Structure and lattice dynamics of titanium hydrides due to thermobaric treatment

Abstract

An in situ X-ray diffraction study of the high-pressure ζ-phase evidenced a fct Ti sublattice. In the fco X-phase quenched under pressure, H atoms are displaced to octahedral sites, and the energy of H optic peak (at ～ 75 meV) is half that in other Ti-H phases where H occupy tetrahedral sites. Phase transformations on heating of x-TiH(D)～0.75 were studied by neutron diffraction, small angle and inelastic neutron scattering (INS). Bound multiphonons were observed in the INS spectra of ordered γ-TiH(D).     

A57Fe Mössbauer study of the high pressure hydride phases of Fe and Co

Hydrides of iron and cobalt prepared at pressures between 4.0 and 9.5 GPa were studied by⁵⁷Fe Mössbauer spectroscopy at 4.2 K. Iron hydride was found to be nearly stoichiometric FeH. The two iron sites in its dhep lattice have hyperfine fields of 33.8 and 28.8 T. Practically the same results were found for the deuteride. In hep ε-CoH_x, the hyperfine fields decrease with hydrogen content by about 6% betweenx=0 andx=0.5. In all studied hydrides the electron densities at the⁵⁷Fe nuclei are smaller than in the pure metals.     

Properties of C60 polymerized under high pressure and temperature

60 polymers. Pure and mixed phase polymeric samples were synthesized by simultaneously subjecting microcrystalline C₆₀ powder or pellets to various pressures () and temperatures (). The optical spectra of the orthorhombic, tetragonal, and rhombohedral C₆₀ polymer phases are observed to be quite distinct and rich. These spectra exhibit numerous lines and an overall downshift in frequency relative to C₆₀ is observed, consistent with a loss of double bonds from the fullerene cage. The LDMS spectra of a sample synthesized at under hydrostatic conditions and , exhibited a succession of clear peaks at mass numbers corresponding to , similar to the LDMS data on the C₆₀ photopolymer. This is taken as further evidence for interfullerene bonds in these high-pressure polymers. The XRD pattern of this sample indicates the presence of a strong texture in the sample. Received: 14 November 1996/Accepted: 8 January 1997     

Increase in the superconducting transition temperature in Zr-Hf alloys due to <Emphasis Type="Italic">s-d</Emphasis> electron transfer under pressure

Correlation between the roughness of neighboring interfaces (roughness cross correlation) in a Ni/C X-ray multilayer mirror (XMM) prepared by laser ablation was studied by measuring X-ray diffuse scattering (XDS). The XDS intensities in the vicinity of the first Bragg reflection were measured at different photon energies: slightly below (8.325 keV) and slightly above (8.350 keV) the nickel photoabsorption K edge. The effective screening of the contribution from the deep layers to the XDS cross section due to the strong damping of the wave field at a photon energy higher than the photoabsorption edge allowed information on the character of the in-depth roughness cross correlation in the sample to be obtained. In particular, the characteristic lateral correlation length of the roughness was 0.35 µm at a photon energy of 8.325 keV (the contribution to the XDS cross section of the entire XMM volume), and it increased to 0.4 µm at a photon energy of 8.350 keV (predominantly the contribution from the upper layers). These data give direct evidence for the mechanism of smoothing of the interfacial roughness in the process of Ni/C XMM growth on anomalously large (up to micron) spatial scales. It was found that only rough large-scale defects with sizes of ≥10 µm are reproduced reasonably well from layer to layer. The processes of viscous flow and (or) reevaporation of high-energy target ions during deposition, which is characteristic of the laser method of XMM preparation, may serve as a possible explanation of the observed phenomenon.     

Amorphization of cuprite,Cu2O,due to chemical decomposition under high pressure

JETP Letters - Using synchrotron radiation and a diamond-anvil cell, a structural study of the phase transformations in Cu2O is carried out from diffraction data obtained in situ while heating at a...     

Enhanced superconductivity of the Ti-Zr alloys in the high-pressure BCC phase

A high-pressure study of the crystal structure and superconductivity of Ti-Zr alloys demonstrates an increase in the ω-β-transition pressure from about 30 to 43–57 GPa when the titanium content in the alloys increases from 0 to 50 at. %. The isobaric values of the BCC β-phase superconducting temperature (at 46 GPa) increase from 5.7 to more than 15 K between 0 and 50 at. % Ti, the latter value being the absolute maximum for BCC d-metal alloys. These data correlate with the earlier assumption of an s-d electron transfer in Zr under pressure.     

Study of the solid-state amorphization of (GaSb)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> semiconductors by real-time neutron diffraction and electron microscopy

The spontaneous amorphization of high-pressure quenched phases of the GaSb-Ge system has been studied by neutron diffraction while slowly heating the phases at atmospheric pressure. The sequence of changes in the structural parameters of the initial crystalline phase and the final amorphous phase is established. The behavior of the phases and the correlation in the structural features of the phase transitions and anomalous thermal effects exhibit signs of the inhomogeneous model of solid-state amorphization.     

Features of the pressure-induced phase transitions in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The structural changes induced by a 9-GPa pressure in Eu₂(MoO₄)₃ single crystals at room temperature have been studied using x-ray diffraction. It is established that a structural phase transition from the initial tetragonal phase to the new high-pressure tetragonal phase occurs rather than solid-phase amorphization that was observed previously in polycrystalline samples. The samples in the observed transition remain single-crystalline despite a significant difference (ΔV ～ 18%) between the specific volumes of the initial and final phases. It is shown that the transition from the initial state to the high-pressure phase occurs via the formation of broad transition zones featuring a continuous and smooth change of the crystal lattice parameters.     

A synchrotron study of high-pressure transformations in TiH0·74

Abstract

The crystal structure of the TiH_0·74 alloy was studied by the energy dispersive X-ray diffraction technique in the pressure range to 30·5 GPa at temperatures to 630 K. A phase transformation to the (η + ω) two-phase state was found to occur above 7 GPa at room temperature, then (η+ω)-TiH_0·74 remained stable up to P=30·5 GPa. Another phase transformation resulting in a single-phase state, ζ-TiH_0·74, was found to occur upon heating (η+ω)-TiH_0·74 above T ? 560 K. Both high-pressure phases, η and ζ, were indexed on the basis of the tetragonal sublattices of the Ti atoms with nearly the same specific volumes. It is assumed from the relation of the specific volumes that the hydrogen atoms occupy the tetrahedral interstices in the ζ-phase and the octahedral interstices in the η-phase.     

Hydrogen vibrations in γ-TiH and γ-ZrH under high pressure

Abstract

The effect of high hydrostatic pressure on the vibrational spectra of monohydrides γ-TiH and γ-ZrH was studied by inelastic neutron scattering, the maximum experimental pressure being, respectively, P=15 and 17·5 kbar. There was observed a uniform linear pressure-induced energy shift of each spectrum compared to that at ambient pressure with proportionality factors of 1·02 for y-TiH and 1·03 for γ-ZrH. The observed linear shift of the one-phonon hydrogen modes and the bound multiphonons (biphonons and triphonons) is indicative of a pressure-induced increase of the anharmonicity parameters in the model Hamiltonian representing the strength of two-phonon and three-phonon interactions.