Inelastic Neutron Scattering by Cerium Hydrides. III. Localized Vibrations of Interstitial Hydrogen in the F.C.C. CeH₂ Lattice

Explicit formulas have been derived for the frequencies of the localized modes of an impurity at the octahedral interstitial sites of a f.c.c. CaF₂‐type structure, describing the dynamics of the host lattice by short‐range central forces and assuming that the impurity interacts with the first and second neighbouring atoms of the host lattice. A threefold degeneracy is predicted. A small peak observed at 74 meV in the neutron data on CeH_1.98 is attributed to such localized modes, due to an occupation of a few percent of the octahedral sites by hydrogen atoms. A unique set of the parameter values for the coupling of the interstitial hydrogen to the atoms of the host lattice could not be derived from the position of this peak alone; a set is proposed from a comparison with the parameter values for the host lattice which have been previously determined.