Formation of α-Al2O3 by thermal decomposition of basic aluminium chlorides at low temperatures

Basic aluminium chloride gels and the crystalline salt AlCl3 · 2 Al(OH)3 · 6 H2O are thermally decomposed to an amorphous product in a first step at low temperatures. If the reaction gas is rapidly removed the amorphous decomposition product directly transforms to corundum at increased temperature. Starting from gels the α-Al2O3 formation begins at 420 °C, transformation of the crystalline salt at 530 °C. Structural similarities of precursor compounds and corundum are discussed as a reason for this behaviour. High contents of HCl and H2O in the atmosphere surrounding the specimen result in decomposition to transition aluminas at increasing temperature, which are transformed into α-Al2O3 above 1000 °C.