Cation Diffusion in Single‐Crystal and Polycrystalline BeO

The diffusion of Be was measured in single crystals of BeO in directions normal and parallel to the hexagonal c axis. The diffusion coefficients, D , in the two directions were nearly equal to each other. The values of D , measured between 1500° and 2000°C were quite similar to values reported previously for high‐density, high‐purity, polycrystalline BeO. The single‐crystal data, taken in conjunction with polycrystalline data, show that the observed diffusion in polycrystalline BeO proceeds entirely by volume diffusion with no significant diffusion along grain boundaries, although there is some evidence of rapid surface diffusion along microcracks in the surface. The data are interpreted in terms of extrinsic impurity‐dependent diffusion via mobile vacancies.