Modulated structures in corundum type oxides

Abstract

The miscibility gap for the system Al₂O₃-Cr₂O₃-Fe₂O₃ was determined for a constant Cr₂O₃ content of 16-6 mole %. Dark-field transmission electron microscopy showed dark and light lamellae (approximate width 50–200 Å) in solid solutions annealed within the miscibility gap. X-ray diffraction results indicated that in early stages of annealing a broadening occurred only on the (hkl) reflections with l≠0. The orientation of the lamellae was determined from electron diffraction to be in the [001] direction. In the system Al₂O₃-Cr₂O₃ separation of phases proceeds very slowly and may be enhanced by hydrothermal treatment. The mechanism of phase separation is spinodal and proceeds as follows : crystalline solution →intermediate modulated phase→ equilibrium phases.

Theoretical estimates of the elastic strain energy indicated that composition fluctuations in the [001] direction did not minimize the volume free energy. It is noted that sinco the direction of the composition fluctuation is in part determined by the kinetics of the process, the apparent discrepancy between observation and prediction based on equilibrium considerations is reasonable.