The hydrides with different hydrogen contents were prepared by hydrogenation and dehydrogenation of the Ti1-xMox (x=0.05, 0.10, 0.15, 0.20, 0.25, 0.33) alloys. The phase analysis and morphology observation were carried out using XRD and TEM. When the hydrogen content is between H/M=0.9~1.7, a new bct hydride named as ε-phase was observed under room temperature, but not observed under high temperatures. This ε-phase hydride of Ti-Mo alloy was different from the three bct phases in Ti-H system, thus considered to be a new hydride phase. The basic crystal structure of the ε-hydrides, the phase transition during the annealing dehydrogenation process, and the formation conditions of the ε-hydrides were investigated based on the collected XRD data.
The detailed crystal structure of the ε-phase Ti0.85Mo0.15D1.56 including the occupation of D atoms was derived from the Rietveld refinements of the measured neutron powder diffraction (NPD) and XRD patterns. A double ε-phase model and a ∆a/a microstrain model were used in the Rietveld refinement to fit the uncommon line broadening of the NPD and XRD patterns, and the D concentration inhomogeneity and microstrain were consequently confirmed. The model of dislocation-induced anisotropic line broadening was also used in the Rietveld refinement, and the refinement indicated that the dislocation may be the cause of the microstrain. The crystallographic relationship of the δ-ε phase transition was proposed based on the crystal structure of δ and ε hydrides of Ti-Mo alloys.
The δ-ε phase transition in Ti-Mo-H system was considered to start at a beginning temperature Tb, and a parabola curve of Tb depend on the H content was proposed. The maximum of the Tb was Tc. The axis ratio c/a of fct unit cell of hydrides of Ti-Mo alloy with x=0.25 were obtained from the first principle calculations, which is consist with the experimental observation. The total energy change was used to explain the Tb change dependence on the H content. The electronic structure calculation suggests that the band split nearby the Fermi energy was not the intrinsical cause of the δ-ε phase transition. The average number of the outer electrons was considered to be related to the Tc, which is likely to be associated with the average number of the outer d and s electrons.
The neutron powder diffraction measurements of Ti0.67Mo0.33D1.83 were carried out under different temperatures from 10K to 300K, and the atomic displacement B of the D atoms and the volume expansion coefficients were obtained by Rietveld refinement. The Debye temperature was calculated based on first principle density functional theory. Then, the relationship between the thermal vibration of D atoms and the metal atoms was determined. The static disorder atomic displacements were calculated and the anharmonic thermal vibrations were investigated. A method revealing the relationship between the atomic displacement of different atoms in a polyatomic crystal based on Debye temperature and atomic displacement of one atom was proposed, and the possibility of determination of the relative movement of different atoms caused by thermal vibration based on atomic displacement was discussed.
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