Self-Diffusion of 45Ca and 88Y in Pure and YF3-Doped CaF2 Single Crystals

Tracer self-diffusion coefficients for Ca and Y were measured in pure and YF₃-doped CaF₂ crystals for dopant levels ranging from 2 to 10 mol% and temperatures between 960° and 1212°C. Coefficients for both cations increased approximately linearly with the square of the dopant concentration at constant temperature, with D *_Ca> D *_Y in doped crystals but D *_Ca> D *_Y in pure crystals. The activation energies for diffusion of both cations decreased with increasing dopant concentration. Atomic jump pathways are considered and the decrease in activation energy is explained in terms of reduction in the enthalpy of motion accompanying the formation of Willis anion defect clusters in the more heavily doped crystals.