Magnetic resonance determination of the electronic density of states and the metal—non-metal transition in the LaH2−LaH3 system

Abstract

Proton spin-lattice relaxation rate R1 and 139La Knight shift measurements have been utilized to study the electronic density of states N(EF) at the Fermi level and the metal-non-metal (MNM) transition in the lanthanum hydrides, LaHx(2 ⩽ x ⩽ 3), in samples prepared from highest purity Ames Laboratory lanthanum as well as from lanthanum containing controlled low levels (less than 300 ppm) of gadolinium. In the pure samples, free-electron-like metallic behavior is found for all compositions up to x ≈ 2.8 at temperatures T ⩽ 200 K. In the gadolinium-doped samples the Gd3+ ion spin fluctuations, characterized by the ion spin-lattice relaxation time τi,contribute an additional proton spin relaxation rate. Measurements of this rate for x < 2.8 show that τi ∝ T−1 at all temperatures, indicating metallic behavior. However, for x ⩾ 2.8 and T > 250 K, τi ∝ T−5, showing that Gd3+ spin relaxation is then governed by the two-phonon process typical of insulating solids.