Hydrogen locations, diffusion and the electronic density of states in yttrium dihydrides: A nuclear magnetic resonance investigation

Abstract

We report results of wide-line and pulsed proton magnetic resonance measurements on yttrium dihydrides, YH x (1.63 ⩽ x ⩽ 1.98), covering the temperature range 140–760 K and yielding information on hydrogen locations, diffusion and the density of states in the dihydride phase. Our principal conclusions are as follows. (1) At x = 1.98, wide-line second moment measurements indicate that 15% of the octahedral (O) sites in the f.c.c. CaF 2 structure are occupied by hydrogen. (2) Spin-lattice relaxation time measurements in the rotating frame ( T 1 ρ versus T) yield hydrogen jump frequencies ν j showing three distinct regions in which the activation energy E act for hydrogen diffusion increases with T. (3) For x = 1.98 ν j is consistently higher than for x = 1.92, corroborating the partial occupation of O sites for x values below 2.0. (4) At the lower limiting composition ( x ≈ 1.8) E act becomes anomalously large (approximately 1.1 eV atom −1), compared with the value 0.5 eV atom −1 at x = 1.92, and the diffusion rate is about 10 3 times smaller. (5) At low temperatures the spin-lattice relaxation rate T −1 1 is dominated by the conduction electron contribution T −1 1 e . The quantity (T 1eT) −1 2 which is proportional to the density of states at the Fermi level, is 0.061 ± 0.002 s −1 2 K −1 2 for the dihydride compositions studied, indicating a constant density of states in the dihydride phase.