Low-energy vibrations and octahedral site occupation in Nb95V5H(D)y

Abstract

Nb100−xVxHy is a random b.c.c. alloy system with an H sublattice disordered with respect to site energies. Previous incoherent inelastic neutron scattering (IINS) studies ascertained both octahedral (o) and tetrahedral (t) site occupation by H for x = 50, 10and50. On the basis of spectroscopic and statistical arguments, the low-temperature H vibrational peaks in the Nb-rich, low-H-concentration Nb95V5H1 alloy were assigned to Nb4V2 o sites and NbV3 t sites. To probe the regime of lower H concentration and the effects of D substitution, further IINS spectra were measured for Nb95V5Hy (y = 0.5and0.8) and Nb95V5D2.7. As the H concentration was decreased, the spectra at 4 K indicated an increasingly predominant 38 meV peak, which was assigned to the doubly degenerate H vibrations in the Nb4 plane of the Nb4V2 o site. A portion of the board feature centered at approximately 200 meV was assigned to the much stiffer H vibration polarized along the V-V apical axis. The Nb95V5D2.7 results located the analogous soft D vibration at 29 meV, yielding an H/D vibrational energy ratio of 1.31. To our knowledge, this is the only known case of spectroscopically confirmed o-site occupation by H in a b.c.c. metal (alloy) lattice. Moreover, the soft modes yield the lowest observed vibrational energy for H in a metal hydride.