New polymorphs of alumina

We present the results of high-pressure, high-temperature studies on xeroge l gamma alumina which is made up of nanocrystalline alumina and an amorphou s phase. Not many studies of this type have been reported on xerogel materi als. At about 5 GPa and 1400 degrees C, the xerogel alumina transforms into a polymorphic mixture containing alpha Al2O3, B Al2O3 and C Al2O3 where th e last two phases have B Ln(2)O(3) and C Ln(2)O(3) type structures respecti vely. Here Ln stands for the rare earths. The xerogel alumina containing 1 wt.% Cr2O3 under these conditions transforms into a polymorphic mixture con taining Al2O3, k' Al2O3 and H Al2O3 where H Al2O3 has the H Ln(2)O(3) type structure. For the first time, the observation of rare earth sesquioxide st ructures of alumina is reported. At about 5 GPa at 27 degrees C, the xeroge l aluminal transforms to an amorphous phase. The results are discussed usin g the free energy diagram for xerogels proposed by R. Roy (J. Amer. Cer. Se c., 52, 344, 1969; 67, 468, 1984).