Isotope effect on structural transitions of Ti_1.0Mn_0.9V_1.1H_X(D_X) and Ti_1.0Cr_1.5V_1.7H_X(D_X) with hydrogenation

We have investigated the structural transitions of Ti_1.0Mn_0.9V_1.1H_X(D_X) and Ti_1.0Cr_1.5V_1.7H_X(D_X) upon hydrogenation at 293 K and discussed the effect of hydrogen isotope on their crystal structures. The various hydride samples used for X-ray diffraction (XRD) investigation were obtained after measurement of the P-C isotherms by taking them out of the reactor. The crystal structures, phase abundance and lattice parameters of the hydrides were determined by the Rietveld method using XRD data. Because the hydrides of the alloy Ti_1.0Mn_0.9V_1.1 revealed complex peak profiles, we double-checked the structures through transmission electron microscope (TEM) investigations. The results of the TEM observation agreed well with those of XRD data. The crystal structures of corresponding isotope hydrides, the phase abundance and the lattice parameters do not depend on the kind of hydrogen isotope, but only on the hydrogen content. That is, if the corresponding isotope hydrides have the same hydrogen contents, they have also the same crystal structures, although they show a large difference between the equilibrium pressures in their P-C isotherms. At the experimental temperature, the Ti_1.0Mn_0.9V_1.1 alloy and Ti_1.0Cr_1.5V_1.7 alloy revealed different structural transition processes upon hydrogenation although the crystal structures of these two alloys are both body centered cubic (BCC). The structural transitions of the alloys Ti_1.0Mn_0.9V_1.1 and Ti_1.0Cr_1.5V_1.7 can be summarized by BCC (a=3.0183(1) Å)→body centered tetragonal (BCT) (a=2.874(3) Å, c=3.89(1) Å)→face centered cubic (FCC) (a=4.311(8) Å) in alloy Ti_1.0Mn_0.9V_1.1 and BCC (a=3.0212(9) Å)→FCC (a=4.261(4) Å) in alloy Ti_1.0Cr_1.5V_1.7. The Ti-rich phases with NiTi₂ structure and α-Ti with hexagonal close packed (HCP) structure absorbed hydrogen at relatively low hydrogen pressures and the phase abundance remained almost constant. From this fact, it can be deduced that it is desirable to decrease their amount as far as possible in order to increase the effective hydrogen storage capacities of the alloys.