Low-temperature ¹H nuclear magnetic resonance study of crystal and electronic structures of the nearly stoichiometric yttrium dihydride

Abstract

Proton nuclear magnetic resonance absorption spectra and spin-lattice relaxation rates in yttrium dihydride have been measured in the temperature range from 4.2 to 310K at 36.0MHz. The second moment of the resonance line corresponds to the rigid-lattice regime for YH_1.99 and its value agrees with the anticipated CaF₂ type of structure. The main contribution to the spin-lattice relaxation rate R ₁ arises from conduction electrons and is characterized by R _le = 2.77 × 10⁻³s⁻¹ K⁻¹ × T. Evidence of proton self-diffusion was seen in the linewidth and in R ¹ for YH_1.99+0.1. The onset temperature, about 250 K, of the self-diffusion, is close to the metal-semiconductor transition reported earlier for that hydride. Below that temperature the linewidth becomes temperature independent and the second moment of the line is explained in terms of different structure models. The fit to the temperature dependence of R ₁ in the temperature range 70–310K gives R _le = 2.1 × 10⁻³s⁻¹K⁻¹ × T. The relaxation becomes almost temperature independent below 50 K. Various mechanisms for this behaviour are discussed. In addition, the R ₁ data for the sample prepared with yttrium of 99.9% purity are presented. In contrast with the previous case, where pure yttrium from the Ames Laboratory was used, R ₁ has a large contribution of spin diffusion to the paramagnetic Gd³⁺ ions.