Electron and vibrational spectroscopies using DFT, plane waves and pseudopotentials: CASTEP implementation

The enhanced bandgap and birefringence of rare-earth phosphates XPO4 (X = Sc, Y, La, and Lu): a first-principles investigation

Rare-earth phosphates were thought to be good candidates as ultraviolet/deep ultraviolet optical materials due to their relatively large bandgap and optical properties. In this paper, the authors screened out a family of XPO4 (X = Sc, Y, La, and Lu) compounds with an enhanced bandgap (HSE06 bandgap ≥ 7.61 eV) and birefringence (0.0934-0.2003@1064 nm) using first-principles calculations. The origin of enhanced optical properties was investigated using projected density of states, distortion indices, and Born effective charges. The results show that the PO4 anionic groups and X-O polyhedra give the main contribution in determining the optical properties, and the PO4 anionic groups give more contribution than other functional basic units. The spin-orbit interaction was also investigated. Similar band structures were found after spin-orbit coupling (SOC) was considered, and slightly enhanced birefringence was found when SOC was applied to these rare-earth phosphates.

Experimental and theoretical investigation of Low-Frequency vibrational modes of 4-Amino 3,5 Dinitro Pyrazole in terahertz frequency domain

Pyrazoles have recently received significant attention due to their unique and potential applications in the medical field, agriculture and are also known to be highly stable explosives. The present work describes the terahertz time-domain spectroscopy (THz-TDS) based study of 4-Amino 3,5 Dinitro Pyrazole(ADNP) in between the 0.1 and 3.0 THz ranges. A Toptica-Teraflash fibre-coupled handheld terahertz system has been employed in reflection mode configuration. We ascertained complex refractive index, absorption coefficients, and complex dielectric constants from 0.1 THz to 3.0 THz. The value of the refractive index and absorption coefficients are found to be 1.8 and 10---180 cm -1, respectively. Also, we have analyzed the structural, vibrational, and optical properties of ADNP using the plane-wave pseudopotential method based on Density Functional Theory (DFT) calculations. We have observed six low-frequency optical phonon modes, located at 0.36, 1.20, 1.52, 1.77, 2.40, and 2.75 THz, respectively, exhibiting a redshift compared to the values predicted by the DFT calculations. The possible reasoning for the above might be due to the anharmonicity that is not considered in the DFT calculations. The theoretical calculations align with the experimental results and deliver direction for further investigations and the futuristic application of ADNP.

Improving the magnetic moment of Ca2Ge and promoting the conversion of semiconductors to diluted magnetic semiconductors using Mn-doping

The dilute magnetic properties of materials have important potential applications in the field of electronic science and technology. Intrinsic Ca2Ge is a new environmentally friendly semiconductor material, and exhibits cubic and orthorhombic phases. The crystal structure characteristics of Ca2Ge indicate that the modulation of its dilute magnetic properties can theoretically be achieved by doping with magnetic elements. The study of band structures shows that Ca2Ge is a semiconductor, while Mn doped Ca2Ge is a semi-metal. The results of density of states and atomic population analysis show that Mn doped Ca2Ge exhibits ferrimagnetism with a magnetic moment of 5 μB, and the orbital splitting energy of the Mn atom is 1.0 eV. Mn-doping changes the cubic crystal field of Ca2Ge, and the charge transfer and electron polarization of Ca d and Ge p orbitals are affected by Mn atoms. The Ca d orbital is split into dzz, dzy, dzx, dxx-yy and dxy orbitals, and the contribution of spin of each d split orbital to the magnetic moment of the Ca d orbital is in the order dxy > dyz > dxz > dxx-yy > dzz. The Ge p orbital is split into px, py and pz orbitals, and the spin contribution of each p orbital to the magnetic moment of the Ge p orbital is in the order py > pz > px. The analysis of atom populations shows that the charge transfer and spin of Ca and Ge change with Mn doping, and the difference between spin up and spin down increases, improving the magnetism of Ca2Ge and forming a dilute magnetic semiconductor.

Effect of Low to High Pressure on the Structural, Mechanical, Electrical, and Optical Properties of Inorganic Material Ca3AsBr3: An Ab Initio Investigation

Inorganic metal halide solar cells made from perovskite stand out for having outstanding efficiency, cheap cost, and simple production processes and recently have generated attention as a potential rival in photovoltaic technology. Particularly, lead-free Ca3AsBr3 inorganic materials have a lot of potential in the renewable industry due to their excellent qualities, including thermal, electric, optoelectronic, and elastic features. In this work, we thoroughly analyzed the stress-driven structural, mechanical, electrical, and optical properties of Ca3AsBr3 utilizing first-principles theory. The unstressed planar Ca3AsBr3 compound's bandgap results in 1.63 eV, confirming a direct bandgap. The bandgap within this compound could have changed by applying hydrostatic stress; consequently, a semiconductor-to-metallic transition transpired at 50 GPa. Simulated X-ray diffraction further demonstrated that it maintained its initial cubic form, even after external disruption. Additionally, it has been shown that an increase in compressive stress causes a change of the absorption spectra and the dielectric function with a red shift of photon energy at the lower energy region. Because of the material's mechanical durability and increased degree of ductility, demonstrated by its stress-triggered mechanical characteristics, the Ca3AsBr3 material may be suitable for solar energy applications. The mechanical and optoelectronic properties of Ca3AsBr3, which are pressure sensitive, could potentially be advantageous for future applications in optical devices and photovoltaic cell architecture.

Rb8Nb10Ge6O41: a new niobium-germanate crystal featuring unique one-dimensional [Nb7O30]∞ chains and wide mid-IR transparency

Germanate oxides have garnered considerable interest owing to their diverse structural configuration and intriguing properties. Herein, we present a novel niobium germanate crystal, Rb8Nb10Ge6O41, extracted through the process of spontaneous crystallization. It showcases a unique three-dimensional (3D) structural framework composed of one-dimensional (1D) twisted [Nb7O30]∞ chains and isolated [Ge3O9] rings, arising from the divergent polymerized manifestations of [NbO6] and [GeO4] basic building blocks, respectively, marking the first instance of such a topography in germanate materials. Notably, the title compound exhibits exceptional thermal stability up to 1250 °C with a good congruent melting nature. Moreover, it achieves a short ultraviolet edge at 306 nm and a favorable infrared edge cutoff exceeding 6.2 μm, thus indicating a wide transparency window. Additionally, this study elucidates the microscopic birefringence of Rb8Nb10Ge6O41 and clarifies the intricate relationship between its structure and properties. Our findings suggest that the polymerization of distinct structural motifs within a single compound is an effective strategy for exploring novel inorganic materials.

Novel antimony phosphates with enlarged birefringence induced by lone pair cations

Phosphates, whose obvious disadvantage is the relatively small birefringence, can be overcome by the introduction of post-transition metal cations containing stereochemically active lone-pair electrons. In this paper, two new compounds were successfully explored in the A-Sb-P-O system, i.e. Cs2Sb3O(PO4)3 (CsSbPO) and (NH4)2Sb4O2(H2O)(PO4)2[PO3(OH)]2 (NH4SbPOH). Transmission spectra show that CsSbPO has a surprising transmission range with a UV cutoff edge of 213 nm. First-principles calculations show that both compounds have a wide band gap (5.02 eV for CsSbPO and 5.30 eV for NH4SbPOH) and enlarged birefringence (Δn = 0.034@1064 nm for CsSbPO and Δn = 0.045@1064 nm for NH4SbPOH). The results of real-space atom-cutting investigations show that the distorted [SbOx] polyhedra originating from the asymmetric lone pair electrons give the main contribution to the total birefringence and overcome the disadvantage of small birefringence of phosphates but maintain wide transition windows.

Design of Deep-Ultraviolet Zero-Order Waveplate Materials by Rational Assembly of [AlO2F4] and [SO4] Groups

Zero-order waveplates are widely used in the manufacture of laser polarizer waves, which are important in polarimetry and the laser industry. However, there are still challenges in designing deep-ultraviolet (DUV) waveplate materials that satisfy large band gaps and small optical anisotropy simultaneously. Herein, three cases of aluminum sulfate fluorides: Na2AlSO4F3, Li4NH4Al(SO4)2F4, and Li6K3Al(SO4)4F4, with novel [AlSO4F3] layers or isolated [AlS2O8F4] trimers were designed and synthesized by the rational assembly of [AlO2F4] and [SO4] groups through a hydrothermal method. Experiments and theoretical calculations imply that these three possess short cutoff edges (λ < 200 nm) and small birefringence (0.0014-0.0076 @ 1064 nm), which fulfils the prerequisite for potential DUV zero-order waveplate materials. This work extends the exploration of DUV zero-order waveplate materials to the aluminum sulfate fluoride systems.

On the Use of Solomon Echoes in 27 Al NMR Studies of Complex Aluminium Hydrides

The quadrupole coupling constant CQ and the asymmetry parameter η have been determined for two complex aluminium hydrides from 27 Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The thus obtained data for KAlH4 (CQ =(1.30±0.02) MHz, η=(0.64±0.02)) and NaAlH4 (CQ =(3.11±0.02) MHz, η<0.01) agree very well with data previously determined from MAS NMR spectra. The accuracy with which these parameters can be determined from static spectra turned out to be at least as good as via the MAS approach. The experimentally determined parameters (δiso , CQ and η) are compared with those obtained from DFT-GIPAW (density functional theory - gauge-including projected augmented wave) calculations. Except for the quadrupole coupling constant for KAlH4 , which is overestimated in the GIPAW calculations by about 30 %, the agreement is excellent. Advantages of the application of the Solomon echo sequence for the measurement of less stable materials or for in situ studies are discussed.

Structural, electronic, optical, elastic, thermodynamic and thermal transport properties of Cs2AgInCl6 and Cs2AgSbCl6 double perovskite semiconductors using a first-principles study

In this study, we employ the framework of first-principles density functional theory (DFT) computations to investigate the physical, electrical, bandgap and thermal conductivity of Cs2AgInCl6-CAIC (type I) and Cs2AgSbCl6-CASC (type II) using the GGA-PBE method. CAIC possesses a direct band gap energy of 1.812 eV, while CASC demonstrates an indirect band gap energy of 0.926 eV. The CAIC and CASC exhibit intriguingly reduced thermal conductivity, which can be attributed to the notable reduction in their respective Debye temperatures, measuring 182 K and 135 K, respectively. The Raman active modes computed under ambient conditions have been compared with real-world data, showing excellent agreement. The thermal conductivity values of CAIC and CASC compounds exhibit quantum mechanical characteristics, with values of 0.075 and 0.25 W m-1 K-1, respectively, at 300 K. It is foreseen that these outcomes will generate investigations concerning phosphors and diodes that rely on single emitters, with the aim of advancing lighting and display technologies in the forthcoming generations.

Threefold coordinated germanium in a GeO2 melt

The local structure around germanium is a fundamental issue in material science and geochemistry. In the prevailing viewpoint, germanium in GeO2 melt is coordinated by at least four oxygen atoms. However, the viewpoint has been debated for decades due to several unexplained bands present in the GeO2 melt Raman spectra. Using in situ Raman spectroscopy and density functional theory (DFT) computation, we have found a [GeOØ2]n (Ø = bridging oxygen) chain structure in a GeO2 melt. In this structure, the germanium atom is coordinated by three oxygen atoms and interacts weakly with two neighbouring non-bridging oxygen atoms. The bonding nature of the chain has been analyzed on the basis of the computational electronic structure. The results may settle down the longstanding debate on the GeO2 melt structure and modify our view on germanate chemistry.

Novel carbon allotropes in all-sp2 bonding networks: self-assembling design and first-principles calculations

Compared with traditional structure prediction methods, the purposeful bottom-up approach is better able to obtain structures with specified performance. In this study, we established two novel carbon phases in purely sp2-bonded networks, termed H61-carbon and H62-carbon, using a self-assembling approach. These carbyne-connected carbon allotropes had helix chains joined by cumulative double-bond chains. We certified the new carbon allotropes to be dynamically and mechanically stable. Both of these carbon allotropes exhibited excellent mechanical properties, and they had metallic and superconductive characteristics featuring superconducting transition temperatures of 10 K (H61-carbon) and 7.4 K (H62-carbon), respectively. These results provide an important strategy for the design of novel carbon allotropes with specified properties.

Preliminary Tc Calculations for Iron-Based Superconductivity in NaFeAs, LiFeAs, FeSe and Nanostructured FeSe/SrTiO3 Superconductors

Many theoretical models of iron-based superconductors (IBSC) have been proposed, but the superconducting transition temperature (T_c) calculations based on these models are usually missing. We have chosen two models of iron-based superconductors from the literature and computed the T_c values accordingly; recently two models have been announced which suggest that the superconducting electron concentration involved in the pairing mechanism of iron-based superconductors may have been underestimated and that the antiferromagnetism and the induced xy potential may even have a dramatic amplification effect on electron-phonon coupling. We use bulk FeSe, LiFeAs and NaFeAs data to calculate the T_c based on these models and test if the combined model can predict the superconducting transition temperature (T_c) of the nanostructured FeSe monolayer well. To substantiate the recently announced xy potential in the literature, we create a two-channel model to separately superimpose the dynamics of the electron in the upper and lower tetrahedral plane. The results of our two-channel model support the literature data. While scientists are still searching for a universal DFT functional that can describe the pairing mechanism of all iron-based superconductors, we base our model on the ARPES data to propose an empirical combination of a DFT functional for revising the electron-phonon scattering matrix in the superconducting state, which ensures that all electrons involved in iron-based superconductivity are included in the computation. Our computational model takes into account this amplifying effect of antiferromagnetism and the correction of the electron-phonon scattering matrix, together with the abnormal soft out-of-plane lattice vibration of the layered structure. This allows us to calculate theoretical T_c values of LiFeAs, NaFeAs and FeSe as a function of pressure that correspond reasonably well to the experimental values. More importantly, by taking into account the interfacial effect between an FeSe monolayer and its SrTiO₃ substrate as an additional gain factor, our calculated T_c value is up to 91 K and provides evidence that the strong T_c enhancement recently observed in such monolayers with T_c reaching 100 K may be contributed from the electrons within the ARPES range.

Li3B8O13X (X = Cl and Br): Two New Noncentrosymmetric Crystals with Large Birefringence Induced by BO3 Units

Enthusiasm for the exploration of nonlinear alkali metal borates remains high. Focusing on the Li-B-O-X (X = Cl and Br) system, two examples of noncentrosymmetric borates, Li₃B₈O₁₃Cl and Li₃B₈O₁₃Br, were obtained using a high-temperature solution method under vacuum conditions. Structurally, the Li₃B₈O₁₃X crystals exhibit two independent alternately arranged three-dimensional B-O network structures formed by the basic building block unit B₈O₁₆. The performance measurements demonstrate their short ultraviolet cutoff edges. The theoretical calculation indicates that the BO₃ units dominate the contribution to their large optical anisotropy with the birefringence, 0.094 and 0.088@1064 nm for Li₃B₈O₁₃Cl and Li₃B₈O₁₃Br, respectively.

M(NH2SO3)2·xH2O (M = Ca, Pb, x = 0, 1, 4): Effect of Hydrogen Bonding on Structural Transformations and Second Harmonic Generation of Metal Sulfamates

Nonlinear optical (NLO) crystals are very important for laser technology, but the performances of available NLO crystals are still insufficient for increasing demand. Recently, the exploration of new NLO crystals in non-π-conjugated systems with the heteroatomic tetrahedra is attracting a lot of interest. In this work, we systematically explore the metal sulfamates containing [NH₂SO₃] groups and four metal sulfamates, namely, Ca(NH₂SO₃)₂·4H₂O, Ca(NH₂SO₃)₂·H₂O, Pb(NH₂SO₃)₂·H₂O, and Pb(NH₂SO₃)₂ were synthesized by aqueous solution and hydrothermal methods. Notably, these metal sulfamates exhibit different crystal structures and optical properties owing to the diverse arrangement of the functional groups in their structures. In addition, due to hydrogen bond regulation, the centrosymmetric (CS) compound Ca(NH₂SO₃)₂·4H₂O can transform into noncentrosymmetric (NCS) Ca(NH₂SO₃)₂·H₂O, leading to NLO activity. Experimental characterizations and theoretical analysis reveal that these metal sulfamates are ultraviolet transparent and suitable for developing new NLO materials.

Shedding Light on the Structure and Characterization of K2ZnGe2O6: A Phase-Matchable Nonlinear Optical Crystal

Nonlinear optical (NLO) materials have recently aroused great interest owing to their capability of frequency conversion in solid-state lasers. Herein, we report an acentric zinc germanate K₂ZnGe₂O₆ obtained successfully through spontaneous crystallization methods. It affords a novel three-dimensional (3D) framework comprised of [GeO₄] and [ZnO₄] motifs with K atoms located in the tunnels. K₂ZnGe₂O₆ displays a moderate second-harmonic-generation (SHG) intensity (0.73 × KDP) with phase-matchable behavior. Optical characterization demonstrated that it has a UV cutoff edge located at 368 nm with a large energy band of 3.23 eV, accompanied by a wide transmission window, covering a 3-5 μm atmospheric window. Moreover, thermal properties implied that it possesses intriguing thermal stability of 987 °C and a congruent melting nature. Additionally, first-principles calculations unveiled that the NLO performance was primarily attributed to the collective effect of [GeO₄] and [ZnO₄] building units. These findings indicate that K₂ZnGe₂O₆ is a potential NLO crystal.

Ag2Mo2O7: an oxide solid-state Ag+ electrolyte

Ag₂Mo₂O₇ powders and micro-crystals were prepared at 400 °C for 24 h and 500 °C for 6 h using solid-state reactions. The Ag₂Mo₂O₇ samples crystalized in a triclinic P1̄ space group with the cell parameters a = 6.0972(1) Å, b = 7.5073(1) Å, c = 7.6779(2) Å, α = 110.43(1)°, β = 93.17(1)°, γ = 113.51(1)°, and V = 294.17(1) ų from Rietveld refinements. Ag₂Mo₂O₇ powder is homogeneous with size of 2-8 μm and the ceramic pellets are in good sintering conditions with a relative density ∼93%. The indirect band gaps E _g(i) of Ag₂Mo₂O₇ from reflectance measurements and DFT calculations are 2.63(1) and 1.80 eV. The vibrational modes of Ag₂Mo₂O₇ were investigated by first-principles (DFT) calculations and Raman spectrum measurements with 24 of 33 predicted Raman modes recorded. According to DOS analyses, the valence bands (VB) of Ag₂Mo₂O₇ are mainly constituted of O-2p and Ag-4d orbitals, while the conduction bands (CB) are mainly composed of Mo-4d and the O-2p orbitals. Regarding the impedance analysis, Ag₂Mo₂O₇ is a silver oxide ion electrolyte with a conductivity of ∼5 × 10^-4 S cm^-1 at 450 °C. The carrier activation energy of Ag₂Mo₂O₇ is 0.88(3) eV from the temperature dependent conductivity measurements.

Electronic structure, phonons and optical properties of baryte type scintillators TlXO4(XCl, Br)

This article thoroughly addresses the structural, mechanical, vibrational, electronic band structure and the optical properties of the unexplored thallous perchlorate and perbromate fromab initiocalculations. The zone centered vibrational phonon frequencies shows, there is a blue shift in the mid and high frequency range from Cl → Br due to change in mass and force constant with respect to oxygen atom. From the band structure it is clear that the top of the valence band is due to thalliumsstates, whereas the bottom of the conduction band is due to halogensand oxygenpstates, showing similar magnitude of dispersion and exhibits a charge transfer character. These characteristics and the band gap obtained are consistent with that of a favourable scintillators. Our findings deliver directions for the design of efficient TlXO₄based scintillators with high performance which are desirable for distinct applications such as medical imaging, high energy physics experiments, nuclear security.

A thermodynamic criterion for the choice of flux and its validity in NaBO2-fluxed β-BaB2O4 crystal growth

This work provides a promising theoretical methodology to solve the most fundamental problem in flux crystal growth.

Synergistic Effect of π-Conjugated [C(NH2)3] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation

The search for nonlinear optical (NLO) crystals with excellent comprehensive properties is a formidable challenge. In this work, two guanidine antimony fluorides, C(NH₂)₃Sb₂F₇ and C(NH₂)₃SbF₄, were obtained by conjunction of [C(NH₂)₃] groups with π-conjugated configuration and stereochemically active Sb³⁺ cations. Due to the different coordination modes of Sb-F bonds and H-F hydrogen bonds, the crystal structure of C(NH₂)₃Sb₂F₇ is centrosymmetric (CS), while C(NH₂)₃SbF₄ is noncentrosymmetric (NCS). Optical measurements show that the UV cutoff wavelengths of the title compounds were both less than 240 nm. Thermal studies indicate that these crystals are stable up to 250 °C. In addition, the second harmonic generation (SHG) response of C(NH₂)₃SbF₄ is 2 times that of KH₂PO₄ (KDP) with the phase-matchable capacity. Theoretical calculations reveal that the large SHG effects of C(NH₂)₃SbF₄ were attributed to the synergy between the planar [C(NH₂)₃] units and the distorted [SbF₄] groups. These results demonstrate that the guanidine antimony fluorides will have potential value as UV NLO materials.

Synthesis and Characterizations of Two Tellurides β-BaGa2Te4 and Ba5Ga2Ge3Te12 with Flexible Chain Structure

Demands for IR birefringent materials are increasing due to the rapid developments of IR laser applications. Herein, two new chain tellurides β-BaGa₂Te₄ and Ba₅Ga₂Ge₃Te₁₂ have been discovered. β-BaGa₂Te₄ crystallizes in the orthorhombic space group Imma (no. 74) with unit cell constants of a = 23.813(3) Å, b = 11.9673(19) Å, and c = 6.7215(9) Å, while Ba₅Ga₂Ge₃Te₁₂ crystallizes in the monoclinic space group P2₁/c (no. 14) with unit cell constants of a = 13.6540(3) Å, b = 9.6705(2) Å, and c = 23.1134(7) Å. The structure of β-BaGa₂Te₄ can be considered to be the antiparallel arrangement of one-dimensional (1D) [GaTe₂] chains formed by edge-sharing GaTe₄ tetrahedra, which are separated by Ba²⁺ cations. In the crystal structure of Ba₅Ga₂Ge₃Te₁₂, two kinds of 1D chains, namely chain 1 _∞¹[(GaGe)₃Te₈] and chain 2 _∞¹[(GaGe)₂Te₄], are stacked alternately and put together by the coulomb force with Ba²⁺ cations. In addition, First-principles calculations indicate that β-BaGa₂Te₄ has a large birefringence, ∼0.325 at 2050 nm, derived from the superposition of the polarizabilities of GaTe₄ tetrahedra, implying that it has potential as an IR birefringent material. This work may provide some guidance for exploring new IR birefringent crystals.

Ultra-broadband terahertz fingerprint spectrum of melatonin with vibrational mode analysis

Melatonin (MLT), as a neurotransmitter and an endogenous neurohormone, plays an important role in physiological functions through interactions with specific receptors. The conformations of MLT are closely related to its biological activities and functions. However, the internal relationship between the structure and interaction of MLT and its allosteric transition remains unclear. In this work, we obtain the broadband fingerprint terahertz (THz) spectrum of MLT in the range of 0.5-18 THz using the air-plasma terahertz time-domain spectroscopy (THz-TDS) system. DFT calculations are employed to analyze the vibration characteristics of MLT. The result shows that the low-frequency vibrations mainly come from the strong coupling between inter- and intramolecular vibrations, and the contribution of intramolecular vibrations gradually dominates with increasing frequency. Meanwhile, the local vibrations of the different functional groups distribute widely in the THz low-frequency band, relating to the diversity of conformational changes in the molecule. The intermolecular hydrogen bonds (HBs) have distinct resonant responses and play critical roles in the THz low-frequency vibrations. The study reveals the complex characteristics of the resonant coupling of MLT with THz electromagnetic waves. The results will help to understand the conformational preferences of MLT in neural signal transmission processes.

Comparative DFT study of vibrational, electronic, and optical properties of energetic alkali metal salts based on nitrogen-rich 5-aminotetrazole

This article presents a thorough density functional theory based comparative study on nitrogen-rich 5-aminotetrazole alkali metal salts M 5-At (M = Li, K, Rb, Cs). The calculated structural parameters using plane-wave pseudopotential method are consistent with the experimental results. The computed vibrational frequencies at ambient pressure show that vibrational modes in high energy region are due the NH bond of NH₂ group. Pressure variation IR spectra of these materials show clear frequency shifts where Li 5-At shows an overall red shift below 900 cm^-1 contrary to the blue shift seen in other materials in this range. The born effective charge values reveal the presence of strong covalency between N, H, and C atoms whereas an increased ionic nature is seen as the atomic number of metal atoms increases. Furthermore, we used full potential linear augmented plane wave (FP-LAPW) method for calculating electronic structure and optical properties with TB-mBJ potential which provides an enhanced band gap for all materials compared to standard GGA functional. Electronic structure calculation reveals that all the compounds are indirect band gap insulators with the exception of Li 5-At. The computed partial density of states show mixed ionic-covalent nature in metal-N/C bonds and covalent nature in NC bonds. In addition, we are also presenting the optical properties such as real and imaginary dielectric constant, absorption, refraction, reflection, loss spectrum as functions of photon energy. From the optical properties we can conclude that all the studied compounds are optically anisotropic in nature and are good absorbers in the ultraviolet (UV) region.

Intriguing Dimensional Transition Inducing Variable Birefringence in K2Na2Sn3S8 and Rb3NaSn3Se8

The birefringent crystals capable of modulating the polarization of lights are of the current research interests. Although many oxide crystals have been discovered and widely used in UV and visible regions, the birefringent crystals in the infrared (IR) region are still rare. Herein, two new chalcogenides, K₂Na₂Sn₃S₈ and Rb₃NaSn₃Se₈, have been synthesized by the solid-state method. We have used the single crystal X-ray diffraction to determine their structures. K₂Na₂Sn₃S₈ crystallizes in the monoclinic space group C2/c and exhibits a three-dimensional framework constructed by the corner-sharing SnS₄ and SnS₅ units, whereas Rb₃NaSn₃Se₈ crystallizes in the tetragonal space group P4/nbm and features a zero-dimensional [Sn₃Se₈]^4- trimer built by the three edge-sharing SnSe₄ tetrahedra. The physical property measurements indicate that Rb₃NaSn₃Se₈ has a wide IR transparent window up to 20 μm and large birefringence, ∼0.196, suggesting its potential application as a birefringent crystal in the IR region. However, compared with Rb₃NaSn₃Se₈, the birefringence of K₂Na₂Sn₃S₈ is relatively small, ∼0.070. The study of their structure-property relationship indicates that the different connection modes of SnQ_n (Q = S, Se; n = 4, 5) polyhedra are the main reason for the large difference of birefringence between the two compounds. These studies will provide a new insight for the origin of birefringence and will facilitate the exploration of new IR birefringent crystals.

The structure-Raman spectra relationships of Mg3(PO4)2 polymorphs: A comprehensive experimental and DFT study

Three Mg₃(PO₄)₂ polymorphs (Mg₃(PO₄)₂-I, II, III) were synthesized at high-pressure and high-temperature conditions. The structures and vibrational properties of Mg₃(PO₄)₂ polymorphs were studied by X-ray diffraction (XRD), Raman spectroscopy, and density functional theory (DFT) calculations. The obvious different PO stretching vibrational modes were experimentally observed for Mg₃(PO₄)₂-I, II, III. The calculated vibrational frequencies were in good agreement with measurements. All the observed vibrational modes for Mg₃(PO₄)₂-I, II, III were well assigned based on the calculations, which provided a support for investigating and comparing vibrational properties of three Mg₃(PO₄)₂ polymorphs.

The crystal growth and properties of novel magnetic double molybdate RbFe5(MoO4)7 with mixed Fe3+/Fe2+ states and 1D negative thermal expansion

Single crystals of new composition RbFe₅(MoO₄)₇ were successfully grown by the flux method, and their crystal structure was determined using the X-ray single-crystal diffraction technique.

A new acentric borate-nitrate Cs3B8O13(NO3) with interpenetrating porous 3D covalent and ionic lattices

A new acentric borate-nitrate Cs3B8O13(NO3) was synthesized by a molten salt method which consists of interpenetrating porous 3D covalent [B8O13]∞ and ionic [(NO3)Cs3]∞ lattices. It shows low ultraviolet cut-off edge (202 nm) and phase-matching second harmonic generation (SHG) intensity (0.7 KDP @1064 nm). First principles calculations showed that the main source of SHG is the cooperation of the B-O and [NO3]- groups.

A two-year water-stable 2D MOF with aqueous NIR photothermal conversion ability

Structural lability in humid air or water severely limits the practical use of MOFs. Developing new MOFs with exceptional water stability is interesting for both industrial applications and academic research. Herein we report a new method to improve the water stability of MOFs by using three-dimensional rigid shielding ligands. A very highly stable two-dimensional MOF (CuCP-MOF) is synthesized in this work, in which [2,2]paracyclophane dicarboxylate ligands are coordinated with Cu(ii) ions to form a paddle wheel structure. CuCP-MOF is a triclinic crystal with unit cell parameters a = 10.065 Å, b = 10.897 Å, c = 10.940 Å, α = 90.676°, β = 91.729°, and γ = 92.725° determined by single crystal X-ray diffraction and DFT simulation. It can easily form MOF nanosheets due to the large interlayer distance and weak interlayer interactions. It shows good aqueous stability, and remains intact after storage in water for two years, as evidenced by FTIR and XRD analyses. CuCP-MOF shows a strong absorption in the NIR range due to the d-d transition of Cu(ii). The aqueous dispersions of CuCP-MOF exhibit high NIR photothermal conversion efficiency, about 17.5% for a laser with an energy density of 5 W cm-2 (808 nm) and 22.0% for a laser of 2 W cm-2 on average.

K3B4PO10 and K2MB4PO10 (M = Rb/Cs): rare mixed-coordinated borophosphates with large birefringence

Mixed-coordinated borophosphates with a large B/P ratio, K₃B₄PO₁₀ and K₂MB₄PO₁₀ (M = Rb/Cs), exhibit deep-UV cut-off edges and a relatively large birefringence.

Structure Evolution of Na2O2 from Room Temperature to 500 °C

Na₂O₂ is one of the possible discharge products from sodium-air batteries. Here, we report the evolution of the structure of Na₂O₂ from room temperature to 500 °C using variable-temperature neutron and synchrotron X-ray powder diffraction. A phase transition from α-Na₂O₂ to β-Na₂O₂ is observed in the neutron diffraction measurements above 400 °C, and the crystal structure of β-Na₂O₂ is determined from neutron diffraction data at 500 °C. α-Na₂O₂ adapts a hexagonal P62m (no. 189) structure, and β-Na₂O₂ adapts a tetragonal I4₁/acd (no. 142) structure. The thermal expansion coefficients of α-Na₂O₂ are a = 2.98(1) × 10^-5 K^-1, c = 2.89(1) × 10^-5 K^-1, and V = 8.96(1) × 10^-5 K^-1 up to 400 °C, and a ∼10% volume expansion occurs during the phase transition from α-Na₂O₂ to β-Na₂O₂ due to the realignment/rotation of O₂^2- groups. Both phases are electronic insulators according to DFT calculations with band gaps (both indirect) of 1.75 eV (α-Na₂O₂) and 2.56 eV (β-Na₂O₂). An impedance analysis from room temperature to 400 °C revealed a significant enhancement of the conductivity at T ≥ 275 °C. α-Na₂O₂ shows a higher conductivity (∼10 times at T ≤ 275 °C and ∼3 times at T > 275 °C) in O₂ compared to in Ar. We confirmed, by dielectric analysis, that this enhanced conductivity is dominated by ionic conduction.

Photocatalytic partial oxidation of methanol to methyl formate under visible light irradiation on Bi-doped TiO2 via tuning band structure and surface hydroxyls

Preparing visible light responsive catalysts for partial oxidation of methanol to methyl formate is a challenging issue. This work addresses the synthesis, characterization and theoretical calculation of Bi doped TiO₂ catalysts as well as their photocatalytic performance and reaction mechanism for MF synthesis from methanol. The catalysts were prepared by a simple wet chemical method. The results of the characterization and theoretical calculation evidenced that bismuth was intercalated in the lattice of anatase by the substitution of titanium. Impurity levels were formed in the valence band, conduction band and between the two bands. The Bi 6s and 5p orbitals contributed to the formation of the impurity levels. The photo-excited electrons transited from the valence band via impurity levels, formed by Bi 6s orbitals, to the conduction band. The doping of Bi enhanced surface hydroxyls, reduced the band gaps and raised the valence band edges (VBE) of the Bi doped catalyst. The Bi doped catalysts were visible light responsive due to the reduced band gap. The surface hydroxyls were beneficial to the methanol conversion, and the rise of the VBE enhanced the redox potential of the photogenerated holes. Only moderate redox potentials and sufficient surface hydroxyls could lead to high methanol conversion and MF selectivity. This study is of great significance to the development of the photocatalytic synthesis theory and provides a green route for MF synthesis from methanol.

Bi-doped titania is well studied for photocatalytic partial oxidation of methanol to methyl formate under visible light irradiation.

Data-driven prediction of diamond-like infrared nonlinear optical crystals with targeting performances

Combining high-throughput screening and machine learning models is a rapidly developed direction for the exploration of novel optoelectronic functional materials. Here, we employ random forests regression (RFR) model to investigate the second harmonic generation (SHG) coefficients of nonlinear optical crystals with distinct diamond-like (DL) structures. 61 DL structures in Inorganic Crystallographic Structure Database (ICSD) are selected, and four distinctive descriptors, including band gap, electronegativity, group volume and bond flexibility, are used to model and predict second-order nonlinearity. It is demonstrated that the RFR model has reached the first-principles calculation accuracy, and gives validated predictions for a variety of representative DL crystals. Additionally, this model shows promising applications to explore new crystal materials of quaternary DL system with superior mid-IR NLO performances. Two new potential NLO crystals, Li2CuPS4 with ultrawide bandgap and Cu2CdSnTe4 with giant SHG response, are identified by this model.

Adsorption mechanism of Pt, Ag, Al, Au on GaAs nanowire surfaces from first-principles

The electronic and optical properties of metal (M) atoms adsorbed GaAs nanowires are systemically investigated utilizing first-principles calculations based on density functional theory. Different materials (M  =  Pt, Ag, Al and Au) and different coverages (1M, 2M, 3M and 4M) are considered to construct surface adsorption models. The calculations show that all metal-adsorbed GaAs nanowire surfaces are stable, and the difficulty of metal atom adsorption on nanowire surfaces follows the rule of Ag  >  Au  >  Al  >  Pt. In addition, the layer distance variation of nanowire surfaces after metal atom adsorption mainly take place near the outmost layer region. In 1M coverage case, the work function is reduced by Pt, Ag, Al adsorption, while increased by Au adsorption. Specially, Pt- and Al-adsorbed GaAs nanowire surfaces are direct band gap semiconductors, but Ag- and Au-adsorbed surfaces are indirect band gap. The adsorption of metals on GaAs nanowire surfaces are via chemisorption. Moreover, metal atom adsorption can enlarger the absorption coefficient of GaAs nanowires, which are gradually enhanced with increasing the coverage of metal atoms.

Time-Domain Terahertz Spectroscopy and Density Functional Theory Studies of Nitro/Nitrogen-Rich Aryl-Tetrazole Derivatives

The paper reports the time-domain THz spectroscopy studies of noncentrosymmetric energetic nitro/nitrogen-rich aryl-tetrazole high-energy molecules. The fingerprint spectra in the THz domain reveal the role of different functional groups attached to position "1" of the tetrazole moiety, which controls the energetic properties. These responses are deliberated through density functional theory (DFT) calculations. The synthesized aryl-tetrazoles exhibit high positive heat of formation (369-744 kJ/mol), high detonation velocities, and pressures (D _v: 7734-8298 m·s^-1; D _p: 24-28 GPa) in comparison to the noncentrosymmetric 2,4,6-trinitrotoluene (TNT). These compounds exhibit variation in the refractive indices and absorption between 0.1 and 2.2 THz range. The DFT studies at the molecular and single-crystal level (using plane wave pseudo potential method) endorse in detecting these bands (with ∼1% deviation). The calculated vibrational frequencies and linear optical properties are found to have good agreement with the experimental data in UV-visible and THz regions.

Structural investigations on two typical lithium germanate melts by in situ Raman spectroscopy and density functional theory calculations

In situ Raman spectroscopy, together with density functional theory calculations, was used to monitor the structural changes of polycrystalline Li₄GeO₄ and Li₆Ge₂O₇ from room temperature to their melting temperatures.

Syntheses, structures and characterization of non-centrosymmetric Rb2Zn3(P2O7)2 and centrosymmetric Cs2M3(P2O7)2 (M = Zn and Mg)

Three phosphates Rb₂Zn₃(P₂O₇)₂ and Cs₂M₃(P₂O₇)₂ (M = Zn and Mg) were successfully synthesized by a high-temperature solution method.

Refractive properties of the α-BaGeO3 crystal and their origins: a density functional theory study

Density functional theory calculations show that α-BaGeO₃ is a promising birefringent crystal used in the mid-IR region; its unique refractive characteristics are associated with the Ba–O bonds and their spatial orientations.

Enhancement of light emission in Bragg monolayer-thick quantum well structures

Control over spontaneous emission rate is important for improving efficiency in different semiconductor applications including lasers, LEDs and photovoltaics. Usually, an emitter should be placed inside the cavity to increase the spontaneous emission rate, although it is technologically challenging. Here we experimentally demonstrate a phenomenon of super-radiance observed in a cavity-less periodic Bragg structure based on InAs monolayer-thick multiple quantum wells (MQW). The collective super-radiant mode shows enhanced emission rate for specific angles and frequencies. This behaviour correlates with the calculations demonstrating individual spots of the enhanced Purcell coefficient near the Bragg condition curve. This study provides a perspective for realization of surface emitting cavity-less lasers with distributed feedback.

Investigations on Forming Ether Coated Iron Nanoparticle Materials by First-Principle Calculations and Molecular Dynamic Simulations

The mechanism of coating effects between ether molecules and iron (Fe) nanoparticles was generally estimated using first-principle calculations and molecular dynamic (MD) simulations coupling with Fe (110) crystal layers and sphere models. In the present work, the optimized adsorption site and its energy were confirmed. The single sphere model in MD simulations was studied for typical adsorption behaviors, and the double sphere model was built to be more focused on the gap impact between two particles. In those obtained results, it is demonstrated that ether molecules were prone to be adsorbed on the long bridge site of the Fe (110) crystal while comparing with other potential sites. Although the coating was not completely uniform at early stages, the formation of ether layer ended up being equilibrated finally. Accompanied with charge transfer, those coated ether molecules exerted much binding force on the shell Fe atoms. Additionally, when free ether molecules were close to the gap between two nanoparticles, they were found to come under double adsorption effects. Although this effect might not be sufficient to keep them adsorbed, the movement of these ether molecules were hindered to some extent.

First-Principles Investigation of Adsorption of Ag on Defected and Ce-doped Graphene

To enhance the wettability between Ag atoms and graphene of graphene-reinforced silver-based composite filler, the adsorption behavior of Ag atoms on graphene was studied by first-principles calculation. This was based on band structure analysis, both p-type doping and n-type doping form, of the vacancy-defected and Ce-doped graphene. It was verified by the subsequent investigation on the density of states. According to the charge transfer calculation, p-type doping can promote the electron transport ability between Ag atoms and graphene. The adsorption energy and population analysis show that both defect and Ce doping can improve the wettability and stability of the Ag-graphene system. Seen from these theoretical calculations, this study provides useful guidance for the preparation of Ag-graphene composite fillers.

Unusual optical isotropy in anisotropic alkali metal perchlorates MClO 4 (M = Li, Na, K, Rb, Cs)

We report a detailed study on structural, vibrational, born effective charge (BEC), electronic and optical properties of the alkali metal perchlorates, MClO ₄(M  =  Li, Na, K, Rb, Cs) based on Density functional theory. The ground state calculations are done using plane wave pseudopotential method by including dispersion corrected method for more accurate prediction of structural and vibrational frequencies. The calculated lattice parameters and bond lengths are consistent with the experimental values. Further, detailed interpretation of the zone centered vibrational modes yields good concurrence between the experimental and calculated values. There is a decrease in wavelength with an increase in frequency (blue shift) from Li  →  Na  →  K  →  Rb  →  Cs. The obtained BEC shows the mixed covalent-ionic character of the compounds. The electronic and optical properties are calculated using the full potential linearized augmented plane wave method by TB-mBJ potential. The TB-mBJ band structure shows indirect band gap with O-2p states dominating in the valence band. In spite of anisotropic structure, alkali metal perchlorates are found to possess optical isotropy.