S. Hayashi, E. Akiba, T. Ueda, and K. Nomura
[Proc. Int. Symp. Mat. Chem. Nucl. Environment, pp. 505-513, 1996]

The hydrogen sites in beta-Ti_1-yV_yH_x were studied by the incoherent, inelastic neutron scattering. Four peaks were observed, whose energy ranges were 13 leq epsilon leq 16 meV, 36 leq epsilon leq 44 meV, 97 leq epsilon leq 110 meV, and 134 leq epsilon leq 152 meV. Those peaks were assigned to the metal lattice vibrations and the local vibrational modes of hydrogen atoms occupying the octahedral site (O site) and two kinds of tetrahedral sites (T₁ and T₂ sites), respectively. With increase in V content, the fraction of the O site increases, while that of T₁ decreases. The composition dependence was analyzed using a cluster model.
The hydrogen diffusion in beta-Ti_1-yV_yH_x was studied by ¹H NMR. Temperature and frequency dependences of ¹H spin-lattice relaxation time (T₁) were measured over the temperature range from 105 to 400 K and at resonance frequencies of 9, 22.5, 52 and 90 MHz. The relaxation mechanism is fluctuation of the dipole-dipole interactions between ¹H and ¹H and between ¹H and ⁵¹V caused by the hydrogen diffusion in the alloys. The activation parameters were determined from the data fitting with Bloembergen-Purcell-Pound (BPP) theory taking into account the distribution of the correlation time. The apparent activation energy decreases with increasing in the vanadium concentration, as shown in Fig.1. This composition dependence was analyzed using the cluster model with the short-range ordering.
Alloys, with the compositions of Ti(33-47 mol%)-V(42-67 mol%)-Fe(0-14 mol%) were investigated to find out the most suitable composition for hydrogen absorbing material under the conditions for tritium recovery. The alloy Ti_43.5V_49.0Fe_7.5 showed the maximum hydrogen capacity of 3.90 wt% ([H]/[M] = 1.90) at 253 K among the alloys studied. The hydrogen absorption rate at 253 K was faster than that of the LaNi₅-H₂ system at 293 K. Conspicuous change in the pressure composition (pc) isotherms was not observed after 50 cycles of hydrogen absorption and desorption. All dehydrated alloys were bcc, but dihydrides of the alloy were fcc. Among the series of Ti-V-X alloys (X = Fe, Co, Ni, Cr, and Pd), substitution of iron was found to be the most effective to improve the hydrogen absorbing properties.