V.K. Fedotov

Atomic ordering in the hcp cobalt hydrides and deuterides

Abstract Cobalt hydrides and deuterides with the hcp metal lattice and H(D)-to-metal atomic ratios 0.18≤ x ≤0.5 were prepared under high pressures of hydrogen or deuterium, respectively, and studied by neutron diffraction in a metastable state at 120 K and ambient pressure. A profile analysis of the spectra showed that in all samples the hydrogen and deuterium atoms occupy octahedral interstitial sites. In the samples with x ≤0.26, the hydrogen and deuterium atoms are randomly distributed over these sites. In the samples with x ≥0.34, they form layered superstructures, occupying every third octahedral base layer at x =0.34 and every second layer at x =0.38 and 0.5.

Neutron spectroscopy of γ manganese hydride

Abstract The vibrational spectrum of fcc γ -MnH 0.41 synthesized under high pressure of gaseous hydrogen was studied by inelastic neutron scattering at 2 K in the range of energy transfers from 25 to 400 meV. The fundamental band of optical hydrogen vibrations consists of a peak at 111 meV with a broad shoulder towards higher energies, which extends up to about 140 meV. At higher energy transfers, the spectrum originates from multiphonon neutron scattering and exhibits approximately harmonic behaviour. The results are compared with the available data for other metal hydrides.

A real-time neutron diffraction study of phase transitions in the Ti-D system after high-pressure treatment

Phase transitions in TiDapproximately 0.75 subjected to high-pressure treatment were investigated by simultaneous real-time measurements of neutron diffraction and small-angle neutron scattering. The neutron spectra were taken on heating the samples in the temperature ranges 100 to 300 K and 300 to 760 K followed by cooling to 430 K. A sequence of structural transitions was observed, which involves seven different phases and intermediate states with the hexagonal close-packed, face-centred cubic, face-centered orthorhombic or body-centred cubic metal sublattices and hydrogen atoms distributed on octahedral or ordered/disordered tetrahedral interstices.

Displacive ordering in the hydrogen sublattice of yttrium trihydride

Powder samples of YH3 and YD3 have been studied by neutron diffraction (ND) with a much higher statistical accuracy than obtained previously. The profile analysis of the obtained ND patterns confirmed the high-symmetry HoH3-type structure of YH3 and ruled out the ‘broken symmetry’ structures proposed recently to explain the insulating properties and lattice dynamics of this compound. At the same time, it was demonstrated that the HoH3type structure is only the structure of the mean lattice of YH3 .L argestatic displacements of H atoms from the symmetrical positions in this structure do occur, and ordering of these displacements on a short-range scale can settle the controversies between the crystal structure and physical properties of YH3.

Neutron diffraction investigation of γ manganese hydride

Abstract A profile analysis of the neutron diffraction spectrum of the fcc high pressure hydride λ-MnH 0.41 measured under ambient conditions showed that hydrogen is randomly distributed over the octahedral interstices of the fcc metal lattice and that the hydride is an antiferromagnet with the same collinear spin structure as pure λ-Mn, but with a smaller magnetic moment of about 1.9 Bohr magnetons per Mn atom.

Neutron scattering studies of the structure and lattice dynamics of a solid solution of hydrogen in -manganese

A solid solution of hydrogen in -Mn, -MnHx with xD 0:073, was prepared at 623 K and a hydrogen pressure of 0.85 GPa. A profile analysis of the neutron diffraction patterns measured at 225 K and at 300 K showed that hydrogen in-MnH0:073 randomly occupies the interstitial positions 12e (0, 0, 0.538) of the -Mn host lattice. An inelastic neutron scattering study of -MnH0:073 at 90 K revealed a pronounced peak at 6.4 meV and a rather broad optical hydrogen band with peaks at 73, 105 and 123 meV. The splitting of the optical band into three different modes agrees with the low symmetry, 2, of the 12e hydrogen sites. A tentative interpretation is given for the peak at 6.4 meV.

Hydrogen Solubility in Amorphous Silica at Pressures up to 75 kbar

The solubility of hydrogen in amorphous silica at a temperature of 250 °C and pressures up to 75 kbar is studied using a quenching technique. The molar ratio H(2)/SiO(2) is found to linearly increase with pressure from X = 0.16 at P = 6 kbar to X = 0.53 at P = 75 kbar. An investigation of a sample with X = 0.47 by Raman spectroscopy demonstrated that hydrogen dissolves in silica in the form of H(2) molecules, and these molecules occupy voids of, at least, two different types in the silica network. An X-ray diffraction study showed that the hydrogen molecules penetrating in the silica glass network prevented its irreversible densification occurring if the silica glass is compressed to the same pressure without hydrogen.

Phase transitions and equilibrium hydrogen content of phases in the water–hydrogen system at pressures to 1.8 kbar

Using a volumetric technique, the hydrogen solubility X in liquid water (L), low-pressure hexagonal ice (I h), and high-pressure cubic clathrate ice (sII) is studied at hydrogen pressures up to 1.8 kbar and temperatures from−36 to+20 °C. The triple point of the L+I h+sII equilibrium is located at P=1.07(5) kbar and T=−10(1) °C. The hydrogen concentrations of phases at the triple point are X L =0.17(5), and X sII=2.3(1) wt.% H2. The thermal stability and the process of decomposition of the clathrate phase at ambient pressure are studied by neutron diffraction.

An in situ structural study of the high-pressure transformations in TiH0.74

Structural phase transitions in the TiH0.74 alloy have been studied in the pressure range extending to 30.5 GPa at temperatures of up to 630 K, using diamond anvils and energy dispersive x-ray diffraction. Two phase transitions were observed at high pressures. At room temperature, the TiH0.74 alloy undergoes a transition to the (η+ω) two-phase state at above 7 GPa, and then this state persists up to 30.5 GPa. The other phase transition occurs upon heating (η+ω)-TiH0.74 to T>560 K to give a single-phase state, ζ-TiH0.74. The ω-phase corresponds to the high-pressure phase of pure titanium. The Ti sublattices of the η- and ζ-phases are indexed within the tetragonal symmetry, their specific volumes being much the same. Analysis of the specific volumes and the hydrogen contents in these two phases suggests that hydrogen atoms are likely to occupy tetrahedral interstices in the ζ-phase and octahedral interstices in the η-phase.

Giant tunnelling effect of hydrogen and deuterium in α manganese

The crystal structure of a solid solution of 3.5 at.% deuterium in α-Mn was determined by neutron diffraction and the spectrum of optical deuterium vibrations at 5 K was studied by inelastic neutron scattering (INS). The results give conclusive evidence of the tunnelling origin of the strong peaks at 6.4 and 1.6 meV observed earlier at temperatures up to 100 K in the INS spectra of hydrogen and deuterium solutions in α-Mn, respectively.