Nuclear Magnetic Resonance (NMR) methods have been used to study the nonstochiometric dihydrides and dideuterides of high purity scandium. For the first time, measurements have been made of the spin ...relaxation times T(,1) and T(,2) of all three nuclear species present (('45)Sc, ('2)D, ('1)H) in a metal hydride (deuteride) system, permit- ting a comparison of the main features of both atomic and vacancy motion on the hydrogen sublattice. In the regions of the conventional diffusion induced ('45)Sc- and ('2)D-T(,1) minima (<800 K), the relaxation rates are dominated by fluctuations of the quadrupolar interaction reflecting vacancy motion. The symmetric ('1)H-T(,1) minimum can be well described by a Lorentzian (Debye) spectral density function. In contrast, the ('45)Sc-T(,1) minimum shows for 1n(Sc-T(,1)) versus reciprocal temperature a substantially smaller slope on the high temperature side at high vacancy concentrations. (('45)Sc-T(,1))(,min) saturates rapidly for vacancy concentrations larger than c(,v) > 0.03 and shows a weaker frequency dependence than expected. The jump-attempt frequencies (nu)(,o) obtained from the ('45)Sc-T(,1) data agree well with the values obtained from neutron scattering measurements, whereas the ('1)H-T(,1) data yield anomalously low jump frequency prefactors. We interpret the departure of the ('45)Sc results from the Lorentzian model as indicating the formation of vacancy pairs and the importance of particle-particle interactions. The ('45)Sc- and ('2)D- T(,1) data also reveal the importance of three particle correlations and conduction elec- tron screening for the quadrupolar relaxation mechanism. At high temperatures (>800K), we have observed a new, previously unfore- seen and essentially frequency independent decrease of T(,1) and T(,2) for all three nuclear species (('45)Sc, ('2)D and ('1)H). This second, high temperature T(,1) minimum suggests the formation of short-lived clusters on the H(D) sublattice similar to that occurring in superionic fluorides and the existence of highly correlated modes of motion. Large amplitude vibrations of the hydrogen (deuterium) atoms at high temperatures lead to an effective slowdown of atom diffusion and the T(,1) decrease. Several other high temperature relaxation mechanisms have been considered and can be excluded on both theoretical and experimental grounds. ('1)DOE REport IS-T-1237. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.
