Hydrogen in nonstoichiometric cubic niobium carbides: Neutron vibrational spectroscopy and neutron diffraction studies

Abstract

The vibrational spectra and positions of H(D) atoms in NbC 1− y H x (D x ) (0.19 ≤ y ≤ 0.29, 0.04 ≤ x ≤ 0.30) have been studied by inelastic neutron scattering (INS) and neutron diffraction. The analysis of the neutron diffraction data for NbC 0.76H x (D x ) and NbC 0.71H x (D x ) has revealed a number of different structures depending on the carbon concentration and the presence of absorbed H(D) atoms: the partially ordered cubic P m 3 ¯ m structure for NbC 0.76, the partially ordered orthorhombic Pmmm structure for NbC 0.76D 0.17 and NbC 0.76H 0.18, the disordered cubic F m 3 ¯ m structure for NbC 0.71 and NbC 0.71D 0.30, and the disordered tetragonal I4/ mmm structure for NbC 0.71H 0.28. The INS spectra of NbC 0.81H x and NbC 0.76H x (D x ) in the energy transfer range 40–140 meV are found to consist of a single fundamental peak due to hydrogen optical vibrations (centered at 98 meV for H and at 65 meV for D) and a single peak due to carbon optical vibrations (centered at 78 meV). In addition to these peaks, the INS spectrum of NbC 0.71H 0.28 exhibits a peak at 130 meV, suggesting that H atoms in this compound occupy the sites displaced from the centers of carbon vacancies.