Electronic and structural transitions in stoichiometric and nearly-stoichiometric lanthanum trihydride and trideuteride. A 139La and 1H nuclear magnetic resonance study.

Abstract

We report the results of NMR measurements of the 139La and 1p resonances in stoichiometric and nearly-stoichiometric lanthanum trihydride, LaH x, and trideuteride, LaD x, 2.89⩽x⩽3.00, covering the temperature range, 4.2 K⩽T⩽350 K. The Knight shift of the 139La resonance is consistent with metallic behavior for T⩾210 K and with semiconducting behavior characterized by a band gap, E g=0.10 eV, for T<210 K. In contrast, the 1H T 1 follows T 1T=constant, consistent with metallic behavior, at least down to 77 K. Both the 139La lineshape at T<210 K and the second moment of the 1H resonance at 4.2 K confirm that the octahedral site hydrogen (deuterium) are randomly displaced from the geometrical centers of the octahedral sites.