Electron density-of-states and the metal–insulator transition in LaHx

Abstract The temperature dependence of the Gd 3+ impurity ion spin relaxation rate, τ i −1 , in lanthanum hydrides, LaH x (2.0≤ x ≤3.0), probes the metal–insulator (MI) transition in this system. Because Gd 3+ is an S-state ion, τ i −1 depends on interaction with conduction electrons in the metallic state, resulting in Korringa-type behavior, τ i T =constant, and ( τ i T ) −1/2 ∝ N ( E F ), the electronic density-of-states at the Fermi level. In the non-metallic state, weak phonon processes result in a temperature dependence, τ i −1 ∝ T n , with 3 n τ i −1 has been measured via the Gd 3+ -induced contribution, R 1p , to the proton spin-lattice relaxation rate, R 1 , by comparing the measured rate in pure LaH x with that in LaH x containing controlled low levels of Gd. The results show that in the metallic state, N ( E F )∝(2.91− x ) 1/3 , consistent with earlier results based on the 139 La Knight shift which also support N ( E F )∝(2.91− x ) 1/3 .