Preparation and crystal structure of U-phase Ln3(Si3 –xAl3 +x)O12 +xN2 –x(x≈ 0.5, Ln = La, Nd)

Abstract

U-phase Ln₃(Si_{3 –x}Al_{3 +x})O_{12 +x}N_{2 –x}(Ln = La, Nd) occurs as a crystalline phase in rare-earth sialon ceramics formed by devitrification of grain-boundary glasses at 1000–1400 °C. The crystal structure of Nd U-phase has been determined from Cu-Kα X-ray powder diffractometer data and refined by the Rietveld full-profile technique to R_F= 0.028. The space group is P321 and the cell dimensions are a= 7.974(1)Å, c= 4.873(1)Å and V= 268.29 ų. The structure is isomorphic with the La₃Ga₅GeO₁₄ structure, and exhibits corner-shared layers of (Si,Al)(O,N)₄ tetrahedra interconnected by AlO₆ octahedra. The rare-earth cations occupy sites between the tetrahedral layers. Transmission electron microscopy and lattice imaging studies support the X-ray structural findings. The structural relationship of the U-phase to other nitrogen-containing ceramic phases is discussed.