Boehmite derived γ-alumina system.: 1.: Structural evolution with temperature, with the identification and structural determination of a new transition phase, γ′-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, similar to69% octahedral and similar to31% tetrahedral, was observed for material calcined between 500 and 900 degreesC. Gamma alumina was found to be present between 450 and 750 degreesC. Its structure was tetragonally distorted but showed a reduced tetragonal distortion with increasing temperature. A cubic gamma-Al2O3 phase was never detected. Above 750 degreesC, delta-Al2O3 was not observed, but instead a new phase was identified and designated gamma-prime-alumina (gamma'-Al2O3). Similarly to delta-Al2O3, gamma'-Al2O3 was determined to be a triple cell of gamma-Al2O3 and was described using the P (4) over bar m2 space group. The cation ordering in this structure is more obvious than that for gamma-Al2O3, with fewer site positions being occupied with increasing calcination temperature.