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

Inelastic and elastic neutron-scattering techniques have been used to study the structure and optical hydrogen vibrations of the ordered niobium hydrides NbHx 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 epsilon 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 NbHx phase diagram and the anharmonicity of the hydrogen potential in these hydrides.