Ab initio study of iron and iron hydride: III. Vibrational states of H isotopes in Fe, Cr and Ni

Adiabatic potentials, energy levels and wavefunctions for collective vibrational states of hydrogen isotopes in monohydrides of transition metals with face-centred cubic lattices, gamma-FeH and NiH, and with body-centred cubic lattices, alpha-FeH and CrH, are investigated by means of ab initio total-energy calculations in the local density and local spin-density approximations (LDA, LSDA). The study for the different transition-metal monohydrides, including PdH and NbH studied earlier, yields a general insight into the microscopic vibrational potential wells: the topology of their spatial shapes is specific to the metal lattice, but their depths and curvatures change quantitatively in a systematic manner through the transition-metal series. The calculated excitation energies agree very well with results of inelastic neutron scattering (INS) experiments both for non-magnetic NiH, treated within the LDA, and ferromagnetic gamma-FeH, treated within the LSDA. The theoretical data for the two considered hydrides, with bcc structure, for which corresponding experimental data do not exist, provide an ab initio database for the construction of metal-hydrogen interaction models, e.g., for studies of self-trapped vibrational states of isolated H atoms in transition metals.