Tracer diffusion of selected cations in single crystal MgO

Abstract

The diffusion of various cations in single crystal magnesium oxide has been measured over a wide temperature range using radioactive tracers and a sectioning technique. Self-diffusion studies may be interpreted in such a manner as to suggest that the enthalpy of formation of a Schottky defect in this oxide is (3.8 ± 0.3) eV, while the enthalpy of motion of a cation vacancy is (1.56 ± 0.08) eV. Intrinsic diffusion occurs above 1900 °C, with a 'knee' temperature, for the material used here, of 1800 °C. Impurityprecipitation-controlled diffusion has been observed below 1500 °C. Atmosphere, apparently, has no effect on diffusion rates. The radius of a diffusing ion seems to have no detectable influence on its enthalpy of motion in MgO, whereas electronic properties (polarisability and charge) are evidently important in this respect. Most impurities diffuse by a vacancy mechanism, but beryllium diffusion may possibly proceed via interstitial sites. Short-circuit enhancement of diffusion in MgO involves two (or more) mechanisms, one being dependent on the presence of impurity precipitates at dislocations, while the other is independent of this phenomenon and operates for ions which are large compared with the host cation. Such enhancement is believed to have obscured precipitationcontrolled lattice diffusion in many studies and also accounts for the variation in activation energies reported by different authors for the diffusion of a given impurity.