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 Kalpha single-crystal data in Imm2 with a = 8.230(l) Angstrom, b = 9.655(2) Angstrom, c = 4.9093(8) Angstrom, and V = 390.1 Angstrom(3), to a weighted R(F-obs(2)) = 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 similar to0.34 Angstrom along the c axis and the Ba atoms are coordinated by 10 N/O atoms at 2.85 to 3.35 Angstrom, whereas the Sr atoms are coordinated by 8 N/O atoms at 2.66 to 3.22 Angstrom. Powder samples of Ba S-phases (x approximate to 0 and 2) and an S-phase with 10% of the Ba atoms substituted by Eu atoms (x approximate to 2) were prepared by spark plasma sintering and postheat treatment at 1600 degreesC for 15 h. The S-phases were studied by Si-29 and Al-27 magic-angle spinning NMR and the results are discussed in relation to proposed distributions of Si/Al and O/N atoms in the structure.