Boehmite Derived (gamma)-Alumina System. 1. Structural Evolution with Temperature, with the Identification and Structural Determination of a New Transition Phase, -Alumina

Variations in the structure of gamma-alumina ((gamma) -Al2O3), derived from well-crystalline boehmite, calcined at various temperatures in air were investigated. Consistent distribution of cation coordination, ~69% octahedral and ~31% tetrahedral, was observed for material calcined between 500 and 900 C. Gamma alumina was found to be present between 450 and 750 C. Its structure was tetragonally distorted but showed a reduced tetragonal distortion with increasing temperature. A cubic(gamma)-Al2O3 phase was never detected. Above 750 C,(theta)-Al2O3 was not observed, but instead a new phase was identified and designated gamma-prime-alumina ((gamma)-Al2O3). Similarly to (gamma)-Al2O3, (theta)-Al2O3 was determined to be a triple cell of(gamma)-Al2O3 and was described using the Pm2 space group. The cation ordering in this structure is more obvious than that for -Al2O3, with fewer site positions being occupied with increasing calcination temperature.