In-situ X-ray diffraction study of alumina α-Al₂O₃ thermal behavior under dynamic vacuum and constant flow of nitrogen

We determined the crystalline characteristics of the hexagonal α-alumina phase by X-ray diffraction at high temperatures between 293 and 2308 K under dynamic vacuum then under constant flow of nitrogen. The experimental values obtained under dynamic vacuum allow to deduce, an anisotropy of the 'c' direction compared to the 'a' direction of approximately 12% in the range of 293 K–1760 K. On the other side, no anisotropy remains above 1765 K. This anisotropy reflects the fact that aluminum atoms occupy 2/3 of the octahedral sites in the compact hexagonal sublattice formed by the oxygen atoms. We have also reacted alumina when heated under a constant flow of nitrogen. Nitriding alumina occurs around 1765 K and then forms aluminum mononitride, AlN, ZnS-Wurtzite type hexagonal structure. The thermal expansion of this mononitride cell is linear and is characterized by an anisotropy of the direction 'a' in comparison to the direction 'c' of about 17%. - Highlights: • Anisotropic expansion of alumina cell between 293 and 1765 K. • Isotropic Expansion alumina mesh above 1765 K. • The isotropy reflects the fact that all compact hexagonal octahedral sites are identically occupied by the oxygen atoms. • Nitriding alumina occurs around 1765 K, which means in the area of isotropic expansion.