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

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.