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 (γ-Al 2 O 3 ), 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 γ-Al 2 O 3 phase was never detected. Above 750 °C, δ-Al 2 O 3 was not observed, but instead a new phase was identified and designated gamma-prime-alumina (γ‘-Al 2 O 3 ). Similarly to δ-Al 2 O 3, γ‘-Al 2 O 3 was determined to be a triple cell of γ-Al 2 O 3 and was described using the P 4̄ m 2 space group. The cation ordering in this structure is more obvious than that for γ-Al 2 O 3, with fewer site positions being occupied with increasing calcination temperature.