Phase Transformations in the Ti—H and Zr—H Systems at High and Atmospheric Pressures*

Anin situstructural study of the high-pressure transformations in TiH0.74

“…Under pressure, the eutectoid temperature and the transition temperature of the γ -phase decrease, as shown in the right-hand part of figure 1, and a new phase, ζ , appears in the alloys near the eutectoid point at P = 2.05 GPa, x = H/Ti ≈ 0.74 and T ≈ 560 K [2,6,8]. Early structural studies of the ζ -phase were limited to x-ray measurements [8], and several diffraction patterns were registered photographically between P = 5-8 GPa and T = 520-620 K. It was reported that Ti atoms in the ζ -phase form an FCT sublattice with axial ratio c/a = 0.89 [8].…”

“…Hydrogen atoms in all these phases are randomly distributed over tetrahedral interstitial sites (tetrasites) [1][2][3]. There is one more phase, monohydride γ -TiH, found to be stable under atmospheric pressure [4][5][6]. A neutron diffraction study of γ -TiD [3,7] demonstrated that Ti atoms form a face-centred orthorhombic sublattice with axial ratios [2].…”

“…Further studies attempted to obtain the high-pressure phase by thermobaric treatment, which involved thermal treatment of the TiH x samples at T 560 K and P 5 GPa, quenching to liquid nitrogen temperature, and recovery to atmospheric pressure (see figure 1). The quenched samples were single-phase, and this state, designated as the χ -phase, was metastable under atmospheric pressure below 95 K [6]. Extensive studies of the quenched metastable state at T 90 K by the neutron scattering technique [2,3,7,9] have shown that hydrogen atoms in the χ -phase randomly occupy octahedral interstitial sites (octasites) of the FCT Ti sublattice, whereas the tetrasites are occupied in the stable phases.…”

High-Pressure Synthesis of Novel Polyhydrides of Zr and Hf with a Th4H15-Type Structure

“…At room temperature and below, these metals form the following phases with hydrogen: an α phase, which is a primary hydrogen solution in an hcp metal lattice; a γ phase, which is a stoichiometric monohydride with a distorted fcc metal lattice and hydrogen ordered at the tetrahedral interstitials; a δ phase, which is a substoichometric (H/Me ≈ 1.6) dihydride with a CaF 2 -type structure; and a nearly stoichiometric ε-phase dihydride, which has a tetragonally distorted CaF 2 -type structure, also known as the ThH 2 -type structure. The stability of the γ phase was debated for a long time, but eventually it was demonstrated that the γ phase is stable in the Ti–H and Zr–H systems …”

Pressure effect on the hydrogen vibrations in γ-TiH and γ-ZrH

“…The fifth phase in these systems, monohydride γ -MeH, has also been reported to be stable at atmospheric pressure below 441 K in the Ti-H system [5] and below about 510 K in the Zr-H system [6]. The γ -MeH(D) phases have an ordered crystal structure where the hydrogen atoms occupy the tetrahedral interstitial sites (with unit-cell fractional coordinates 1 [4,7,8].…”