Abstract
We had reported observation of polymorphs of alumina having rare earth sesquioxide type structures in xerogel γ alumina (XGA) at high pressures and temperatures (High Pressure Research, 1998). In this paper we report observation of two additional phases at somewhat different pressures and temperatures. The XGA quenched from 5.2 GPa and 1450°C showed, besides α Al2O3, a new hexagonal metastable polymorph μAl2O3. Over a period of 10 to 12 weeks μ A12O3 transforms to a stable hexagonal phase λ Al2O3. The cell parameters of λ Al2O3 are comparable with those of k Al2O3 found by Saalfield on dehydration of gibbsite A12O33H2O in air at 800°C. The XGA containing 1 wt% of Cr2O3 yielded similar results at 4.56GPa and 1100°C. Further the XGA containing Cr showed complete transformation to n Al2O3 at 5.2 GPa and 1520°C.