Lattice dynamics of β-V2O5: Raman spectroscopic insight into the atomistic structure of a high-pressure vanadium pentoxide polymorph

We report here the Raman spectrum and lattice dynamics study of a well-crystallized β-V(2)O(5) material prepared via a high-temperature/high-pressure (HT/HP) route, using α-V(2)O(5) as the precursor. Periodic quantum-chemical density functional theory calculations show good agreement with the experimental results and allow one to assign the observed spectral features to specific vibrational modes in the β-V(2)O(5) polymorph. Key structure-spectrum relationships are extracted from comparative analysis of the vibrational states of the β-V(2)O(5) and α-V(2)O(5) structures, and spectral patterns specific to the basic units of the two V(2)O(5) phases are proposed for the first time. Such results open the way for the use of Raman spectroscopy for the structural characterization of vanadium oxide-based host lattices of interest in the field of lithium batteries and help us to greatly understand the atomistic mechanism involved in the α-to-β phase transition of vanadium pentoxide.     

Effect of the Structure and Size Factor on the Catalytic Properties of Cobalt–Zirconium Oxide Nanoparticles in Deep Oxidation of Methane

We have used X-ray phase analysis, transmission electron microscopy, and temperature-programmed reduction to study cobalt–zirconium catalysts. The most active samples are characterized by higher dispersity of both the support (L = 12-13 nm) and the active component (L 3 nm); modification of the zirconium dioxide with yttrium oxide increases the thermal stability of the catalysts. We have shown that the activity of the catalysts is determined by both the strength of oxygen–catalyst binding and the acidic properties of the catalyst surface.     

Theoretical study of structural phase transition in a RbMnCl<Subscript>3</Subscript> crystal by the Kim-Gordon method

Symmetry analysis of the low-temperature phase of RbMnCl₃ crystals with the monoclinic axis perpendicular to the sixfold axis of the high-temperature phase showed that its space group is either C _2h ³ or C _2h ⁶ The distribution of normal vibrations over the irreducible representations of the high-temperature phase is refined, and the symmetry relationships for normal vibrations of all possible low-temperature phases are tabulated. The model of the potential function of the crystal is obtained by the nonempirical Kim-Gordon method. This model allows one to establish the existence of the saddle point of the potential surface and several harmonically unstable modes corresponding to correlated rotations of rigid MnCl₆ and Mn₂Cl₉ polyhedra. The absolute energy minimum is determined by deforming the lattice along the eigenvector of the E_1g mode within the sp. gr.. C _2h ⁶ .     

Effect of structural,redox and acid characteristics of M<Subscript>x</Subscript>O<Subscript>y</Subscript>/ZrO<Subscript>2</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) (M-Mn,Co, Cr) oxide nanocomposites on their catalytic properties in the deep oxidation of methane

Alumina-and zirconia-manganese catalysts for the deep oxidation of methane were studied by X-ray phase analysis, temperature-programmed hydrogen reduction, and IR spectroscopy. The most active catalyst has the optimal combination of dimensional, redox, and acid characteristics. The zirconia-manganese catalysts formed on the surface of a kaolin-aerosil honeycomb matrix provides for 80–100% conversion of methane to CO₂ at 690–750 °C. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 6, pp. 367–372, November–December, 2007.     

A Nano-Effect in the Generation of Alumomanganese Catalysts for the Deep Oxidation of Methane

Alumomanganese catalysts for the deep oxidation of methane have been investigated by X-ray phase analysis and temperature programmed reduction. Application of the active component and stabilizing additives (La, Ba, Sr) under non-equilibrium conditions facilitates retention of nano-particles of the carrier and stabilization of low-temperature modifications of Al₂O₃ ( and phases), which determines the high activity and thermal stability of the catalysts. A phase dimension effect was observed during high temperature treatment of the catalysts (900 °C, 5 h), which includes a decrease in the phase transitions of aluminum oxide with a decrease in the size of its particles and the formation of the metastable -phase in the more dispersed -Al₂O₃ carrier (L = 4 nm).     

In Situ Measurements of Thermal Distortions in Synchrotron Optics under High Heat Load

Modern synchrotron X-ray sources produce many kilowatts of power, most of which are absorbed by the first optical elements of the monochromators. As a result, the monochromator crystal deforms (creating a heat bump), leading to severe degradation of the monochromator performance. This problem has been known for decades and various methods have been developed to lessen the effect of heat bumps by implementing various internal cooling methods for the crystal as well as using crystals with superior heat conductivity [1].     

Dynamics of bimodal growth in pentacene thin films

Previous studies have established that pentacene films deposited on silicon oxide consist of a substrate-induced "thin-film" phase, with the bulk phase of pentacene detected in thicker films only. We show that the bulk phase nucleates as early as the first monolayer, and continues to nucleate as film growth progresses, shadowing the growth of the thin-film phase. Moreover, we find that the transition between the "thin-film" and the bulk phase is not a continuous one, as observed in heteroepitaxial systems, but rather the two phases nucleate and grow independently.     

X-ray standing wave analysis of the effect of isotopic composition on the lattice constants of Si and Ge

The x-ray standing wave (XSW) technique is used to measure the isotopic mass dependence of the lattice constants of Si and Ge. Backreflection allows substrates of moderate crystallinity to be used while high order reflection yields high accuracy. The XSW, generated by the substrate, serves as a reference for the lattice planes of an epilayer of different isotopic composition. Employing XSW and photoemission, the position of the surface planes is determined from which the lattice constant difference Deltaa is calculated. Scaled to DeltaM = 1 amu we find (Deltaa/a) of -0.36x10(-5) and -0.88x10(-5) for Ge and -1.8x10(-5) and -3.0x10(-5) for Si at 300 and 30 K, respectively.     

Termination effect in X-ray diffraction studies of disordered systems

The effect of termination on the structural parameters of the radial distribution function of atoms has been analyzed with the use of the structure factor calculated for the model of soft spheres. It is shown that in the range of the experimentally attainable magnitudes of the diffraction vector S _max, the position R ₁ and the area Z ₁ ^sym of the first maximum of the radial distribution function of atoms linearly depends on 1/S _max. The method is suggested for taking into account the effect of termination on the structure parameters.     

The Raman spectrum of the γ′‐V2O5 polymorph: a combined experimental and DFT study

Structure and vibrational dynamics of γ′‐V₂O₅ synthesized from a pristine γ‐LiV₂O₅ sample via a chemical oxidation route was studied by means of Raman spectroscopy and quantum‐chemical calculations. The calculations based on density functional theory reliably reproduce the experimental structure of the γ′‐V₂O₅ lattice. The calculated Raman spectrum agrees remarkably well with the experimental one. Making use of the agreement, a complete assignment of Raman bands to vibrations of particular structural units is proposed. The comparison of Raman spectra and structural features of α‐V₂O₅ and γ′‐V₂O₅ polymorphs allowed establishing reliable ‘structure–spectrum’ correlations and identifying Raman peaks characteristic for different structural units. Copyright © 2015 John Wiley & Sons, Ltd.