Diffusion of Kr Isotopes in Solid Ar

The diffusion coefficient D=D⁰e^-Q⁰RT and isotope effect for Kr⁷⁸ and Kr⁸⁶ in solid Ar have been measured using a tracer technique. The activation energy QKr^_⁰ depends on the interchange parameter w, but is estimated to lie in the range 3500-3900 cal mole^-1, with the most reliable w giving the lower value. Thus, the two calculated QKr^_⁰ (3853 and 4050 cal mole^-1) for the vacancy mechanism using a two-body (12, 6) potential appear somewhat high, but in reasonable agreement with experiment. The isotope effect gives fΔK≡(D₇₈D₈₆-1)[(86/78)1/2-1]=0.48±0.25 which, since QKr^_⁰QAr^_⁰, suggests that ΔK~0.6 for a single-vacancy mechanism. {Here f is the correlation factor and ΔK≡(ω₂^_⁷⁸ω₂^_⁸⁶-1)[(86/78)1/2-1], where ω₂ is the tracer jump rate.}