Manganese Ions as Paramagnetic Probes in Magnesium Oxide. II. Crystal Growth and Ionic Diffusion

Abstract

The Mn2+ e. s. r. signal in MgO samples, containing 1-2000 at - ppm of manganese, is used as a paramagnetic probe to monitor the behavior of fresh MgO powders following thermal activation at 800 K - 1200 K in vacuum or in air. Two distinct Mn2+ signals are generally observed after evacuation and they correspond respectively to substitutional Mn2+ ions and clusters of Mn2+ ions near the surface. From the spectral intensity and the zero-field splitting term, D, informations have been obtained on the particle growth in metastable fresh MgO powders thermally treated in the range 800 - 1200 K, and on the progressive diffusion of surface Mn2+ ions into the bulk of MgO. The D-term decrease with lower manganese content or higher temperature thermal treatment is interpreted as a diffusion of Mn2+ ions towards cationic sites with more accurate cubic symmetry, i.e., more distant from the surface. After activation in air at 1300 K, all the apparent Mn2+ ions are found to be in perfectly octahedral sites in the bulk of MgO. A mechanism by which manganese ions are incorporated at substitutional sites in the lattice is discussed.