Preparation and crystal structure of a new Sr containing sialon phase Sr2AlxSi12-xN16-xO2+x (x approximate to 2)

A nitrogen-rich sialon phase containing Sr or Eu, named the S-phase, has be en reported to form in the M'-Si-Al-O-N systems with M'=Sr and Eu. A sample with overall composition Sr2Al2.5Si10O4N14.5 hot-pressed at 1800 degrees C for 2 h contained approximately 85 vol% of the S-phase, and in addition th e alpha- and beta-sialon phases and an amorphous phase. Its structure was s olved from X-ray synchrotron powder data (lambda = 1.1608 Angstrom), using direct methods, and was refined by the Rietveld method from 131 reflections in the 2 theta range 10-59 degrees to R-F=2.7%, with the assumed compositi on Sr2Al2Si10O4N14, space group Imm2, a = 8.2788(9), b = 9.5757(9), c = 4.9 158(4) Angstrom, V = 389.7 Angstrom(3). The structure model was confirmed b y its electron diffraction pattern and by high-resolution electron microsco py studies. The structure exhibits a tetrahedral network with high connecti vity, each tetrahedron sharing corners with seven surrounding tetrahedra, a nd the Sr atoms, irregularly coordinated by eight O/N atoms, are found in t unnels extending along [001].