Inelastic neutron scattering studies of vibrational excitations of hydrogen in Nb and Ta

Inelastic incoherent neutron scattering techniques have been used to study the optic vibrations in hydrides of Nb and Ta for hydrogen concentrations between 44 and 49 at.% in the case of Nb and 42 at.% for Ta and at temperatures between 10 and 295 K. In all cases the first harmonic of the singlet fundamental vibration could be observed with an anharmonic shift of 3-5% to lower energies. In addition, evidence is presented for the first time of a combination band corresponding to the simultaneous excitation of the singlet and doublet fundamental vibration corresponding to out-of-plane and in-plane motion, respectively, of the proton on its tetrahedral site. These modes are coupled by anharmonic interactions which are described by a three-dimensional anharmonic-oscillator model of tetragonal symmetry (4̄2m) appropriate to hydrogen in bcc metals. This analysis using both the present data as well as previous work by other authors gives a consistent picture of anharmonicity in the bcc metal hydrides. In addition, splittings and/or broadening of the observed vibrational peaks for some concentrations or temperatures could be correlated with recent, new information on the high-concentration, low-temperature portion of the NbHₓ phase diagram.