Exploiting phase transitions in catalysis: reaction ofCO2andH2on dopedVO2-polymorphs

VO2is well known for its reversible transition between two phases with tetragonal rutile and monoclinic structure. In a previous theoretical study (Stahl and Bredow 2022ChemPhysChem23e202200131) we showed that the adsorption energy of CO is different on surfaces of the two Mo-stabilized polymorphs. This can be exploited to promote catalytic reactions by removing CO from the catalyst surface. As proof-of-principle, we investigated the hydrogenation reaction ofCO2. For this purpose, the adsorption energies ofCO2and possible intermediates and productsH2O, HCOOH,H2COand CO were calculated. Significant differences were found for the reaction energies of the hydrogenation ofCO2to formic acid and formaldehyde on the two polymorphs. This shows that it is in principle possible to alter the reaction thermodynamics by applying reaction conditions which stabilize a particular polymorph. In order to investigate the influence of the polymorph on kinetic properties, the reactions barriers of a step-wise reaction ofCO2+2H2→H2CO+H2Owas calculated using the nudged elastic band method.VO2was found to reduce the reaction barriers compared to the gas phase. Additionally, the minimum energy path of the bulk phase transition of undopedVO2was calculated using the distinguished reaction coordinate method. A catalytic cycle exploiting the phase transition is proposed based on the theoretical results.

Structural, electronic and magnetic properties of greigite Fe3S4by GGA and GGA+Uversus SCAN meta-GGA density functionals

The performance of exchange-correlation functional of density functional theory represented in generalized gradient approximation (GGA) and in the strongly constrained and appropriately normed (SCAN) meta-GGA scheme to study structural, electronic, and magnetic properties of greigite (Fe3S4) was investigated. The effects of inclusion of strong electron correlations represented by on-site Hubbard correctionU, and nonlocality of the long-range van der Waals (vdW) interactions were also considered. Geometry optimization yielded the inverse spinel structure and lattice parameter of greigite in good agreement with experimental data. Calculated electronic structure revealed a half-metallic nature of the greigite bands for the applied functionals except for GGA, which predicts metallic behavior. Antiferromagnetic coupling of iron ions in tetrahedral and octahedral coordinations makes the overall crystal structure ferrimagnetic. In general the GGA+Uand SCAN show comparable performance in prediction physical properties of greigite. Inclusion of the vdW correction does not change the character of the bands.

Rb2Ln[Si2O6]F (Ln = Y, Eu-Lu): Flux Synthesis, Structure Determination, and DFT-Calculated Formation Enthalpies of a Series of Mixed-Anion Rare Earth Cyclosilicates

A new series of mixed-anion alkali rare earth silicate fluorides with composition Rb2Ln[Si2O6]F (Ln = Y, Eu-Lu) has been synthesized via an alkali chloride/fluoride eutectic flux synthetic route. All synthesized compositions crystallize in the tetragonal space group P42/mnm with a 3D framework consisting of LnO4F2 octahedra, tetrasilicate rings, and 1D channels containing alkali metals. A combination of powder X-ray diffraction, single-crystal X-ray diffraction, and luminescence emission spectroscopy was performed to characterize the reaction products. In addition, density functional theory (DFT) calculations were utilized to calculate the 0 K formation enthalpies of the synthesized phases and of hypothetical trivalent actinide analogues to probe the likelihood of the successful synthesis of such trivalent transuranic containing phases, specifically Am and Cm, in the future.

Negative Thermal Expansion in the Polymorphic Modification of Double Sulfate β-AEu(SO4)2 (A-Rb+, Cs+)

New polymorphic modifications of double sulfates β-AEu(SO₄)₂ (A-Rb⁺, Cs⁺) were obtained by the hydrothermal method, the structure of which differs significantly from the monoclinic modifications obtained earlier by solid-state methods. According to single-crystal diffraction data, it was found that the compounds crystallize in the orthorhombic system, space group Pnna, with parameters β-RbEu(SO₄)₂: a = 9.4667(4) Å, b = 13.0786(5) Å, c = 5.3760(2) Å, V = 665.61(5) ų; β-CsEu(SO4)2: a = 9.5278(5) Å, b = 13.8385(7) Å, c = 5.3783(3) Å, V = 709.13(7) ų. The asymmetric part of the unit cell contains one-half Rb⁺/Cs⁺ ion, one-half Eu³⁺ ion, both in special sites, and one SO₄^2- ion. Both compounds exhibit nonlinear negative thermal expansion. According to the X-ray structural analysis and theoretical calculations, the polarizing effect of the alkali metal ion has a decisive influence on the demonstration of this phenomenon. Experimental indirect band gaps of β-Rb and β-Cs are 4.05 and 4.11 eV, respectively, while the direct band gaps are 4.48 and 4.54 eV, respectively. The best agreement with theoretical calculations is obtained using the ABINIT package employing PAW pseudopotentials with hybrid PBE0 functional, while norm-conserving pseudopotentials used in the frame of CASTEP code and LCAO approach in the Crystal package gave worse agreement. The properties of alkali ions also significantly affect the luminescent properties of the compounds, which leads to a strong temperature dependence of the intensity of the ⁵D₀ → ⁷F₄ transition in β-CsEu(SO₄)₂ in contrast to much weaker dependence of this kind in β-RbEu(SO₄)₂.

Developing Waste Forms for Transuranic Elements: Quaternary Neptunium Fluorides of the Type NaxMNp6F30 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Al, Ga)

A series of quaternary Np(IV) fluorides was synthesized using a mild hydrothermal synthesis approach. The compositions are all of the type Na_xMNp₆F₃₀, where M = Ti(III), V(III), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Al(III), and Ga(III) and x = 4 for divalent metals, x = 3 for trivalent metals. The compounds all crystallize in the P-3c1 space group and are isotypic with actinide analogues Na_xMAn₆F₃₀ (An = Ce, U, Th, Pu). Structure data from the neptunium crystals were combined with data from the other actinide analogues to establish the tetravalent, nine-coordinated ionic radii of neptunium (1.030(2) Å), plutonium (1.014(1) Å), and cerium (1.012(2) Å). Radiation damage studies were also carried out on a surrogate material, the cerium analogue Na₃AlCe₆F₃₀, which indicates that the structure type has low resistance to amorphization. Density functional theory calculations were carried out to compute the band gaps and enthalpies of formation variations among the isotypic cerium and actinide structures to compare the stability of the structures.

Enhanced Electron Correlation and Significantly Suppressed Thermal Conductivity in Dirac Nodal-Line Metal Nanowires by Chemical Doping

Enhancing electron correlation in a weakly interacting topological system has great potential to promote correlated topological states of matter with extraordinary quantum properties. Here, the enhancement of electron correlation in a prototypical topological metal, namely iridium dioxide (IrO₂ ), via doping with 3d transition metal vanadium is demonstrated. Single-crystalline vanadium-doped IrO₂ nanowires are synthesized through chemical vapor deposition where the nanowire yield and morphology are improved by creating rough surfaces on substrates. Vanadium doping leads to a dramatic decrease in Raman intensity without notable peak broadening, signifying the enhancement of electron correlation. The enhanced electron correlation is further evidenced by transport studies where the electrical resistivity is greatly increased and follows an unusual T $\sqrt T $ dependence on the temperature (T). The lattice thermal conductivity is suppressed by an order of magnitude via doping even at room temperature where phonon-impurity scattering becomes less important. Density functional theory calculations suggest that the remarkable reduction of thermal conductivity arises from the complex phonon dispersion and reduced energy gap between phonon branches, which greatly enhances phase space for phonon-phonon Umklapp scattering. This work demonstrates a unique system combining 3d and 5d transition metals in isostructural materials to enrich the system with various types of interactions.

Exploiting Phase Transitions in Catalysis: Adsorption of CO on doped VO2 -Polymorphs

VO₂ is well known for its low-temperature metal-insulator transition between two phases with tetragonal rutile and monoclinic structure. The adsorption of CO on the two polymorphs of Mo-doped VO₂ is calculated to investigate the effect of a substrate phase change on the adsorption energy. The system is investigated theoretically at density-functional theory level using a hybrid functional with London dispersion correction. We establish a computational protocol applicable for the study of physisorption on open-shell transition metal oxides. The main task is to control the spin state of open-shell slab models used to model adsorption of closed-shell molecules in order to obtain numerically stable adsorption energies and to reduce spin contamination within the broken-symmetry unrestricted Kohn-Sham approximation. Applying this procedure, it is possible to identify the most stable adsorption positions of CO on both phases of VO₂ . CO adsorbs vertically with the C atom on a surface V atom in the monoclinic phase with an adsorption energy of -56 kJ/mol. The same adsorption position has an adsorption energy of only -46 kJ/mol on the rutile phase. Similar differences were obtained with multireference methods using an embedded cluster model. This effect may inspire experimental strategies exploiting the rutile ↔ ${ \leftrightarrow }$ monoclinic VO₂ phase transition in catalytic processes where CO is formed as product or as an intermediate.

Crystallization of A3Ln(BO3)2 (A = Na, K; Ln = Lanthanide) from a Boric Acid Containing Hydroxide Melt: Synthesis and Investigation of Lanthanide Borates as Potential Nuclear Waste Forms

A series of alkali metal rare-earth borates were prepared via high-temperature flux crystal growth, and their structures were characterized by single crystal X-ray diffraction (SXRD). Na₃Ln(BO₃)₂ (Ln = La-Lu) crystallize in the monoclinic space group P2₁/n, the potassium series K₃Ln(BO₃)₂ (Ln = La-Tb) crystallize in the orthorhombic space group Pnma, while the Ln = Dy, Ho, Tm, Yb analogues crystallize in the orthorhombic space group Pnnm. To demonstrate the generality of this synthetic technique, high-entropy oxide (HEO) compositions K₃Nd_0.15(1)Eu_0.20(1)Gd_0.20(1)Dy_0.22(1)Ho_0.23(1)(BO₃)₂ and K₃Nd_0.26(1)Eu_0.29(1)Ho_0.22(1)Tm_0.14(1)Yb_0.10(1)(BO₃)₂ were obtained in single crystal form. Radiation damage investigations determined that these borates have a high radiation damage tolerance. To assess whether trivalent actinide analogues of Na₃Ln(BO₃)₂ and K₃Ln(BO₃)₂ would be stable, density functional theory was used to calculate their enthalpies of formation, which are favorable.

The Role of Local DFT+U Minima in the First-Principles Modeling of the Metal-Insulator Transition in Vanadium Dioxide

The DFT+U method is frequently employed to improve the first-principles description of strongly correlated materials. However, it is prone to deliver metastable electronic minima. While these local minima of the DFT+U method are often considered to be computational artifacts, their physical meaning and relationship to true excited states remains unclear. In this work, the possibility of theoretically modeling transformations in the solid state that require thermal or optical excitations of electrons is explored, taking into account the metastable states of the computationally undemanding DFT+U formalism. For this purpose, we choose to examine the example of the VO₂ metal-insulator transition. Metastable states that are located on different electronic potential energy surfaces are found to correspond to experimentally observed VO₂ phases. The identified metastable electronic states can be used to model the collapse of the VO₂ band gap at elevated temperatures and upon photoexcitation as well as other monoclinic-monoclinic phase transformations. The results suggest that local DFT+U minima can indeed carry physical meaning, while they remain under-reported in theoretical literature on transition metal oxides like VO₂.

Effect of Doping on Rutile TiO2 Surface Stability and Crystal Shapes

Transition-metal-(TM-)doped TiO₂ has been considered as promising electrode material for the oxygen evolution reaction (OER). OER activity is expected to depend on the coordination of the surface atoms. In this study, we theoretically investigate the stability of low-index surfaces of TM-doped rutile, (110), (100), (101) and (001), with 50 % of the Ti atoms substituted by Sc, Y, V, Nb or Ta. For Sc and Y, we also consider models with O vacancies providing the most stable oxidation state of Sc and Y. Surface energies are calculated with DFT(+U). Based on the Gibbs-Wulff theorem, the shape of the single crystals is predicted. It is observed that p-doping leads to spontaneous oxygen loss and O vacancies cause surface reconstruction. The Wulff shapes of n-doped TiO₂ have smaller contributions of the (110) facet and, for Nb and Ta, larger contributions of other facets. Given the higher coordinative unsaturation of the TM atoms in the latter, a higher catalytic activity is expected.

The Origin of the Thermochromic Property Changes in Doped Vanadium Dioxide

Vanadium dioxide is a promising material for novel smart window applications due to its reversible metal-insulator transition which is accompanied by a change in its optical properties. The transition temperature (TMIT) can be controlled via elemental doping, but the reduction of TMIT is generally coupled with a decrease of the optical contrast between the two phases. To better understand how the contrast is fundamentally connected to TMIT, the thermochromic properties of doped VO2 were theoretically investigated across the metal-insulator transition from first principles. Different dopants and their interaction with the VO2 host structure as well as different modes of doping were studied in detail. It was found that the transition temperature change is mainly related to the stabilization of the high-temperature metallic phase due to lattice deformations which are caused by the presence of the dopant ion. Inherent limitations to the thermochromic performance of VO2 substitutionally doped by the replacement of vanadium cations with other species were found, and alternative approaches were proposed. Specifically, a charge-neutral substitution of oxygen or an oxygen substitution in combination with interstitial doping without net charge transfer between the dopant atoms and VO2 were identified as promising avenues to ensure a low TMIT and no loss of optical contrast in vanadia-based smart window materials.

The mechanism of semiconductor to metal transition in the hydrogenation of VO2: A density functional theory study

VO2 is a glamorous material with specific metal-semiconductor-transition (MST). The hydrogenation of VO2 could make it a promising material applying in the ambient environment. In this work, we reveal the...

First principles studies of the electronic and structural properties of the rutile VO2(110) surface and its oxygen-rich terminations

We present a density functional theory (DFT) study of the structural and electronic properties of the bare metallic rutile VO₂(110) surface and its oxygen-rich terminations. Due to the polyvalent nature of vanadium and abundance of oxide phases, the modelling of this material on the DFT level remains a challenging task. We discuss the performance of various DFT functionals, including PBE, PBE +U(U= 2 eV), SCAN and SCAN + rVV functionals with non-magnetic and ferromagnetic spin ordering, and show that the calculated phase stabilities depend on the chosen functional. We predict the presence of a ring-like termination that is electronically and structurally related to an insulating V₂O₅(001) monolayer and shows a higher stability than pure oxygen adsorption phases. Our results show that employing the spin-polarized SCAN functional offers a good compromise, as it offers both a reasonable description of the structural and electronic properties of the rutile VO₂bulk phase and the enthalpy of formation for oxygen rich vanadium phases present at the surface.

Surfaces of VO2 -Polymorphs: Structure, Stability and the Effect of Doping

Vanadium dioxide is an interesting and frequently applied material due to its metal‐insulator phase transition. However, there are only few studies of the catalytic activity and surface properties of different VO₂ polymorphs. Therefore, we investigated the properties of the surfaces of the most stable VO₂ phases theoretically at density‐functional theory level using a self‐consistent hybrid functional which has demonstrated its accuracy for the prediction of structural, electronic and energetic properties in a previous study. We found that the surfaces of the rutile R phase of VO₂ are not stable and show a spontaneous phase transition to the monoclinic M₁ phase. Doping with Mo stabilizes the surfaces with rutile structure even for small dopant concentrations (6.25 %). Both M₁ and R surfaces strongly relax, with and without doping. In particular the metal‐metal distances in the uppermost layers change by up to 0.4 Å. Mo segregates in the topmost layer of both R and M₁ phases. The electronic structure is only slightly changed upon doping.

S = 1/2 Chain in BiVO3F: Spin Dimers versus Photoanodic Properties

BiVO₃F was prepared, characterized, and identified as a unique example of bismuth vanadyl oxyhalide with paramagnetic V⁴⁺ centers. Its crystal structure shows 1D magnetic units with rare alternation of edge-sharing O-O and F-F μ₂ bridges along the octahedral chains. Structural pairing across the O₂ edges induces antiferromagnetic spin dimers (S = 0) with J/K_b ≈ 300 K, ∼15 times greater than the exchange across the F₂ bridges, within a non-ordered magnetic ground state. Despite multiple compositional, structural, and electronic analogies with the BiVO₄ scheelite compound, one of the most promising photoanodes for solar water splitting, the photoactivity of BiVO₃F is relatively modest, partially due to this electronic pairing benefitting fast electron-hole recombination. Similar to monoclinic VO₂, the V⁴⁺ spin dimerization deters the singlet → triplet electronic photoexcitation, but results in potential carrier lifetime benefits. The reduction of the bandgap from an E_g of ∼2.4 eV to ∼1.7 eV after incorporation of d¹ cations in BiVO₄ makes BiVO₃F an inspiring compound for local modifications toward an enhanced photoactive material. The direct d → d transition provides a significant enhancement of the visible light capture range and opens a prospective route for the chemical design of performant photoanodes with a mixed anionic sublattice.

In Situ Conversion of Metal-Organic Frameworks into VO2 -V3 S4 Heterocatalyst Embedded Layered Porous Carbon as an "All-in-One" Host for Lithium-Sulfur Batteries

Although lithium-sulfur batteries exhibit a fivefold higher energy density than commercial lithium-ion batteries, their volume expansion and insulating nature, and intrinsic polysulfide shuttle have hindered their practical application. An alternative sulfur host is necessary to realize porous, conductive, and polar functions; however, there is a tradeoff among these three critical factors in material design. Here, the authors report a layered porous carbon (LPC) with VO₂ /V₃ S₄ heterostructures using one-step carbonization-sulfidation of metal-organic framework templates as a sulfur host that meets all the criteria. In situ conversion of V-O ions into V₃ S₄ nuclei in the confined 2D space generated by dynamic formation of the LPC matrix creates {200}-facet-exposed V₃ S₄ nanosheets decorated with tiny VO₂ nanoparticles. The VO₂ /V₃ S₄  @ LPC composite facilitates high sulfur loading (70 wt%), superior energy density (1022 mA h g^-1 at 0.2 C, 100 cycles), and long-term cyclability (665 mA h g^-1 at 1 C, 1000 cycles). The enhanced Li-S chemistry is attributed to the synergistic heterocatalytic behavior of polar VO₂ and conductive V₃ S₄ in the soft porous LPC scaffold, which accelerates polysulfide adsorption, conversion, and charge-transfer ability simultaneously.