Variation of the coefficient of self-diffusion in a crystal caused by changes in the isotope composition

Variation of the parameters of self-diffusion in a crystal of simple substance caused by changes in the isotope composition have been theoretically studied in the regions of high and low temperatures. The results of numerical calculations performed for diamond and bcc lithium crystals are presented. It is shown that the isotope dependence of the self-diffusion coefficient D is weak at high temperatures but becomes substantial at low temperatures: at T = 0, the D value increases by an order of magnitude on the passage from 7Li to 6Li and decreases by three orders of magnitude on the passage from 12C to 13C. This isotope effect is related to the phenomena of quantum tunneling and zero-point oscillations of atoms in crystals at T = 0. The isotope dependence of the interatomic interaction potential significantly influences the self-diffusion coefficient only at high temperatures.