Crystal structure of rare-earth silicon-oxynitride J-phases, Ln4Si2O7N2

Rare-earth silicon-oxynitride J-phases, Ln4Si2O7N2 (Ln=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), were prepared by the N2 gas-pressured sintering method at 1 MPa of N2 and 1500–1700 °C. The Rietveld analysis was carried out for X-ray powder diffraction data measured at room temperature. The crystal structures of Ln4Si2O7N2 were refined with the structure model of La4Si2O7N2 for Ln=La, Pr, Nd, and Sm, and with that of Lu4Si2O7N2 for Ln=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The refined monoclinic unit-cell parameters (lengths a, b, c, angles β, and volume V) increased linearly in their two series of Ln with increasing ionic radii of rare-earth atoms. Discontinuities of the unit-cell parameters were found between the two Ln series.