Nuclear Magnetic Resonance ofH1andNb93in the Niobium-Hydrogen System

The nuclear magnetic resonance of H and ${\mathrm{Nb}}^{93}$ have been observed in the $\mathrm{Nb}{\mathrm{H}}_x$ system as a function of hydrogen concentration and temperature. It is shown that line narrowing of the steady-state absorption resonance is limited at high temperatures by inhomogeneous broadening due to a distribution of fields caused by bulk paramagnetism of the samples. A sharp, though not discontinuous, change of hydrogen diffusion is observed at temperatures that range from 60 to 100°C and that depends on hydrogen concentration. Niobium resonance linewidth and intensity changes indicate that sharp change in proton diffusion is associated with the phase transition to the high-temperature cubic phase of the hydrides. The activation energy of Nb${\mathrm{H}}_{0.7}$ is $E_a=5\mathrm{}$ kcal/mole in the low-temperature (orthorhombic) phase and $E_a=3.7\mathrm{}$ kcal/mole in the high-temperature (cubic) phase. The niobium resonance is observable at all hydrogen concentrations. As the hydrogen concentration increases, the Nb Knight shift decreases. The fractional change in shift is $\frac{\Delta K}{K}=-9\%$ at Nb${\mathrm{H}}_{0.7}$.