V. P. Dmitriev

Polarized Raman spectra of Cs5H3(SO4)4· H2O single crystals

The Raman spectra of the superprotonic conductor Cs5H3(SO4)4·H2O have been investigated in the 300-500 K range. An assignment of bands due to internal and external vibrations in term of approximate type of motions is proposed. The spectroscopic results are used for discussing crystal structures of different phases, the type of structural disorder and phase transitions.

Raman spectra of crystals in the proton glass state

Polarized Raman spectra of (PCHS) single crystals were measured in the frequency range at 20 - 300 K, i.e. both below and above the glass transition temperature . The evolution of the spectra with decreasing temperature indicates that the orientational disorder of ions and molecules changes at from the dynamical to a static one. A scenario for the glass transition and a model of the resulting glass state is suggested, in which the water molecules (H bonds) play the role of binding constituents of the isolated sulphate complexes .

Raman scattering in the proton conductor Rb3H(SeO4)2

Abstract Careful polarised Raman scattering studies have been performed with the aim of analysing structural and dynamic aspects of the superionic transition in Rb 3 H(SeO 4 ) 2 . The results, in full agreement with the structural data, show convincingly the off-centre positions of protons in the OH … O bonds and confirm the orientational disorder of the SeO 4 2− groups as well as the formation of a dynamic network of hydrogen bonds in the high-temperature superionic phase.

Local mechanism for crystal-quasicrystal transformations

Abstract It is shown that the transformation mechanism which takes place at a crystal-quasicrystal transformation preserves a lattice of nodes in whose vicinity the crystal and quasicrystal structures are similar. Between the nodes fusion of neighbouring positions and creation of new positions occur, resulting in a disordering of the quasicrystal state.

X-ray absorption spectroscopy determination of the products of manganese borohydride decomposition upon heating

Manganese borohydride Mn(BH4)2 powder is heated in a hydrogen atmosphere in vacuum. The long-range order in the structure is monitored in situ by means of X-ray absorption spectroscopy and X-ray diffraction; short-range order, via Mg K-edge X-ray absorption near-edge structure spectroscopy. Above 120°C, the X-ray diffraction pattern disappears and an irreversible phase transition occurs, accompanied by sample amorphization and profuse hydrogen desorption. In the hydrogen atmosphere, the phase transition occurs at a temperature of ∼110°C. The standard scheme of borohydride decomposition suggests hydrogen desorption and the formation of metallic manganese and boron. However, a theoretical analysis of X-ray absorption spectra shows that the most likely products of Mn(BH4)2 decomposition are manganese borides.

X-ray diffraction study of a sequence of phase transitions in Cs2HgCl4 crystals

The temperature behavior of lattice parameters and diffraction patterns of the reciprocal lattice in Cs2HgCl4 crystals is studied by x-ray diffraction in the temperature range from 4.2 to 300 K. A sequence of phase transitions is observed and attributed to the evolution of incommensurate and commensurate modulations along the crystallographic a and c axes of a unit cell in the initial Pnma structure.

Vibrational spectra and particularities of the phase transitions in alkaline trihydrogenselenites

The changes in the Raman spectra of alkaline trihydrogen selenites are studied at their ferroelectric phase transitions. It is attempted to establish a relation between these changes and the characteristic properties of the phase transitions.

Hydride phase formation in carbon supported palladium hydride nanoparticles by in situ EXAFS and XRD

In the current work we present a detailed analysis of the hydride phase formation in industrial Pd/C nanocatalysts by means of combined in situ X-ray absorption spectroscopy (EXAFS), X-ray diffraction (XRD) and volumetric measurements for the temperatures from - 10 to 50 °C in the hydrogen pressure range from 0 to 1000 mbar. α- and β- hydride phases are clearly distinguished in XRD. For the first time, H/Pd atomic ratio were obtained by theoretical fitting of the near-edge region of the absorption spectra (XANES) and compared with volumetric measurements.

Crystal geometry and phenomenological models of reconstructive phase transitions

Abstract The basic concepts of the phenomenological theory of reconstructive phase transitions are presented. The nonlinear order parameter concept is introduced and the connection between the classical form of the Landau theory and a density wave approach to phase transitions is given. This new phenomenological approach is applied both to standard examples of reconstructive phase transformations (BCC-HCP, BCC-FCC, HCP-FCC, BCC-w) and to new problems like the formation of quasicrystals.

High pressure x-ray diffraction study of nickel–copper chromites solid solutions

A high-pressure synchrotron radiation diffraction study has been carried out on Ni(1-x)Cu(x)Cr(2)O(4) solid solutions. Observed pressure-controlled phase transitions, along with data previously collected for temperature-induced phase transitions, are analyzed in the framework of the unified phenomenological model that results in mapping of the generic phase diagram for the whole family of Ni(1-x)Cu(x)Cr(2)O(4) solid solutions.