Synthesis of pure 15R-SiAlON polytype phase & its crystal structure under carbothermal-reduction-nitridation

Synthesis of pure 15R-SiAlON polytype phase, unexplored so far, through carbothermal-reduction-nitridation (CRN) of a mixture of alumina with stoichiometrically proportionate silica has been successfully attempted. The probable reaction sequences of formation are described in terms of the obtained phases. It is argued that the role of CRN process is limited to the formation of intermediate phases, AlN and beta-SiAlON, which finally react to form the polytype phase. FTIR studies provide supportive evidence for the different chemical reactions taking place. Formed particles are mostly agglomerated which are actually consisted of fine crystallites (100-300 nm). The powder is well sinterable without external densification aid under spark plasma sintering process producing plate-like elongated grains. The refinement of X-Ray data revealed lattice parameters, lattice volume, crystallite size, microstrain as well as atom coordinates and the corresponding Wycoff positions of the obtained 15R-SiAlON. Six independent atom sites in the unit cell, one (Si/Al) site at 3a, two (Si/Al) sites at 6c and three (O/N) sites at 6c are found. The crystal structure as revealed is silicon rich, ordered but distorted with respect to both bond length and direction for the presence of N4- electronic state.