PREPARATION AND CRYSTAL-STRUCTURE OF LAAL(SI6-ZALZ)N10-ZOZ

A new nitrogen-rich phase has been observed to form in rare-earth-metal stabilised alpha-sialon preparations containing Sm and Nd. The X-ray powder pattern of this phase overlapped to a great extent that of alpha-sialon. Large quantities of this phase were also found in samples of the overall composition Ln(2)Al(4)Si(11)N(18)O(4) with Ln = Ce and La. The La compound was selected for crystal structure determination, and the obtained X-ray pattern could be indexed with an orthorhombic unit cell; a = 9.4303(7), b = 9.7689(8), c = 8.9386(6) Angstrom. A structural model was refined in the space group Pbcn using the Rietveld method. Based on 267 theoretical reflections in the 2 theta range 10-82 degrees, the following R-values were obtained for the composition LaAl(Si6-xAlz)N10-xOz with z=1; R(wp) = 1.8, R(I) = 6.6 and R(F) = 5.1%. The Al atoms and the (Si,Al) atoms are tetrahedrally coordinated by (N,O) atoms, yielding an Al(Si,Al)(6)(N,O)(10)(3-) network. The La atoms are found in tunnels extending along the [001] direction and are irregularly coordinated by seven (N,O) atoms. The structure of this phase contains structural fragments which show strong similarities with some present in LaSi3N5; their compositional relationship is evident if their chemical formulae are expressed as LaN-AlN-2(Si,Al)(3)(N,O)(4) and 2LaN-2Si(3)N(4).