New polymorphs of alumina: Part II mu and lambda alumina

We had reported observation of polymorphs of alumina having rare earth sesq uioxide type structures in xerogel gamma alumina (XGA) at high pressures an d temperatures (High Pressure Research, 1998), In this paper we report obse rvation of two additional phases at somewhat different pressures and temper atures. The XGA quenched from 5.2 GPa and 1450 degrees C showed, besides a Al2O3, a new hexagonal metastable polymorph mu Al2O3 Over a period of 10 to 12 weeks mu Al2O3 transforms to a stable hexagonal phase lambda Al2O3. The cell parameters of lambda Al2O3 are comparable with those of, k Al2O3 foun d by Saalfield on dehydration of gibbsite Al2O3 . 3H(2)O in air at 800 degr ees C. The XGA containing 1 wt% of Cr2O3 yielded similar results at 4.56 GP a and 1100 degrees C. Further the XGA containing Cr showed complete transfo rmation to a Al2O7 at 5.2 GPa and 1520 degrees C.