Relationship between Crystallite Size and Bond Lengths in Boehmite

Boehmite with a crystallite size between 1 and 27 nm was prepared by annealing a boehmite precipitate under hydrothermal conditions at different temperatures. The crystalline structure, measured with X-ray powder diffraction, was refined with the Rietveld method. From the refinement bond lengths and bond angles between oxygen and aluminum atoms were calculated, which revealed that boehmite bond lengths and local symmetry depended on crystal dimensions. When boehmiteʹs crystallite size decreased, Al–OH interaction increased, and the local octahedral symmetry tended to be tetrahedral. The hydrogen bond, sustaining boehmiteʹs crystalline structure, and the bonds between oxygen atoms and hydroxyls inside the octahedra double layers, building the crystalline structure, became weaker as crystallite size decreased, which could explain why boehmiteʹs transformation temperature into a transitional alumina also decreased with crystallite size. Alumina crystallite size also depended of the corresponding size of boehmite. It was shown that desorbed water from annealed samples came from the crystal surface and not from the bulk.