Diffusion of Cobalt Ion in Single Crystals of Silver-Bromide
The diffusion of cobalt has been studied in high -purity single crystals of silver bromide as a function of temperature, using the sectioning technique. The experiment consists of diffusing the tracer from a thin layer of radioactive Co('57) deposited on the face of a carefully prepared AgBr specimen. The diffusion coefficient is determined from the penetration profile of tracer concentration as a function of depth from the active layer. The measurements were made in the temperature range 243(DEGREES)C to 408(DEGREES)C, and the diffusion coefficients ranged from about 2 x 10(' -10) cm('2)/s to 2 x 10('-7) cm('2)/s. The Arrhenius plot of the logarithm of the diffusion coefficient versus reciprocal temperature showed a definite non-linearity, with an activation energy of 1.34 eV at temperature above 321(DEGREES)C and 1.16eV at lower temperatures. The diffusion of Co('2+) in AgBr was interpreted in terms of a vacancy mechanism and the Lidiard-Debye-Huckel model. By including the well -established temperature-dependent Frenkel defect formation energy and assuming an association energy of 0.2 eV, the degree of association of Co('2+) with a cation vacancy was found to increase from 0.08 at 243(DEGREES)C at 0.65 at 408(DEGREES)C. The enthalpy and entropy for the jump of Co('2+) into a nearest-neighbor cation vacancy are determined to be respectively 0.79 eV and 4.4k. This value of jump energy appears to be consistent with our understanding of the variation in solute mobility due to the crystal field in ionic crystals.
- Pub Date:
- Physics: Condensed Matter