Lattice defects in rare-earth metals

Abstract

First, a review is given of defects created by low-temperature electron irradiation in the h.c.p. heavy lanthanides Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, in the double-h.c.p. light lanthanides Pr and Nd, and in their light homologues Se and Y. The wealth of the experimental data permits to establish general relationships between specific Frenkel-pair characteristics and crystallographic parameters, such as between the threshold energy for displacement, the interatomic distance and the c/a ratio. Another interesting empirical rule is the linear dependence of the interstitial migration temperature (normalized to the melting point) on the deviation of the c/a ratio from its ideal value |c/a — (8/3)½|, which seems to have general validity for almost all h.c.p. metals. (Notable exceptions are Zn and Cd, with their extreme c/a ratios.) A further result is the observation that the specific Frenkelpair resistivity p _F consists of two parts: a ‘normal’ phonon contribution and, in magnetic lanthanides, a contribution from spin scattering.

Second, we treat a specific impurity, H, which forms solutions RH _x with several rare-earths up to relatively high values, for example x = 0·35 atoms of H per atom of Sc. The hydrogen atoms, which occupy tetrahedral interstitial sites in the h.c.p. unit cell, have a tendency to order at temperatures below 150–180 K, giving rise to resistivity anomalies and Snoek-like peaks in internal-friction spectra. A detailed analysis of the ordering using diffuse neutron scattering yields peculiar quasi-linear configurations of chains consisting of H–H pairs on second-neighbour tetrahedral sites along the c- axis surrounding a metal atom.