Study of Sialon S-phases M2AlxSi12-xN16-xO2+x, M = Ba and Ba0.9Eu0.1, by X-ray Single Crystal Diffraction, X-ray Powder Diffraction, and Solid-State Nuclear Magnetic Resonance

The crystal structure of the sialon Ba S-phase Ba2AlxSi12-xN16-xO2+x was refined, for x 2, using Mo K single-crystal data in Imm2 with a = 8.230(1) A, b = 9.655(2) A, c = 4.9093(8) A, and V = 390.1 A3, to a weighted R(Fobs2) = 9.5% for 520 unique reflections. The Si/Al tetrahedral framework is very similar to that for the iso-structural Sr S-phase. The Ba atom positions are shifted relative those of the Sr atoms by ~0.34 A along the c axis and the Ba atoms are coordinated by 10 N/O atoms at 2.85 to 3.35 A, whereas the Sr atoms are coordinated by 8 N/O atoms at 2.66 to 3.22 A. Powder samples of Ba S-phases (x 0 and 2) and an S-phase with 10% of the Ba atoms substituted by Eu atoms (x 2) were prepared by spark plasma sintering and postheat treatment at 1600 C for 15 h. The S-phases were studied by 29Si and 27Al magic-angle spinning NMR and the results are discussed in relation to proposed distributions of Si/Al and O/N atoms in the structure.