Investigation of the self-diffusion in solid sodium using quasielastic neutron scattering

By means of quasielastic neutron scattering the self-diffusion in solid sodium has been investigated in the temperature range between 76 and 97°C. The experiment was carried out using a backscattering spectrometer with an energy resolution of 3.2 × 10−7 eV. From the width of the quasielastic line recorded at small scattering vectors (Q < 0.52 Å−1), a diffusion constant : D=(0.12 ± 0.01) exp−(0.43 ± 0.01)/kT(cm2 s−1) was obtained. The activation energy is given in units of electron volts. This is in good agreement with tracer results of Mundy, Barr and Smith (1967). Single-crystal measurements at large Q values showed an orientational and Q-dependence of the linewidth with deviations from the predictions of the simplest diffusion model. This assumes a monovacancy diffusion mechanism with (1/2 1/2, 1/2) jumps for sodium atoms between the regular sites of the b.c.c. lattice. For an intermediate momentum transfer Q in the 〈100〉 direction, there appeared discrepancies far beyond the error limits. They can be interpreted through an admixture of 20% (1, 0, 0) jumps at 91°C. We assume that such jumps can occur if, in addition to the monovacancy diffusion, the migration of divacancies takes place as has been proposed by Seeger and Mehrer (1969).