of the V₁(5-9) structures are found to be closely related to their local distortions. This analysis of the fundamental properties of defective graphene under compression suggests a number of strategies for generating regular defect patterns with tuneable magnetic and electronic properties and may, therefore, be used as a novel technique to achieve more precise control of graphene electronic structure for various application scenarios such as transistors, strain sensors, and directed chemisorption.

Reaction of Pertechnetate in Highly Alkaline Solution: Synthesis and Characterization of the Nitridotrioxotechnetate Ba[TcO3 N]

The preparation of novel technetium oxides, their characterization and the general investigation of technetium chemistry are of significant importance, since fundamental research has so far mainly focused on the group homologues. Whereas the structure chemistry of technetium in strongly oxidizing media is dominated by the Tc O 4 - ${{\left[{\rm { Tc}}{{\rm { O}}}_{{\rm { 4}}}\right]}^{-}}$ anion, our recent investigation yielded the new Tc O 3 N 2 - ${{\left[{\rm { Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N}}\right]}^{{\rm { 2}}-}}$ anion. Brown single crystals of Ba[TcO₃ N] were obtained under hydrothermal conditions starting from Ba(OH)₂  ⋅ 8H₂ O and NH₄ [TcO₄ ] at 200 °C. Ba [ Tc O 3 N ] ${{\rm { Ba[Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N]}}}$ crystallizes in the monoclinic crystal system with the space group P2₁ /n (a=7.2159(4) Å, b=7.8536(5) Å, c=7.4931(4) Å and β=104.279(2)°). The crystal structure of Ba [ Tc O 3 N ] ${{\rm { Ba[Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N]}}}$ consists of isolated Tc O 3 N 2 - ${{\left[{\rm { Tc}}{{\rm { O}}}_{{\rm { 3}}}{\rm { N}}\right]}^{{\rm { 2}}-}}$ tetrahedra, which are surrounded by Ba²⁺ cations. XANES measurements complement the oxidation state +VII for technetium and Raman spectroscopic experiments on Ba[TcO₃ N] single crystals exhibit characteristic Tc-O and Tc-N vibrational modes.

From symmetry breaking in the bulk to phase transitions at the surface: a quantum-mechanical exploration of Li6PS5Cl argyrodite superionic conductor

Lithium superionic conductor electrolytes may enable the safe use of metallic lithium anodes in all-solid-state batteries. The key to a successful application is a high Li conductivity in the electrolyte material, to be achieved through the maintenance of intimate contact with the electrodes and the knowledge of the chemical nature of that contact. In this manuscript, we tackle this issue by a theoretical ab initio approach. Focusing on the Li₆PS₅Cl, a thiophosphate with high ionic conductivity, we carry on thorough modeling of the surfaces together with the prediction of the thermal and elastic behaviour. Our investigation leads to some new findings: the bulk structure, as reported in the literature, appears to be metastable, with spontaneous symmetry breaking. Moreover, the relevant stoichiometric surfaces identified for stable and metastable crystal structures are not up-down symmetry related and they expose from one side Li₂S and LiCl. Surface reconstructions can be interpreted as local phase transitions. We also predict entirely ab initio the morphology of crystallites, charge, and electrostatic potential at surfaces, together with the effect of temperature on structural properties and the elastic behaviour of this material. Such findings may constitute the relevant groundwork for a better understanding of ionic transport in Li-ion conductors at the electrolyte/anode and electrolyte/cathode interfaces.

Computational Characterization of β-Li3PS4 Solid Electrolyte: From Bulk and Surfaces to Nanocrystals

The all-solid-state lithium-ion battery is a new class of batteries being developed following today's demand for renewable energy storage, especially for electric cars. The key component of such batteries is the solid-state electrolyte, a technology that promises increased safety and energy density with respect to the traditional liquid electrolytes. In this view, β-Li₃PS₄ is emerging as a good solid-state electrolyte candidate due to its stability and ionic conductivity. Despite the number of recent studies on this material, there is still much to understand about its atomic structure, and in particular its surface, a topic that becomes of key relevance for ionic diffusion and chemical stability in grain borders and contact with the other device components. In this study, we performed a density functional study of the structural and electronic properties of β-Li₃PS₄ surfaces. Starting from the bulk, we first verified that the thermodynamically stable structure featured slight distortion to the structure. Then, the surfaces were cut along different crystallographic planes and compared with each other. The (100) surface is confirmed as the most stable at T = 298 K, closely followed by (011), (010), and (210). Finally, from the computed surface energies, the Wulff nanocrystals were obtained and it was verified that the growth along the (100) and (011) directions reasonably reproduces the shape of the experimentally observed nanocrystal. With this study, we demonstrate that there are other surfaces besides (100) that are stable and can form interfaces with other components of the battery as well as facilitate the Li-migration according to their porous structures.

Strontium Nitridoborate Hydride Sr2BN2H Verified by Single-Crystal X-ray and Neutron Powder Diffraction

Combining different anions in one material allows tuning of its structural, magnetic, and electronic properties. We hereby present the mixed anion compound Sr₂BN₂H, expanding the less-known class of nitridoborate hydrides. Solid-state reaction of Sr₂N, BN, and SrH₂ at 850 °C in a tube furnace yielded a gray, air- and moisture-sensitive powder of Sr₂BN₂H. It crystallizes as colorless platelets in the orthorhombic space group Pnma (no. 62) with a = 9.9164(2), b = 3.9079(1), and c = 10.1723(2) Å and Z = 4. An initial structural model was obtained from single-crystal X-ray diffraction data and corroborated by neutron powder diffraction data of the corresponding deuteride. Further validation by ¹H and ¹¹B MAS NMR, FTIR, and Raman spectroscopy complements the structural proof of anionic hydrogen present in the compound. Quantum chemical calculations support the experimental findings and reveal the electronic structure of Sr₂BN₂H.

Anharmonic Coupling of Stretching Vibrations in Ice: A Periodic VSCF and VCI Description

The anharmonicity of O-H stretching vibrations of water ice is characterized by use of a periodic implementation of the vibrational self-consistent field (VSCF) and vibrational configuration interaction (VCI) methods, which take phonon-phonon couplings explicitly into account through numerical evaluation of high-order terms of the nuclear potential. The low-temperature, proton-ordered phase of water ice (namely, ice XI) is investigated. The net effect of a coupled anharmonic treatment of stretching modes is not just a rigid blue-shift of the respective harmonic spectral frequencies but rather a complex change of their relative spectral positions, which cannot be captured by simple scaling strategies based on harmonic calculations. The adopted techniques allow for a hierarchical treatment of anharmonic terms of the nuclear potential, which is key to an effective identification of leading factors. We show that the anharmonic independent-mode approximation─only describing the "intrinsic anharmonicity" of the O-H stretches─is unable to capture the correct physics, and that couplings among O-H stretches must be described. Inspection of harmonic normal coordinates allows identification of specific features of the O-H stretching motions which most likely enable strong mode-mode couplings. Finally, by coupling O-H stretches to all other possible modes of ice XI (THz collective vibrations, molecular librations, bendings), we identify specific types of motion which significantly affect O-H stretching states: in particular, molecular librations are found to affect the stretching states more than molecular bendings.

Designing 3d metal oxides: selecting optimal density functionals for strongly correlated materials

Transition metal oxides (TMOs) have remarkable physicochemical properties, are non-toxic, and have low cost and high annual production, thus they are commonly studied for various technological applications. Density functional theory (DFT) can help to optimize TMO materials by providing insights into their electronic, optical and thermodynamic properties, and hence into their structure-performance relationships, over a wide range of solid-state structures and compositions. However, this is underpinned by the choice of the exchange-correlation (XC) functional, which is critical to accurately describe the highly localized and correlated 3d-electrons of the transition metals in TMOs. This tutorial review presents a benchmark study of density functionals (DFs), ranging from generalized gradient approximation (GGA) to range-separated hybrids (RSH), with the all-electron def2-TZVP basis set, comparing magneto-electro-optical properties of 3d TMOs against experimental observations. The performance of the DFs is assessed by analyzing the band structure, density of states, magnetic moment, structural static and dynamic parameters, optical properties, spin contamination and computational cost. The results disclose the strengths and weaknesses of the XC functionals, in terms of accuracy, and computational efficiency, suggesting the unprecedented PBE0-1/5 as the best candidate. The findings of this work contribute to necessary developments of XC functionals for periodic systems, and materials science modelling studies, particularly informing how to select the optimal XC functional to obtain the most trustworthy description of the ground-state electron structure of 3d TMOs.

In Situ Observation of the Structure of Crystallizing Magnesium Sulfate Heptahydrate Solutions with Terahertz Transmission Spectroscopy

Terahertz time-domain spectroscopy in a transmission geometry combined with visual analysis was used to investigate the crystallization process of MgSO₄ solution. Careful spectral analysis of both a feature at 1.6 THz and the overall magnitude of absorption allowed the extraction of information about the liquid phase before and during crystallization, aiding the investigation of solvation dynamics and the behavior of molecular species at phase boundaries. The method was reproducibly applied to a number of measurements on a series of solutions of three chosen concentrations at different temperatures. When increasing temperature at the end of the measurement, the dissolution of crystals was observed as well. The temperature-dependent absorption data of the semicrystalline systems were converted to the solvent concentrations using a recently developed method. Solutions of a series of concentrations were also investigated in the temperature range of 4-25 °C. The results were compared to the theoretical calculated values, and the consistent differences proved the existence of a hydration shell around the salt ions whose behavior is different from bulk water. Future work will focus on triggering nucleation at specific positions in order to study the very beginning of the crystallization process. MgSO₄ heptahydrate is used as a model system in this study, while the concept and the setup can be applied to other systems.

Real Temperature Model of Dynamic Disorder in Molecular Crystals

Charge carrier mobilities in ordered organic semiconductors are limited by inherent vibrational phonons that scatter carriers. To improve a material's intrinsic mobility, restricting particularly detrimental modes with molecular substitutions may be a viable strategy. Here, we develop a probabilistic temperature-dependent displacement model that we couple with the density functional dimer projection protocol to predict effective electronic coupling fluctuations. The phonon-induced deviations from the equilibrium electronic couplings are used to infer the detriment of low-frequency phonons on charge carrier mobilities in a set of organic single crystals. We show that asymmetric sliding motions in pentacene and 2,6-diphenylanthracene induce large electronic coupling fluctuations, whereas seesawlike motions cause large fluctuations in rubrene, 9,10-diphenylanthracene, and, 2,6-diphenylanthracene. Vibrational analyses revealed that the asymmetric sliding phonon in rubrene persists only in the low-mobility direction of the crystal. Therefore, rubrene's intrinsic high mobility is likely due to the absence of this source of disorder in its high-mobility conduction channels. This model can be used to identify particularly harmful or helpful phonons in crystalline materials and may provide design rules for developing materials with intrinsically low disorder.

On the use of a volume constraint to account for thermal expansion effects on the low-frequency vibrations of molecular crystals

A volume-constraint method is presented as a means to capture the influence of thermal expansion on the low-frequency vibrations in molecular crystals. In particular, the room-temperature terahertz absorption spectra of L-tartaric acid, α-lactose monohydrate, and α-para-aminobenzoic acid (PABA) have been simulated using dispersion-corrected, solid-state density functional theory (DFT-D). By comparing the normal modes obtained with a unit cell optimised without constraints to those obtained with a unit cell optimised while constrained to keep its experimental volume, wholesale improvements to the resultant spectrum is achieved when using the constrained geometry by inhibiting cell contraction. These improvements are demonstrated over a range of popular density functionals and basis sets up to triple-zeta complexity. A correlation method is then presented as a means to quantitatively compare the vibrational pattern of normal modes obtained from both unit cells. This analysis reveals that thermal expansion can effect the character and relative frequency of normal modes, with the choice of geometry ultimately affecting the assignment of the experimental absorptions. The sensibility of using the experimental volume as an approximation is then discussed, where it is speculated that large basis sets or hybrid functionals are necessary to ensure that the thermal expansion effect is not overestimated. The low-frequency absorption spectrum of PABA is then fully characterised using the PBE-D3BJ/6-311G(2d,2p) method.

Photocatalytic Degradation of Rhodamine B Dye and Hydrogen Evolution by Hydrothermally Synthesized NaBH4—Spiked ZnS Nanostructures

Metal sulphides, including zinc sulphide (ZnS), are semiconductor photocatalysts that have been investigated for the photocatalytic degradation of organic pollutants as well as their activity during the hydrogen evolution reaction and water splitting. However, devising ZnS photocatalysts with a high overall quantum efficiency has been a challenge due to the rapid recombination rates of charge carriers. Various strategies, including the control of size and morphology of ZnS nanoparticles, have been proposed to overcome these drawbacks. In this work, ZnS samples with different morphologies were prepared from zinc and sulphur powders via a facile hydrothermal method by varying the amount of sodium borohydride used as a reducing agent. The structural properties of the ZnS nanoparticles were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques. All-electron hybrid density functional theory calculations were employed to elucidate the effect of sulphur and zinc vacancies occurring in the bulk as well as (220) surface on the overall electronic properties and absorption of ZnS. Considerable differences in the defect level positions were observed between the bulk and surface of ZnS while the adsorption of NaBH₄ was found to be highly favourable but without any significant effect on the band gap of ZnS. The photocatalytic activity of ZnS was evaluated for the degradation of rhodamine B dye under UV irradiation and hydrogen generation from water. The ZnS nanoparticles photo-catalytically degraded Rhodamine B dye effectively, with the sample containing 0.01 mol NaBH₄ being the most efficient. The samples also showed activity for hydrogen evolution, but with less H₂ produced compared to when untreated samples of ZnS were used. These findings suggest that ZnS nanoparticles are effective photocatalysts for the degradation of rhodamine B dyes as well as the hydrogen evolution, but rapid recombination of charge carriers remains a factor that needs future optimization.

Monomolecular Cracking of Propane: Effect of Zeolite Confinement and Acidity

The effect of zeolite pore geometry and intrinsic acidity on the activation energy of propane monomolecular cracking was investigated for six topologically distinct zeolites with different pore sizes. Periodic density functional theory calculations were used to calculate the activation energy, while cluster models were used to calculate deprotonation energies. The computed intrinsic activation energies showed a smaller variation with topology than the adsorption energies. No correlation was found between the computed deprotonation and ammonia adsorption energies at the acid site and the intrinsic activation energy. Detailed analysis of the computed structures and properties suggests that acid sites with different pore topologies impose geometrical constraints on the ion-pair formed by the ammonium molecule, which differs significantly from those that affect the propane reaction.

Characteristic Spectral Features of Terra Preta (TP) in the 5-15 Terahertz Range

Terra preta is a fertile anthropogenic soil found in the Amazon basin. One of the most significant differences between the terra preta and surrounding soils is that terra preta is rich in aromatic carbons. Previous infrared investigations of terra preta were reported at energies above 1000 cm^-1 where many other forms of carbon also have absorption lines. No measurements have been reported below 800 cm^-1, where many absorptions associated with aromatic carbons occur in the absence of aliphatic carbon lines. We employ Fourier transform infrared spectroscopy between 150 cm^-1 and 500 cm^-1. A comparison was made between the spectra of terra preta, several pure aromatic compounds, organic fertilizers developed to replicate terra preta and several Australian soils, some of which containing char from bushfires. The spectra in the 150-500 cm^-1 range were very similar between terra preta and the organic fertilizers, while they were very different for the natural soils. These findings indicate that the content of aromatic carbons in terra preta and organic fertilizers is different than in natural soils containing the bushfire chars, but also soils produced entirely by bacterial and fungal activities. This point to the importance of the preparation conditions of the biochars, which are essential ingredients of terra preta and organic fertilizers used in this study.

Strategies for the optimization of the structure of crystalline compounds

When different proposals exist (or can reasonably be formulated) for the size of the unit cell (in terms of number of atoms) and space group of crystalline compounds, a strategy for exploring with simulation methods the various cases and for investigating their relative stability must be defined. The optimization schemes of periodic quantum mechanical codes work in fact at fixed space group and number of atoms per unit cell, so that only the fractional coordinates of the atoms and the lattice parameters are optimized. A strategy is here presented, based on four standard tools, used synergistically and in sequence: (1) the optimization of inner coordinates and unit cell parameters; (2) the calculation of the vibrational frequencies not only at Γ , but also at a set of k → points (in the example presented here they are eight, generated by a shrinking factor 2), looking for possible negative wavenumbers. The latter correspond to maxima, rather than minima, along the coordinate described by the corresponding normal mode; (3) the exploration of the total energy along the mode with negative wavenumber, looking for the minimum of the curve; (4) the identification of the new space group corresponding to the reduced symmetry resulting from the previous step. The strategy is illustrated with reference to the KMnF₃ perovskite compound, for which many space groups are proposed in the literature, ranging from cubic Pm 3 ¯ m to tetragonal P 4 m bm or I 4 m cm and orthorhombic (Pnma and Cmcm) down to monoclinic (P2₁ /m). The corresponding primitive cells contain 5, 10, and 20 atoms in the various cases, and the point symmetry reduces from 48 to 4 operators. In nature, the KMnF₃ phase transitions also include the magnetic phases. For simplicity, here we limit the analysis to the ones that take place between ferromagnetic phases, as they are sufficiently rich for illustrating the proposed strategy. As the total energy differences involved can be as small as, say, 10-50 μHartree, a high numerical accuracy at each one of the steps mentioned above is required. The present calculations, performed with the CRYSTAL code, by using an all electron basis set and the Hartree-Fock and B3LYP functionals, document such an accuracy. The energy difference between the tetragonal I 4 m cm and cubic Pm 3 ¯ m phases is 225 μHartree, with a volume reduction of 0.58 ų ; the differences between the orthorhombic and tetragonal phases are an order of magnitude smaller, being 23 μHartree and 0.06 ų for total energy and cell volume, respectively.

Structural Properties and Magnetic Ground States of 100 Binary d-Metal Oxides Studied by Hybrid Density Functional Methods

d-metal oxides play a crucial role in numerous technological applications and show a great variety of magnetic properties. We have systematically investigated the structural properties, magnetic ground states, and fundamental electronic properties of 100 binary d-metal oxides using hybrid density functional methods and localized basis sets composed of Gaussian-type functions. The calculated properties are compared with experimental information in all cases where experimental data are available. The used PBE0 hybrid density functional method describes the structural properties of the studied d-metal oxides well, except in the case of molecular oxides with weak intermolecular forces between the molecular units. Empirical D3 dispersion correction does not improve the structural description of the molecular oxides. We provide a database of optimized geometries and magnetic ground states to facilitate future studies on the more complex properties of the binary d-metal oxides.

Kinetically Controlled Reduction of β-Vanadyl(V) Orthophosphate: Synthesis and Characterization of New Metastable Polymorphs of Vanadium(III) Phosphate

Two thermodynamically metastable polymorphs of vanadium(III) phosphate, V^IIIPO₄-m1 and VPO₄-m2, have been obtained via reduction of β-V^VOPO₄ by moist hydrogen. The XRPD pattern of VPO₄-m1 can be assigned based on the crystal structure of β-V^VOPO₄, though with distinctly different lattice parameters (VPO₄-m1/β-VOPO₄: Pnma, a = 7.3453(12)/7.7863(5) Å, b = 6.4001(12)/6.1329(3) Å, c = 7.3196(13)/6.9673(5) Å). The XRPD pattern of VPO₄-m2 was found to be very similar to that of Fe₂(VO)(P₂O₇)(PO₄) (VPO₄-m2: P2₁/m, Z = 2, a = 8.792(4) Å, b = 5.269(2) Å, c = 10.398(6) Å, β = 112.60(4)°). The crystal structure models for VPO₄-m1 and VPO₄-m2 have been optimized by DFT calculations. Polymorph m1 contains the unprecedented butterfly shaped [V^IIIO₄] chromophore and has been further characterized by magnetic measurements, by powder reflectance spectroscopy (NIR/vis/UV), and IR spectroscopy. For six polymorphic forms of VPO₄ (m1', m1'', m2, m3, m4, and m5), DFT calculations have been performed. For the existence of VPO₄-m1', -m1'', and -m2, our experiments provide evidence. VPO₄-m3, -m4, and -m5 were obtained by structure optimization based on reduced β-VOPO₄. Their stability is predicted by the DFT calculations.

The ferromagnetic and anti-ferromagnetic phases (cubic, tetragonal, orthorhombic) of KMnF3. A quantum mechanical investigation

Many space groups are proposed in the literature for the KMnF₃ perovskite (see, for example, Knight et al., J. Alloys Compd., 2020, 842, 155935), ranging from cubic (C) (Pm3̄m) to tetragonal (T) ( or I4/m) down to orthorhombic (O) (Pbnm). The relative stability ΔE of these phases, both ferromagnetic (FM) and antiferromagnetic (AFM), has been investigated quantum mechanically by using both the B3LYP hybrid functional and the Hartree-Fock Hamiltonian, an all-electron Gaussian type basis set and the CRYSTAL code. The O phase is slightly more stable than the T phase which in turn is more stable than the C phase, in agreement with experimental evidence. The C to T to O transition is accompanied by a volume reduction. The mechanism of stabilization of the AFM solution with respect to the FM one is discussed. Spin density maps and profiles, Mulliken charges, magnetic moments and bond population data are used for supporting the proposed mechanism. The IR and Raman spectra of the FM and AFM C, T and O cells are discussed; the only noticeable difference between the C, T and O spectra appears at wavenumbers lower than 150 cm^-1. The effect of pressure is also explored in the 0-20 GPa interval. The stability order (O > T > C) at 0 GPa persists also at high pressure, and the differences between the phases increase.

Generalizing Continuum Solvation in Crystal to Nonaqueous Solvents: Implementation, Parametrization, and Application to Molecules and Surfaces

We present an extension of a generalized finite-difference Poisson-Boltzmann (FDPB) continuum solvation model based on a self-consistent reaction field treatment to nonaqueous solvents. Implementation and reparametrization of the cavitation, dispersion, and structural (CDS) effects nonelectrostatic model are presented in CRYSTAL, with applications to both finite and infinite periodic systems. For neutral finite systems, computed errors with respect to available experimental data on free energies of solvation of 2523 solutes in 91 solvents, as well as 144 transfer energies from water to 14 organic solvents are on par with the reference SM12 solvation model for which the CDS parameters have been developed. Calculations performed on a TiO₂ anatase surface and compared to VASPsol data revealed an overall very good agreement of computed solvation energies, surface energies, as well as band structure changes upon solvation in three different solvents, validating the general applicability of the reparametrized FDPB approach to neutral nonperiodic and periodic solutes in aqueous and nonaqueous solvents. For ionic species, while the reparametrized CDS model led to large errors on free energies of solvation of anions, addition of a corrective term based on Abraham's acidity of the solvent significantly improved the accuracy of the proposed continuum solvation model, leading to errors on aqueous pK_a of a test set of 83 solutes divided by a factor of 4 compared to the reference solvation model based on density (SMD). Overall, therefore, these encouraging results demonstrate that the generalized FDPB continuum solvation model can be applied to a broad range of solutes in various solvents, ranging from finite neutral or charged solutes to extended periodic surfaces.

On the Interactions of Melatonin/β-Cyclodextrin Inclusion Complex: A Novel Approach Combining Efficient Semiempirical Extended Tight-Binding (xTB) Results with Ab Initio Methods

Melatonin (MT) is a molecule of paramount importance in all living organisms, due to its presence in many biological activities, such as circadian (sleep-wake cycle) and seasonal rhythms (reproduction, fattening, molting, etc.). Unfortunately, it suffers from poor solubility and, to be used as a drug, an appropriate transport vehicle has to be developed, in order to optimize its release in the human tissues. As a possible drug-delivery system, β-cyclodextrin (βCD) represents a promising scaffold which can encapsulate the melatonin, releasing when needed. In this work, we present a computational study supported by experimental IR spectra on inclusion MT/βCD complexes. The aim is to provide a robust, accurate and, at the same time, low-cost methodology to investigate these inclusion complexes both with static and dynamic simulations, in order to study the main actors that drive the interactions of melatonin with β-cyclodextrin and, therefore, to understand its release mechanism.