Synthesis of Ln₄Si₂O₇N₂(Ln=rare earth) and ressessment of their lattice parameters

Abstract

A series of rare-earth silicon-oxynitride J-phases, Ln₄Si₂O₇N₂ (Ln = rare earth) has been prepared by the N₂ gas-pressure sintering method under 1 MPa of N₂ at 1973-2173 K. The cell parameters of the J-phases have been refined by Rietveld analysis for X-ray powder diffraction data measured at room temperature. A group of Ln₄Si₂O₇N₂ with Ln = La, Pr, Nd, and Sm possesses the high-temperature Y₄Al₂O₉ structure and other group with Ln = Y and Gd-Lu has the low-temperature Y₄Al₂O₉ structure. The cell parameters of the J-phases in each Ln group increase linearly with increasing ionic radii of rare-earth elements. Discontinuities of the cell parameters have been found between the two Ln groups.