INCOHERENT STRUCTURE FACTOR OF HYDROGEN IN NBH0.11

We show that the measurement of the quasielastic energy spectrum S(Q, ω) of incoherently scattered neutrons provides a direct and precise method to determine the location of the H atom diffusing on interstitial sites in a metal lattice. At the reciprocal lattice points the spectrum contains a sharp elastic line with a relative intensity which is equal to the structure factor of the occupied sites. In a neutron scattering experiment performed on a NbH0.11 single crystal, we have measured the structure factor at the (110), (200) and (222) reflection at elevated temperatures (148°C and 280°C). Our results give clear evidence of tetrahedral site occupancy and exclude an appreciable partial occupation of octahedral (≦3%) and triangular sites (≦7%) at 280°C. These findings support the basic assumption of the Chudley Elliott theory in the study of jump diffusion models of H assuming a negligible flight" time in the jump process between neighbouring sites. © 1979 Springer-Verlag."