Preparation and crystal structure of LaAl(Si6 –zAlz)N10 –zOz

Abstract

A new nitrogen-rich phase has been observed to form in rare-earth-metal stabilised α-sialon preparations containing Sm and Nd. The X-ray powder pattern of this phase overlapped to a great extent that of α-sialon. Large quantities of this phase were also found in samples of the overall composition Ln₂Al₄Si₁₁N₁₈O₄ 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)Å. A structural model was refined in the space group Pbcn using the Rietveld method. Based on 267 theoretical reflections in the 2θ range 10–82°, the following R-values were obtained for the composition LaAl(Si_{6 –z}Al_z)N_{10 –z}O_z with z= 1; R_Wp= 1.8, R₁= 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–2Si₃N₄.