Site and barrier energy distributions that govern the rate of hydrogen motion in quasicrystalline Ti45Zr38Ni17Hx

The first application of a recent theory linking nuclear magnetic resonance spin-lattice relaxation rates to interstitial atom motion in disordered systems is presented. Laboratory and rotating frame relaxation rate data taken as a function of temperature for 1H moving in quasicrystalline Ti45Zr38Ni17H163 are fitted with the new theory, yielding a hydrogen site energy distribution of Gaussian shape and width 47±5 meV. The energy barriers for hydrogen motion show a Gaussian distribution of width 50±5 meV, and the difference between the means of the distributions is 0.42±0.01 eV. This is the first time relaxation rates have been analysed to provide information on both hydrogen site energy and barrier energy distributions simultaneously. The data are also fitted using an approach popular for disordered systems: the integration of the Bloembergen, Purcell, and Pound relaxation theory over a distribution of activation energies. The relative merits of this traditional approach and the recent theory in fitting the relaxation data, and also in fitting measurements of the static and magic angle spinning linewidths, are discussed. Although the traditional approach can fit all the data self-consistently, the theory's unsupported assumptions are undermined by the new approach.