First-principles calculations of the stability and local structure of -sialon ceramics on the line Si3N4–½Ca3N2:3AlN

The atomic structures of calcium sialons CaxSi12-mAlmN16 (m = 0.5, 1, 2, 4) with the - or -Si3N4-type structure were calculated, using the ab initio DFT approach, including relaxations of both lattice parameters and positions of the atoms. For all m values the -Si3N4-type structure has a lower energy than the -type. For m = 1, 2 and 4 the -type structure is also stable against decomposition into the metal nitrides, in agreement with experimental data. For m = 0.5 stability becomes uncertain. Al ions prefer positions close to Ca. Rather strong structural relaxations occur around the Ca ion.