NMR Studies of the Hydrides of Disordered and Amorphous Alloys

Abstract

Nuclear magnetic resonance (NMR) spectroscopy has had extensive applications to the characterization of metal-hydrogen systems as is well documented in numerous previous review papers¹⁻⁷. Structural information on hydrogen site occupancies can be obtained from the NMR lineshapes in the “rigid-lattice” (i. e., immobile nuclei) limit. The hyperfine interactions with conduction electrons can be monitored through Knight shifts (σ_K) and spin-lattice relaxation time (T_le) contributions. The various nuclear relaxation times are usually very sensitive¹⁻⁷ to translational diffusion. Under suitable conditions, the hydrogen diffusion constants can be directly measured via various spin-echo techniques^2,3,5,6. NMR studies have been conducted on all three hydrogen isotopes (i. e., H, D, and T) as well as many host metal nuclei (e. g., ⁴⁵Sc, ⁵¹V, ⁸⁹Y, ⁹³Nb, and ¹³⁹La). Although most attention has been primarily focused upon the binary hydride phases (i. e., TiH_x, ZrH_x, PdH_x, etc.), the hydrides formed by crystalline alloys and interraetallics with nominal stoichiometries A₂B, AB, AB₂, and AB₅ have been the subjects of NMR measurements during the past ten years or so. This interest was mainly stimulated by the potential applications of various ternary hydrides (e. g., TiFeH_x, LaNi₅H_x, Mg₂NiH_x) as reversible hydrogen storage systems. However, very few reports of NMR experiments on amorphous hydrides formed from metallic glasses have been published to date ⁸⁻¹⁶ in spite of the recent proliferation of papers on other aspects of these materials. The relative absence of NMR results for amorphous metal-hydrogen systems is partially due to the rather low sensitivity of the technique (i. e., many spectrometers typically require at least 0.5–1.0g samples — which Is often considered to be quite demanding in the metallic glass preparation field). Furthermore, few NMR experiments had been done for H/M ratios below about 0.1 in any crystalline hydride. In fact, most published NMR results⁸⁻¹⁵ for the amorphous hydrides have been for samples with hydrogen-to-metal (H/M) atomic ratios around unity.