Proton Magnetic Resonance Studies of Structure, Diffusion, and Resonance Shifts in Titanium Hydride

Abstract

Measurements of the proton magnetic resonance at ∼26.9 Mc in titanium hydride samples, ranging in composition from TiH_1.61 to TiH_1.97, have been made in the temperature range from —196° to about 200°C. The second moment of the proton resonance at the lower temperatures shows that the hydrogen atoms are randomly distributed among the lattice sites which are located tetrahedrally with respect to the titanium atoms. Self‐diffusion of the hydrogen atoms narrows the proton resonance above room temperature. The temperature dependence of the correlation frequency for the proton motions, obtained from the linewidths, leads to diffusional activation energies which increase with hydrogen content from 9.4 kcal/g atom for TiH_1.607 to 10.2 for TiH_1.923. Moreover, the diffusion rate is directly proportional to the number of unfilled tetrahedral holes in the metallic lattice, which indicates that the self‐diffusion takes place via a vacancy mechanism. Proton resonance shifts to higher applied magnetic fields were observed. They were measured at room temperature for all specimens and were found to increase from 0.01% for TiH_1.607 to about 0.032% for TiH_1.969. For these two extreme compositions, the temperature dependence of the shift was measured between —95°C and 190°C and was found to be similar to the bulk susceptibility, the shifts for TiH_1.969 exhibiting an anomaly at about 13°C as does the susceptibility. These results are interpreted semiquantitatively in terms of exchange interactions which pair spins of electrons in the conduction band with those of electrons localized on the hydrogen. The results suggest that the hydrogen is held in the lattice by a combination of covalent and ionic bonding, the latter involving a net positive charge on the hydrogen. The general importance of exchange interactions in intermetallic compounds is commented upon.