NEUTRON-SCATTERING STUDIES ON THE VIBRATIONAL EXCITATIONS AND THE STRUCTURE OF ORDERED NIOBIUM HYDRIDES - THE EPSILON-PHASE

Inelastic and elastic neutron-scattering techniques have been used to study the structure and optical hydrogen vibrations of the ordered nio bium hydrides NbHx in the concentration range 0.1<x<0.73 and at temper atures between 4 and 250 K. A splitting of the fundamental vibrational modes with an intensity ratio of 2:1 could be observed in the case of the epsilon phase. The neutron vibrational spectra show clearly that this splitting is due to the existence of two inequivalent hydrogen si tes rather than H-H coupling effects. This is confirmed by neutron dif fraction data, which show the presence of two hydrogen sites with diff erent distortions of the surrounding niobium tetrahedra. Our results a lso provide new information on the NbHx phase diagram and the anharmon icity of the hydrogen potential in these hydrides.