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

A new nitrogen-rich phase has been observed to form in rare-earth-meta l stabilised alpha-sialon preparations containing Sm and Nd. The X-ray powder pattern of this phase overlapped to a great extent that of alp ha-sialon. Large quantities of this phase were also found in samples o f the overall composition Ln(2)Al(4)Si(11)N(18)O(4) with Ln = Ce and L a. The La compound was selected for crystal structure determination, a nd the obtained X-ray pattern could be indexed with an orthorhombic un it cell; a = 9.4303(7), b = 9.7689(8), c = 8.9386(6) Angstrom. A struc tural model was refined in the space group Pbcn using the Rietveld met hod. Based on 267 theoretical reflections in the 2 theta range 10-82 d egrees, 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%. Th e 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 a re found in tunnels extending along the [001] direction and are irregu larly coordinated by seven (N,O) atoms. The structure of this phase co ntains structural fragments which show strong similarities with some p resent in LaSi3N5; their compositional relationship is evident if thei r chemical formulae are expressed as LaN-AlN-2(Si,Al)(3)(N,O)(4) and 2 LaN-2Si(3)N(4).