Proton NMR studies of hydrogen diffusion and electronic structure in crystalline and amorphous titanium-copper hydrides

Abstract

The proton line shapes, spin-lattice and rotating-frame relaxation times, and Knight shifts have been measured in crystalline TiCu${\mathrm{H}}_{0.94}$, ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{1.9}$, and ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{2.63}$, and in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$. The second moments of the rigid-lattice line shapes indicate that protons occupy ${\mathrm{Ti}}_{4}$ interstitial sites in crystalline TiCu${\mathrm{H}}_{0.94}$ and ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{1.9}$ while both ${\mathrm{Ti}}_{4}$ and ${\mathrm{Ti}}_{4}$${\mathrm{Cu}}_{2}$ sites are occupied in crystalline ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{2.63}$. Although a detailed determination of hydrogen site occupancy in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$ was not possible, the proton second moment implies octahedral site occupancy in addition to tetrahedral site occupancy. The host-metal structure and hydrogen site occupancies strongly influence both the hydrogen-diffusion behavior and electronic properties. The proton hyperfine interactions appear to be dominated by the transferred core-polarization terms from the Ti $d$ states near the Fermi level as previously found in $\ensuremath{\gamma}$-phase $\mathrm{Ti}{\mathrm{H}}_{x}$ and other Ti-based ternary hydrides. The density of electron states at the Fermi level as monitored at the proton sites is apparently reduced in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$ when compared with densities for the crystalline samples. It is suggested that this reduction may reflect a smearing of the energy bands with the elimination of long-range order in the amorphous phase. The greatly enhanced hydrogen diffusion (relative to crystalline TiCu${\mathrm{H}}_{0.94}$ and $\ensuremath{\gamma}\ensuremath{-}\mathrm{Ti}{\mathrm{H}}_{x}$) has been confirmed for amorphous $a$-TiCu${\mathrm{H}}_{1.3\ifmmode\pm\else\textpm\fi{}0.1}$. The diffusion behavior in crystalline ${\mathrm{Ti}}_{2}\mathrm{Cu}{\mathrm{H}}_{x}$ supports the view that H-atom diffusion jumps through octahedral sites have lower activation energies than the direct tetrahedral-to-tetrahedral jump paths.