NUCLEAR MAGNETIC-RESONANCE EVIDENCE FOR THE OCCURRENCE OF AN ORDERED STRUCTURE IN THE LA-H SYSTEM NEAR LAH2.65 AT 250 K

We report the first measurements of nuclear magnetic resonance of 139La in lanthanum trihydrides showing the occurrence of a phase transition at a temperature of about 250 K. In particular, we studied the NMR of the central (-l/2↔+ l/2) transition of 139La in lanthanum hydride, LaH2 68, and lanthanum deuteride, LaD262 in the temperature range 77 K–500 K. As consequence of the non-cubic environment of the 139La nuclei in these compositions, its magnetic resonance is strongly perturbed by second-order quadrupole interaction. The spectral features characteristic of quadrupole-perturbed central transitions are well-resolved in a temperature range of about 100 below the transition temperature 250 K. At temperatures above room temperature the very fast diffusive motion of the hydrogen reduces the effective quadrupole interaction of the 139La to a very small value so that the spectral linewidth is reduced to its dipolar value. At a temperature of 77 the resonance features are less pronounced, and the effective linewidth is broadened by the presence of random quadrupole interactions which accompany the “freezing in” of the hydrogen atoms. Below 250 the spectrum consists of two resonances which can be characterized by different interaction frequencies and quite different electric field gradient asymmetry parameters. Comparison of the measured parameters with calculat d values based on a point-ion lattice type calculation indicates that the probable La environment is in part that of an ordered structure corresponding to the ideal composition LaD2.5 as derived from neutron scattering data, and in part that corresponding to sites of higher symmetry which occur in the actual composition, LaD2.62 as additional octahedral interstitial sites are filled. © by Akademische Verlagsgesellschaft