Coherent quasielastic neutron scattering fromNbDx

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Abstract
The quasistatic local lattice distortions around individual interstitial deuterium defects in niobium have been investigated by means of coherent quasielastic neutron scattering. The important experimental issues of this study are discussed in detail. We compare the observed absolute scattering cross section with various model calculations in which many different defect locations and the associated Kanzaki forces are tested. The results show that the heretofore anticipated defect models are insufficient to describe the local defect environment. A new model which takes into account the occurrence of diffusion-induced nonequilibrium distortions is able to describe virtually all of our experimental findings. By this analysis we confirm on one hand that D in Nb is preferentially located at tetrahedral sites. We demonstrate on the other hand that a partial triangular site occupancy has to be taken into account, which is associated with a perturbed flight phase of the highly mobile defect. The evidence in our study of local nonequilibrium distortions sheds new light on the cubic symmetry of the force dipole moment and on other distortion-mediated defect properties.