Abstract
Abstract The self-diffusion of Ca in single crystal CaWO4 doped with Nd and Na has been measured in the temperature range 0.53 Tm−0.9 Tm. From 0.75 Tm−0.9 Tm. D = (4.0 ± 0.3) × 10 −3 exp (− 55,800 ± 620 RT ) cm 2 sec −1 and from 0.53 to 0.75 Tm, D = (1.9 ± 0.4) × 10 −8 exp (− 22,900 ± 1100 RT ) cm 2 sec −1 . In the range 0.75 Tm−0.9 Tm, diffusion appears to be intrinsic lattice diffusion, via a vacancy mechanism. In CaWO4 the predominant intrinsic defect structure appears to be the Schottky defect with vacancies on the Ca2+ and O2− sites. The energy of formation of Schottky defect pairs is estimated to be 52 to 66 kcal/g.mole. In the temperature range 0.53 Tm−0.75Tm, Ca diffusion in CaWO4 appears to be extrinsic. Diffusion profiles for samples annealed at lower temperatures lead to two values of D: (i) a small D near the crystal surface (within 20–30 μ), believed to be bulk extrinsic diffusion, and (ii) a large D deep within the crystal, which probably results from superposition on extrinsic bulk diffusion of contributions from diffusion down the dislocations.
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