Preparation and Crystal Structure of of U-Phase Ln3(Si3-xAl3+x)O12+xN2-x (x≈0.5, Ln=La,Nd)

• U-phase Ln3(Si3 –xAl3 +x)O12 +xN2 –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 RF= 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 Å3. 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.

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Salon material; Grain boundary phase; Rietveld analysis; Nitrogen ceramic

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