Structures and magnetic properties of new fluorite-related quaternary rare earth oxides LnY2TaO7 and LaLn2RuO7 (Ln=rare earths)

Abstract

New fluorite-related quaternary rare earth oxides LnY2TaO7 (Ln=La–Dy) and LaLn2RuO7 (Ln=Eu–Tb) have been prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space groups C2221 and Cmcm, respectively. The results of the Rietveld analysis for LnY2TaO7 (Ln=La–Dy) indicate that there exists no structural ordering of the Ln and Y ions at the eight-coordinate 4b site and the seven-coordinate 8c site, but the larger Ln ions occupy the eight-coordinate 4b site rather than the seven-coordinate 8c site, with increasing the Ln ionic radius. On the other hand, the results of the Rietveld analysis for the X-ray diffraction profiles of LaTb2RuO7 showed that La and Tb atoms are almost situated at the eight-coordinated site (4a site) and seven-coordinated site (8g site), respectively, i.e., cation ordering occurs on the 4a and 8g sites.All compounds LnY2TaO7 (Ln=La–Dy) are paramagnetic down to 1.8K. On the other hand, LaTb2RuO7 shows an antiferromagnetic transition at 17K. In addition, another magnetic anomaly has been found at 10K. Analysis of the magnetic specific heat for LaTb2RuO7 indicates that the magnetic transitions at 10 and 17K are due to the magnetic ordering of Tb3+ and Ru5+ ions, respectively.