Neutron-scattering studies on the vibrational excitations and the structure of ordered niobium hydrides: The ɛ phase

Abstract

Inelastic and elastic neutron-scattering techniques have been used to study the structure and optical hydrogen vibrations of the ordered niobium hydrides ${\mathrm{NbH}}_{x}$ in the concentration range $0.1<x<0.73$ and at temperatures 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 \ensuremath{\varepsilon} phase. The neutron vibrational spectra show clearly that this splitting is due to the existence of two inequivalent hydrogen sites rather than H-H coupling effects. This is confirmed by neutron-diffraction data, which show the presence of two hydrogen sites with different distortions of the surrounding niobium tetrahedra. Our results also provide new information on the ${\mathrm{NbH}}_{x}$ phase diagram and the anharmonicity of the hydrogen potential in these hydrides.