Solid-state proton nuclear magnetic resonance studies of hydrogen site occupancies, electronic structure properties, and diffusion behavior in transition metal hydrides

by Bowman, Robert Clark

Abstract (Summary)
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[subscript x], ZrH[subscript x], crystalline TiCuH[subscript 0.94], Ti[subscript 2]CuH[subscript 1.9], and Zr[subscript 2]PdH[subscript x](with x < 2) while both tetrahedral and octahedral sites can be occupied in amorphous a-TiCuH[subscript 1.4], Ti[subscript 2]CuH[subscript 2.6], and Zr[subscript 2]PdH[subscript 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[subscript F]) in Ti[subscript 1-y]V[subscript y]H[subscript x], TiCr[subscript 1.8]H[subscript x], TiCr[subscript 1.9]H[subscript x], TiCuH[subscript x], Ti[subscript 2]CuH[subscriptx], ZrH[subscript x], and Zr[subsript 2]PdH[subscript 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[subscript F) through a Jahn-Teller type mechanism are associated with the temperature and composition dependent tetragonal distortions in Ti[subscript 1-y]V[subscript y]H[subscript 1.95] and ZrH[subscript x] (where 1.75 < x < 2.0). The proton NMR results are consistent with recent band theory calculations and photoemission spectra. Unusual N(E[subscript F]) increases with hydrogen content, which are supported by independent magnetic susceptibility data, have been observed in TiCr[subscript 1.8]H[subscript x], TiCr[subscript 1.9]H[subscript x], and Ti[subscript 2]CuH[subscript x]. The proton parameters suggest that significant differences in N(E[subscript F]) for the crystalline and amorphous phases of TiCuH[subscript x] and Zr[subscript 2]PdH[subscript x] may reflect a smearing of energy levels in the disordered phases. The proton rotating-frame relaxation times for Ti[subscript 1-y]Cu[subscript y]H[subscript x] indicate both crystal structure and hydrogen site occupancies greatly influence diffusion behavior. A significant enhancement in hydrogen mobility for amorphous a-TiCuH[subscript 1.4] has been confirmed; but, short range order is probably retained in the structure of a-TiCuH[subscript 1.4]. Reductions in activation energies are observed when octahedral sites exist in the diffusion paths between tetrahedral sites.
Bibliographical Information:


School:California Institute of Technology

School Location:USA - California

Source Type:Master's Thesis



Date of Publication:08/30/1982

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