Strong Covalency Research Articles

Pr3+ doped BaNb2O6 reddish orange emitting phosphor for solid state lighting and optical thermometry applications

A novel reddish-orange emitting BaNb2O6: Pr3+ phosphor was synthesized by conventional solid-state reaction method and its structural, morphological, optical properties and temperature dependent photoluminescence properties were investigated. The Le Bail profile matching of XRD patterns confirmed the orthorhombic phase presence and the corresponding cell parameters were calculated.Through a profound investigation of UV–visible diffuse reflectance spectra, a decrease in optical-band gap energy and an increase in Urbach tail width were noticed in doped sample as compared to undoped sample. Under the blue light excitation, the emission spectra exhibit peaks in reddish-orange region due to intra f-f transitions of Pr3+ ions. Strong covalence between Pr3+ and O2- was estimated by calculating nephelauxetic ratio, bonding parameter and covalency using the excitation spectra. The involvement of intervalence charge transfer (IVCT) state results in nonradiative relaxation increase, for which the emission peak intensities decreased with increase in temperature. The configurational coordinate model was used to demonstrate this phenomenon. The fluorescence intensity ratio technique was utilized for temperature sensing applications. The suitable CIE coordinates and significant temperature dependent PL makes the BaNb2O6: Pr3+ material to be a potential candidate for contributing red component in WLED and temperature sensing applications.

Copper comeuppance

There are many formally copper(iii) complexes, but X-ray spectroscopy and theory now reveal that they often feature strong covalence and physical oxidation states lower than +iii.

In silico design of pseudo D5h actinide based molecular magnets: role of covalency in magnetic anisotropy

Actinide molecular magnets are of great interest in the area molecular magnetism as they possess strong covalency and spin-orbit coupling that their 4f congeners lack. Despite these known advantages, the actinide based molecular magnets have not been explored in detail due to the limited availability of actinide salts. While theoretical tools are proven to be useful in lanthanide chemistry towards prediction, they are still at an infant stage in actinide chemistry. In this manuscript, we have attempted to utilise CASSCF/RASSI-SO calculations to predict suitable pseudo D5h molecules possessing attractive magnetic properties. To begin with, we have undertaken an extensive benchmarking of the methodology employed by studying two sets of isostructural, [NdTp3], [UTp3], [Nd(COT″)2]-, [U(COT″)2]- {Tp− = trispyrazolylborate, COT″ = bis(trimethylsilyl)cyclooctatetraenyl dianion} complexes. The method assessment reveals that the methodology employed is reliable as this has reproduced the experimental observables. With this, we have moved forward with prediction where pseudo-D5h symmetric [L2M(H2O)5][I]3L2.(H2O) {M = Nd, U, Pu; L = tBuPO(NHiPr)2} systems are modelled from their Nd(III) X-ray structure. Our calculations reveal that the Uranium complex studied possess superior SIM characteristics compared to its lanthanide analogue. Plutonium complex has prolate electron density at the ground state, and hence the ligand environment is unsuitable for yielding SIM behaviour. The role of solvent molecules, counter anions and equatorial and axial ligand are explored and tantalizing prediction with a barrier height of 1403.3 and 989.5 cm−1 are obtained for [tBuPO(NHiPr)2-U-tBuPO(NHiPr)2]3+ and [Pu(H2O)5]3+ models, respectively and this paves the way for synthetic chemist to target such geometries in actinide based SIMs. Is pseudo D5h geometry suitable for Actinide SIMs? In this article, we have extensively studied uranium complex and compared it with its lanthanide analogue. The pseudo D5h symmetric uranium complex possesses oblate electron density which is favourable in the axial ligand field and henceforth it is predicted to have attractive SIM behaviour. Studies on models suggest the low-coordinate actinide chemistry may hold the key to further the blocking temperature beyond liquid N2 temperatures.

Enhanced MIR emission of Er3+/Yb3+ co-doped fluorochloride glass with energy transfer and MIR light source application

The novel Er3+/Yb3+ co-doped fluorochloride glasses with good luminescent properties in the infrared region have been prepared. The middle infrared (MIR) luminescence of fluorochloride glasses is improved by anion substitution and rare-earth ion co-doping strategy. The chloridion doping reduces the phonon state density of the host matrix significantly. The ytterbium ions co-doping promotes the effective energy transfer of Yb3+:2F5/2 → Er3+:4I11/2. The Ω2 value of samples increases with the introduction of Yb3+, which means great asymmetry and strong covalency. The emission cross-section area σem (1.06 × 10−20 cm2) and the absorption cross-sectional area σabs (0.97 × 10−20 cm2) of Er3+/Yb3+ co-doped sample are all larger than that in the previous report. The fluorescence decay characteristics of samples at 980 nm and 1550 nm were investigated. In addition, a MIR light source was prepared preliminarily. Therefore, Er3+/Yb3+ co-doped fluorochloride glass is a potential MIR laser material.

Defect-Induced Enhancement Emission Intensity of Ca4.85(BO3)3F(C4.85BF):0.15Bi3+ by Introducing Cation (Na+, Sr2+, Ba2+) or Anion (Cl-).

Generally, the emission intensity of phosphors can be enhanced by introducing a proper number of defects. To enhance the emission intensity of Ca4.85(BO3)3F(C4.85BF):0.15Bi3+, more Frenkel defects were introduced by Na+, Sr2+, and Ba2+. It is found that the number of Frenkel defects is related to volume and covalence of the crystal, in which the covalence has a greater effect than the volume. Furthermore, the larger the volume of the crystal is, the stronger the covalence of the crystal is, the more Frenkel defects will be produced. The volume of Ca4.85- xSr x(BO3)3F(C4.85- xSr xBF):0.15Bi3+ is larger than that of Ca4.85- xNa x(BO3)3F(C4.85- xNa xBF):0.15Bi3+; however, the covalence of Na+ is similar to that of Sr2+, which leads to the same trap depth ( Eα) and defect density (μg) in the quenching concentration. The results also confirmed that the number of Frenkel defects is mainly influenced by the covalence of crystal. Furthermore, crystal distortion also affects the number of Frenkel defects. C4.85- xSr xBF:0.15Bi3+ and C4.85- xNa xBF:0.15Bi3+ have the same distortion at quenching concentration, which results in the same emission intensity in the quenching concentration. Ca4.85- xBa x(BO3)3F (C4.85- xBa xBF):0.15Bi3+ has a larger volume and stronger covalence; meanwhile, it has deeper trap depth ( Eα) and larger defect density (μg) at the quenching concentration, comparing with C4.85- xSr xBF:0.15Bi3+ and C4.85- xNa xBF:0.15Bi3+. However, the distortion of C4.85- xBa xBF:0.15Bi3+ is in agreement with C4.85- xNa xBF:0.15Bi3+ and C4.85- xSr xBF:0.15Bi3+, which leads to the emission intensity of C4.85- xBa xBF:0.15Bi3+ basically the same as that of C4.85- xNa xBF:0.15Bi3+ and C4.85- xSr xBF:0.15Bi3+ in quenching concentration. And the different rates of distortion result in the different quenching concentrations of C4.85- xNa xBF:0.15Bi3+, C4.85- xSr xBF:0.15Bi3+, and C4.85- xBa xBF:0.15Bi3+. Moreover, for Ca4.85- xMg x(BO3)3F(C4.85- xMg xBF):0.15Bi3+ and Ca4.85(BO3)3F1- yCl y(C4.85BF1- yCl y):0.15Bi3+, there are no Frenkel defects due to weaker covalence and smaller volume of the crystal in C4.85- xMg xBF:0.15Bi3+. However, Frenkel defects can be observed in C4.85BF1- yCl y:0.15Bi3+ due to stronger covalence and larger volume of the crystal, furthermore, and the emission spectra and thermoluminescence spectra of C4.85BF1- yCl y:0.15Bi3+ are similar to those of 0.15Bi3+ doped C4.85- xNa xBF:0.15Bi3+, C4.85- xSr xBF:0.15Bi3+, and C4.85- xBa xBF:0.15Bi3+.

Comparative studies of magnetic properties in osmates with the double perovskite structure

Using the first-principles density functional approach, we comparatively studied the electronic and magnetic properties of two series of ordered double perovskite, $\mathrm{B}{\mathrm{a}}_{2}B\mathrm{Os}{\mathrm{O}}_{6}\phantom{\rule{4pt}{0ex}}(B=\mathrm{Sc},\text{Y},\phantom{\rule{4pt}{0ex}}\text{and In})$ and ${A}_{2}\mathrm{ScOs}{\mathrm{O}}_{6}\phantom{\rule{4pt}{0ex}}(A=\text{Ba},\text{Sr},\phantom{\rule{4pt}{0ex}}\text{and Ca})$. The electronic structure calculations indicate that the Os ion bears the nominal ${t}_{2g}^{3}$ state with oxidation state $5+$ and is the only magnetically active one in all compounds. In nice agreement with the experimental observations, all of them stabilized in the type I antiferromagnetic alignment, which is mediated by the moderate nearest-neighbor antiferromagnetic interactions through the $\mathrm{Os}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}B\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Os}90$\ifmmode^\circ\else\textdegree\fi{} route and accompanied by mild next-nearest-neighbor antiferromagnetic coupling via the $\mathrm{Os}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}B\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Os}180$\ifmmode^\circ\else\textdegree\fi{} route, in spite of the geometric frustration. The mechanism of the antiferromagnetic interaction has been proposed based on the density of states analysis. In addition, the computed magnetic moment on Os ions, $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$, is remarkably smaller than the ideal spin-only value. In conjunction with the modest spin-orbit coupling effect, strong covalency between the $\mathrm{Os}\text{\ensuremath{-}}5d$ and $\mathrm{O}\text{\ensuremath{-}}2p$ states reduces the magnetic moments. More importantly, our results demonstrated that the magnitude of the magnetic couplings as well as magnetic transition temperature ${T}_{N}$ are directly proportional to the reciprocal of the absolute value of energy mismatch between the $B\text{\ensuremath{-}}nd$ and occupied $\mathrm{Os}\text{\ensuremath{-}}{t}_{2g}$ band; in addition, they are also a function of the Sc-O-Os bond angles, i.e., the buckling of the octahedral connectivity. Moreover, comparison of these two series helps to identify the energy mismatch as the efficient factor to adjust the magnetic interactions.

Understanding the kinetics and molecular mechanism of unimolecular gas phase thermal decomposition of the α-ketoester methyl benzoylformate using RRKM and BET theories

The RRKM calculation and bonding evolution theory analysis coupled with quantum theory of atoms in molecules have been used to investigate kinetics and molecular mechanism of gas phase thermal decomposition of methyl benzoylformate. The pressure-dependent rate coefficients, by applying different collisional efficiency values, indicated that the atmospheric pressure is in high-pressure limit of fall-off curve and low-pressure limit rate coefficients are in the range of 10−13-10−12 cm3 molec−1 s−1. Temperature dependence of high-pressure limiting of rate coefficient over the temperature range 733 ± 20% K was estimated to be k∞RRKM(CBS-QB3)=2.92×1013s-1exp(227.4kJmol-1/RT) and k∞RRKM(PBE1PBE)=2.67×1013s-1exp(232.8kJmol-1/RT). Topological analysis of electron localization function and electron density at the B3LYP/6-311G(d,p) level reveal that the reaction can be occurred as going through seven turning points defined as methyl benzoylformate 8-CF†FF†TSFC†[CF†]-0: methyl benzoate + carbon monoxide. The molecular mechanism can be categorized in three fundamental sections A) heterolytic rupture of O3C8 bond and detachment of methoxy part; B) formation of O3C7 bond via donation bond formation mechanism; and C) heterolytic rupture of C7C8 bond and detachment of carbon monoxide. The electron density ρ(3,−1)(r) in the region of bond forming and breaking increases and decreases along the reaction course to reflect bond strengthening and weakening, respectively. AIM parameters revealed that so long as chemical bonds are unformed/ruptured, interactions at related BCPs are covalence in nature, otherwise the nature of chemical bonds are strong shared covalence.

Covalency-driven collapse of strong spin-orbit coupling in face-sharing iridium octahedra

We report $\textit{ab-initio}$ density functional theory calculation and Raman scattering results to explore the electronic structure of Ba$_5$CuIr$_3$O$_{12}$ single crystals. This insulating iridate, consisting of face-sharing IrO$_6$ octahedra forming quasi-one-dimensional chains, cannot be described by the local $j_{\rm eff}$=1/2 moment picture commonly adopted for discussing electronic and magnetic properties of iridate compounds with IrO$_6$ octahedra. The shorter Ir-Ir distance in the face-sharing geometry, compared to corner- or edge-sharing structures, leads to strong covalency between neighboring Ir. Then this strong covalency results in the formation of molecular orbitals (MO) at each Ir trimers as the low-energy electronic degree of freedom. The theoretically predicted three-peak structure in the joint density of states, a distinct indication of deviation from the $j_{\rm eff}$=1/2 picture, is verified by observing the three-peak structure in the electronic excitation spectrum by Raman scattering.

Energy transfer phenomenon of Gd3+ to excited ground state of Eu3+ ions in Li2O-BaO-Gd2O3-SiO2-Eu2O3 glasses.

Li2O-BaO-Gd2O3-SiO2 glasses with different concentration of Eu3+ ions were developed by the traditional melt quenching technique and characterized via FTIR, absorption, excitation, emission and CIE color coordinates analysis for visible red emission application. The FTIR shows strong band at position 740 cm-1 which is attributed to Si-O-Si symmetric stretching mode. Density and molar volume of LBGSEu glasses increases with Eu3+ ions concentration. The covalent nature of bond between the Eu3+ ions and surrounding ligands was confirmed from the bonding parameter (δ). From absorption spectra JO-parameters and oscillator strength are evaluated for LBGSEu6 glass. From JO-parameters, Eu3+ ions have asymmetric coordination environment and stronger covalency. The phonon line PSB (22,522 cm-1) confirm the phonon energy ≈971 cm-1, that corresponds to the energy of one phonon associated with maximum energy of the vibrational mode couple to Eu3+ ions. Under 275 nm and 393 nm excitation, intense red emission was observed at 613 nm, we observe efficient energy transfer phenomena from Gd3+ → Eu3+ in these glasses. Increasing trend of IR with increasing concentration of Eu2O3 indicates the asymmetric environment around Eu3+ ions in LBGS. Moreover, from JO analysis, LBGS glasses have high capability for red laser device with high lasing power and energy extraction ratio. The fluorescence lifetimes show decreasing trend in lifetime with increasing concentration of Eu2O3 is due to radiative transition. From CIE color coordinate, the CIE color coordinates of LBGSEu6 glass fall in reddish region close to orange region and can be useful for optical display devices.

First-principles computational design of unknown flat arsenene epitaxially grown on copper substrate

Two-dimensional materials play essential roles in utilizing surface reactions, such as catalysts, adsorption and separation of chemicals. Especially, group-V mono-elemental materials are highlighted for transistors, optoelectronic devices, and mechanical sensors. Here, we identify unknown honeycomb-type arsenene epitaxially grown on copper substrate using first-principles density functional theory calculations. Key materials properties of lattice mismatch, thermodynamic stability, and surface transport properties are evaluated to verify the feasibility of the structural formation. Furthermore, ab-initio molecular dynamic simulations and scanning tunneling microscopy simulations clearly describe the mechanism of the initial nucleation and growth process. Electronic structure-level calculations characterize a strong covalency between each As atom pair. Our approach combining electronic structure calculations and thermodynamic/kinetic property predictions can be useful for quick screening and plausible design of new low-dimensional materials, which can efficiently functionalize emerging surface systems.

Magnetic Phase Diagrams of R1 – xSrx(Mn1 – x/2Sbx/2)O3 (R = La, Pr, Nd, Sm, Eu) with Trivalent Manganese Ions

The results of neutron-diffraction, magnetic, and magnetotransport studies of La1 ‒ xSrx(Mn 1- /2 3+ Sb /2 5+ )O3 (x ≤ 0.7) pervoskites, which contain only Mn3+ ions, are reported. It is demonstrated that the system undergoes a transition from the antiferromagnetic state to the ferromagnetic one at x ∼ 0.2. Its structure then changes from O′-orthorombic to O-orthorombic, which is attributable to orbital disordering. At x ≥ 0.6, the competition between antiferromagnetic and ferromagnetic interactions and strong diamagnetic dilution trigger the emergence of the cluster spin-glass phase. As the ionic radius of rare-earth ions in R0.7Sr0.3Mn0.85Sb0.15O3 decreases, the ferromagnetic state transforms gradually into spin glass (R = Sm). All ferromagnetic compositions are dielectric in the magnetically ordered phase with strong magnetoresistive effects. It is assumed that ferromagnetism is induced by the strong covalency and orbital disordering.

Tailored optical properties of Dy3+ doped magnesium zinc sulfophosphate glass: Function of silver nanoparticles embedment

Glasses with composition (59.5-x)P2O5–20.0MgO-20.0ZnSO4–0.5Dy2O3-xAgCl (0.0≤x≤ 0.5 mol%) were prepared by melt-quenching method and characterized thoroughly. The role of silver nanoparticles (Ag NPs) inclusion on the spectroscopic and the physical properties of the synthesized glasses were determined. Judd-Ofelt (J-O) theory was used to complement the experimental optical data. TEM images confirmed the existence of Ag NPs of average size ≈4.5 nm. The absorption spectra showed eleven bands aroused from the ground state (6I15/2) to various excited states transitions of dysprosium (Dy3+) ion. The surface plasmon resonance (SPR) peak of Ag NPs was probed around 446 nm. The calculated J-O parameter (Ω2) for the glass without Ag NPs incorporation was the highest, indicating the strong metal-ligand covalence bond. Irrespective of Ag NPs contents, the emission spectra of the studied glasses revealed two intense photoluminescence bands centered at 480 nm (blue: 4F9/2→6H15/2) and 574 nm (yellow: 4F9/2→6H13/2) accompanied by two very weak red bands positioned at 662 and 752 nm. The luminescence intensity of both blue and yellow band was enhanced by a factor of 2 and it was 1.5 fold for the red bands. This enrichment in the luminescence intensity was attributed to the Ag NPs mediated SPR effect and subsequent energy transfer from the Ag NPs to the nearsighted Dy3+ ions.

LiVPO4F1–yOy Tavorite-Type Compositions: Influence of the Concentration of Vanadyl-Type Defects on the Structure and Electrochemical Performance

Mixed-valence LiVPO4F1–yOy materials were obtained for the first time over a large composition range (here 0.35 ≤ y ≤ 0.75) through a single-step solid-state synthesis. Interestingly, the competition between the ionic character of the V3+–F bond and the strong covalency of the V4+═O vanadyl bond originates complex crystal chemistry at the local scale, which allows stabilization of a solid solution between LiVPO4F and LiVPO4O despite a significant deviation from Vegard’s law for the cell parameters. A combined study using IR, Raman, and X-ray absorption spectroscopies highlights the effect of the vanadyl environment on the electronic structure of the vanadium orbitals and a fortiori the electrochemical behavior. Our results underline that the electrochemical performance of LiVPO4F1–yOy-type materials can be controlled by tuning the concentration of vanadyl-type defects, i.e., by playing on the competition between the ionic V3+–F bond and the covalent V4+═O bond.

Enhanced mid-infrared emission of erbium-doped fluoro-bromozirconate glass.

A series of erbium-doped fluoro-bromozirconate glasses modified by various concentrations of Br- was prepared using the melt-quenching method. The mid-infrared fluorescence intensity (2.7μm) was improved by increasing the content of Br-. The differential scanning calorimetry, x-ray diffraction, Fourier-transform infrared spectra, Raman spectra, and mid-infrared luminescence spectra were measured. The decreased phonon density shows that the structural changes due to inserting Br- can enhance the mid-infrared luminescent intensity. From the Judd-Ofelt analysis, it was found that the intensity of Ω2 was enhanced with the introduction of Br- and shows greater asymmetry and stronger covalency. Using the Fuchtbauer-Ladenburg theory and McCumber theory, the emission cross section (2.9×10-20 cm2) and absorption cross section (1.68×10-20 cm2) at 2.7μm were determined. Hence, erbium-doped fluoro-bromozirconate glass is a potential material for application in the mid-infrared region.

Interplay of covalency, spin-orbit coupling, and geometric frustration in the d3.5 system Ba3LiIr2O9

The electronic and magnetic properties of ${d}^{3.5}$ iridate ${\mathrm{Ba}}_{3}{\mathrm{LiIr}}_{2}{\mathrm{O}}_{9}$ have been studied using first-principles electronic structure calculations. The results of the calculations reveal that the system lies in an intermediate spin-orbit coupling (SOC) regime. There is strong covalency of $\mathrm{Ir}\ensuremath{-}5d$ and $\text{O}\ensuremath{-}2p$ orbitals. SOC, together with covalency, conspires to reduce the magnetic moment at the Ir site. By calculating the hopping interactions and exchange interactions, it is found that there is strong antiferromagnetic intradimer coupling within an ${\mathrm{Ir}}_{2}{\mathrm{O}}_{9}$ unit and other antiferromagnetic interdimer interactions make the system frustrated. The anisotropic magnetic interactions are also calculated. The calculations reveal that the magnitude of the Dzyaloshinskii-Moriya interactions parameter is small for this system. The magnetocrystalline anisotropy energy is large for this system and the easy axis lies on the $ab$ plane.

Hole Thermalization Dynamics Facilitate Ultrafast Spatial Charge Separation in CuFeO2 Solar Photocathodes

CuFeO2 is an earth-abundant metal oxide semiconductor showing promising efficiency as a solar photocathode. Using extreme ultraviolet (XUV) and optical spectroscopy, we track the element-specific electron and hole dynamics in CuFeO2 and compare results to those for Fe2O3. Cathodic photocurrent, which is only observed in CuFeO2, is shown to correlate with ultrafast (500 fs) hole thermalization from O 2p to hybridized Cu 3d states, leading to spatial charge separation within the CuFeO2 lattice. These results are supported by DFT calculations confirming strong covalency of Cu 3d and O 2p states at the valence band edge. The ability to elucidate site-specific charge carrier dynamics in real time provides valuable insights for the rational design of catalysts for efficient solar energy harvesting.

Synthesis, Crystal Structure, and Optical Properties of Layered Perovskite Scandium Oxychlorides: Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2.

We report the successful synthesis of three new Ruddlesden-Popper-type scandium oxychloride perovskites, Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2, by conventional solid-state reaction. Small single crystals of Sr2ScO3Cl were obtained by a self-flux method, and the crystal structure was determined to belong to the tetragonal P4/ nmm space group ( a = 4.08066(14) Å, c = 14.1115(8) Å) by X-ray diffraction analysis. The scandium center forms a ScO5Cl octahedron with ordered apical oxygen and chlorine anions. The scandium cation, however, is shifted from the position of the octahedral center toward the apical oxygen anion, such that the coordination geometry of the Sc cation can be effectively viewed as an ScO5 pyramid. These structural features in the oxychloride are different from those of octahedral ScO5F coordinated with a partial O/F anion order at the apical sites in the oxyfluoride Sr2ScO3F. Rietveld refinements of the neutron powder diffraction data of Sr3Sc2O5Cl2 ( I4/ mmm: a = 4.107982(5) Å, c = 23.58454(7) Å) and Ba3Sc2O5Cl2 ( I4/ mmm: a = 4.206920(5) Å, c = 24.54386(6) Å) reveal the presence of pseudo ScO5 pyramids with the Cl anion being distant from the scandium cation, which is similar to the Sc-centered coordination geometry in Sr2ScO3Cl with the exception that the ScO5 pyramids form double layers by sharing the apical oxygen. Density functional calculations on Sr2ScO3Cl indicate the strong covalency of the Sc-O bonds but almost nonbonding interaction between Sc and Cl ions.

Flower-like nanostructured V3S4 grown on three-dimensional porous graphene aerogel for efficient oxygen reduction reaction

A novel flower-like nanostructured V3S4/graphene aerogel (V3S4-GA) hybrid is successfully synthesized as an efficient electrocatalyst for oxygen reduction reaction (ORR). The flower-like V3S4 homogeneously grows on the surface of graphene, which provides a large contact surface area with electrolyte and abundant active sites. The strong covalency of VsbndS bond and the abundant unoccupied d-orbits of vanadium atoms are propitious to the exchange of the adsorbed oxygen molecular. The porous GA exists an excellent continuous electric conductive network for electron transportation and enough nanospace for O2 adsorption and reaction. Such unique nanostructures offer the synthesized V3S4/GA hybrid an excellent ORR catalytic performance, including a high electron transfer per oxygen molecule of 3.97, a high kinetic current density of 9.5 mA cm−2 at −0.5 V vs. Ag/AgCl, a high methanol resistance and a good durability.

Phase transitions of titanite CaTiSiO5 from density functional perturbation theory

Phonon dispersion of titanite ${\mathrm{CaTiSiO}}_{5}$ has been calculated using the variational density functional perturbation theory. The experimentally known out-of-center distortion of the Ti atom is confirmed. The distortion is associated with a ${B}_{u}$ mode that is unstable for wave vectors normal to the octahedral chain direction of the $C2/c$ aristotype structure. The layer of wave vectors with imaginary mode frequencies also comprises the Brillouin zone boundary point $Y$ $(0,1,0)$, which is critical for the transition to the $P{2}_{1}/c$ ground-state structure. The phonon branch equivalent to the imaginary branch of the titanite aristotype is found to be stable in malayaite ${\mathrm{CaSnSiO}}_{5}$. The unstable phonon mode in titanite leads to the formation of transoriented short and long Ti-O1 bonds. The Ti as well as the connecting O1 atom exhibit strongly anomalous Born effective charges along the octahedral chain direction [001], indicative of the strong covalency in this direction. Accordingly and in contrast to malayaite, LO-TO splitting is very large in titanite. In the $C2/c$ phase of titanite, the Ti-O1-Ti distortion chain is disordered with respect to neighboring distortion chains, as all chain configurations are equally unstable along the phonon branch. This result is in agreement with diffuse x-ray scattering in layers normal to the chain direction that is observed at temperatures close to the $P{2}_{1}/c$ to $C2/c$ transition temperature and above. The resulting dynamic chains of correlated Ti displacements are expected to order in two dimensions to yield the $P{2}_{1}/c$ ground-state structure of titanite.

Interplay between structural and magnetic-electronic responses of FeAl2O4 to a megabar: Site inversion and spin crossover

X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl$_{2}$O$_{4}$ transforms to a tetragonal phase at ~18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ~45 GPa, after which a transformation to a Cmcm post-spinel phase is onset. This is attributable to pressure driven Fe-Al site inversion at room temperature, corroborated by signatures in the 57Fe M\"ossbauer spectroscopy pressure data. At the tetragonal to post-spinel transition, onset in the range 45-50 GPa, there is a concurrent emergence of a non-magnetic spectral component in the M\"ossbauer data at variable cryogenic temperatures. This is interpreted as spin crossover at sixfold coordinated Fe locations emanated from site inversion. Spin crossover commences at the end of the pressure range of the tetragonal phase and progresses in the post-spinel structure. There is also a much steeper volume change dV/V ~ 10% in the range 45-50 GPa compared to the preceding pressure regime, from the combined effects of the structural transition and spin crossover electronic change. At the highest pressure attained, ~106 GPa, the M\"ossbauer data evidences a diamagnetic Fe low-spin abundance of ~50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of ~33 T. This is indicative of a magnetic ground state associated with strong covalency, as well as substantive disorder from site inversion and the mixed spin-state configuration. Intriguingly magnetism survives in such a spin-diluted post-spinel lattice at high densities. The R(300 K) data decreases by only two orders of magnitude from ambient pressure to the vicinity of ~100 GPa. Despite a ~26% unit-cell volume densification from the lattice compressibility, structural transitions and spin crossover.