Isotope effect on structural transitions of Ti1.0Mn0.9V1.1HX(D-X) and Ti1.0Cr1.5V1.7HX(D-X) with hydrogenation

We have investigated the structural transitions of Ti1.0Mn0.9V1.1HX(D-X) an d Ti-1.0,Cr1.5V1.7HX(D-X) upon hydrogenation at 293 K and discussed the eff ect of hydrogen isotope on their crystal structures. The various hydride sa mples used for X-ray diffraction (XRD) investigation were obtained after me asurement of the P-C isotherms by taking them out of the reactor. The cryst al structures, phase abundance and lattice parameters of the hydrides were determined by the Rietveld method using XRD data. Because the hydrides of t he alloy Ti1.0Mn0.9V1.1 revealed complex peak profiles, we double-checked t he structures through transmission electron microscope (TEM) investigations . The results of the TEM observation agreed well with those of XRD data. Th e 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 hyd rides have the same hydrogen contents, they have also the same crystal stru ctures, although they show a large difference between the equilibrium press ures in their P-C isotherms. At the experimental temperature, the Ti1.0Mn0. 9V1.1 alloy and Ti1.0Cr1.5V1.7 alloy revealed different structural transiti on processes upon hydrogenation although the crystal structures of these tw o alloys are both body centered cubic (BCC). The structural transitions of the alloys Ti1.0Mn0.9V1.1 and Ti1.0Cr1.5V1.7 can be summarized by BCC (a=3. 0183(1) Angstrom)--> body centered tetragonal (BCT) (a=2.874(3) Angstrom, c =3.89(1) Angstrom)--> face centered cubic (FCC) (a=4.311(8) Angstrom) in al loy Ti1.0Mn0.9V1.1 and BCC (a=3.0212(9) Angstrom)--> FCC (a=4.261(4) Angstr om) in alloy Ti1.0Cr1.5V1.7. The Ti-rich phases with NiTi2 structure and al pha -Ti with hexagonal close packed (HCP) structure absorbed hydrogen at re latively low hydrogen pressures and the phase abundance remained almost con stant. From this fact, it can be deduced that it is desirable to decrease t heir amount as far as possible in order to increase the effective hydrogen storage capacities of the alloys. (C) 2001 Elsevier Science B.V. All rights reserved.