Solid-State Proton Nuclear Magnetic Resonance Studies of Hydrogen Site Occupancies, Electronic Structure Properties, and Diffusion Behavior in Transition Metal Hydrides

Citation

Bowman, Robert Clark, Jr. (1983) Solid-State Proton Nuclear Magnetic Resonance Studies of Hydrogen Site Occupancies, Electronic Structure Properties, and Diffusion Behavior in Transition Metal Hydrides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/w68b-2750. https://resolver.caltech.edu/CaltechETD:etd-05152003-105639

Abstract

Solid-state NMR techniques have been used to measure the proton lineshapes, Knight shifts, and relaxation times in several transition metal hydrides. The objective of these studies is to obtain a better understanding of the roles of host metal structure and substitutional alloying on the hydrogen site occupancy, the electronic structure properties, and diffusion mechanisms.

An improved method for observing the rigid-lattice proton lineshapes and extracting the second moments has been developed. Comparisons of the experimental second moments for polycrystalline samples with the values calculated from Van Vleck formulas for nuclear dipolar interactions have indicated that only tetrahedral interstitial sites are occupied by the protons in TiH_x, ZrH_x, crystalline TiCuH_0.94, Ti₂CuH_1.9, and Zr₂PdH_x (with x < 2) while both tetrahedral and octahedral sites can be occupied in amorphous a-TiCuH_1.4, Ti₂CuH_2.6, and Zr₂PdH_x (x > 2).

The proton Knight shifts and low-temperature spin-lattice relaxation times have been related to the local densities of electron states at the Fermi levels N(E_F) in Ti_1-yV_yH_x, TiCr_1.8H_x, TiCr_1.9H_x, TiCuH_x, Ti₂CuH_x, ZrH_x, and Zr₂PdH_x. The dominant conduction electron hyperfine interaction for protons is a transferred "core-polarization" of the paired hydrogen 1s electrons through spin exchange with the unpaired metal d electrons. The proton NMR parameters have confirmed that decreases in N(E_F) through a Jahn-Teller type mechanism are associated with the temperature and composition dependent tetragonal distortions in Ti_1-yV_yH_1.95 and ZrH_x (where 1.75 ≤ x ≤ 2.0). The proton NMR results are consistent with recent band theory calculations and photoemission spectra. Unusual N(E_F) increases with hydrogen content, which are supported by independent magnetic susceptibility data, have been observed in TiCr_1.8H_x, TiCr_1.9H_x, and Ti₂CuH_x. The proton parameters suggest that significant differences in N(E_F) for the crystalline and amorphous phases of TiCuH_x and Zr₂PdH_x may reflect a smearing of energy levels in the disordered phases.

The proton rotating-frame relaxation times for Ti_1-yCu_yH_x indicate both crystal structure and hydrogen site occupancies greatly influence diffusion behavior. A significant enhancement in hydrogen mobility for amorphous a-TiCuH_1.4 has been confirmed; but, short range order is probably retained in the structure of a-TiCuH_1.4. Reductions in activation energies are observed when octahedral sites exist in the diffusion paths between tetrahedral sites.

Thesis Files