Inhomogeneous distribution of Cr impurities in α–Al₂O₃ during refractory aging

Aged alumina refractories have been investigated by EPR (Electron Paramagnetic Resonance) and EPMA (Electron Probe Micro-Analysis). EPMA shows that Cr is homogeneously distributed inside the corundum grains within spatial resolution (1 μm). Cr³⁺ concentration at the atomic scale is obtained by EPR by the magnitude of dipolar broadening due to Cr-Cr interactions. This local concentration appears to be systematically higher (up to a factor of 10) than the average chromium content of the corundum crystals determined by EPMA. Cr³⁺ ions may be preferentially concentrated at the vicinity of extended lattice defects which help in accommodating the lattice strain due to the size difference between Al³⁺ and Cr³⁺ cations. A random location of these defects explains the apparent homogeneous chromium distribution found by EPMA in these aged refractories.