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摘要
Hydrogen and Na-23 NMR were used to probe diffusive motions of the ions in several NaH powders. In three NaH samples, the H resonance is a superposition of broad and narrow components, reflecting the presence of relatively immobile and mobile H, respectively. The fraction of mobile H grows from 23 to 250 degrees C; this pattern has been observed previously in other ionic hydrides. By 300 degrees C, the formerly broad hydrogen component has itself motionally narrowed. In these samples, the observation of a smaller amount of Na-23 line narrowing by 300 degrees C indicates that only the H- ions are mobile at 300 degrees C, leaving Na-23-Na-23 dipole interactions unaveraged. A deep minimum in the hydrogen rotating-frame relaxation time T-1 rho is observed near 325 degrees C, as expected from the onset of motional averaging. In a fourth sample, the entire H line is narrowed already by 150 degrees C. In this sample, the Na-23 is also partially narrowed at this temperature and by 175-225 degrees C, further narrowing of the Na-23 resonance indicates that now both ions are in rapid motion. In all the samples, the spin lattice relaxation times T-1 for hydrogen and sodium decrease monotonically with temperature, in qualitative accord with relaxation by physical diffusion of spin magnetization to relaxation centers.
- 出版日期2012-9-6
