Structural data from x-ray powder diffraction of a new phase formed in theSi3N4-La2O3-Y2O3 system after oxidation in air

A new phase formed at high temperature with composition (Y2/3La1/3)(2)Si2O7 was found among the reaction products of a hot pressed Si3N4-based materia l (with Y2O3 and La2O3 as sintering aids) after oxidation experiments. This phase was also directly synthesized from oxide precursors (SiO2, Y2O3, La2 O3) at 1600 degrees C, under the condition of a silica excess. After identi fying a proper monoclinic cell, the structure of such a compound was succes sfully refined by Rietveld method starting from the existing model of G-for m of La2Si2O7. The new phase turned out to be isostructural with it, showin g however a characteristic inhomogeneous substitution of Y atoms by La ones in only half the cell positions and a rather distorted arrangement of Si-O tetrahedra. (C) 1999 Elsevier Science Limited All rights reserved.