Crystal structures and magnetic properties of fluorite-related oxides Ln3NbO7 (Ln=lanthanides)

Crystal structures and magnetic properties of the ternary oxides Ln₃NbO₇ (Ln=La, Pr, Nd, Sm-Lu) are reported. Their powder X-ray diffraction measurements and Rietveld analyzes show that they have the fluorite-related structures with space group Pnma (Ln=La, Pr, Nd), C222₁ (Ln=Sm-Tb), or Fm-3m (Ln=Dy-Lu). Magnetic susceptibility measurements were carried out from 1.8 to 400 K. The Ln₃NbO₇ compounds for Ln=Pr, Gd, Dy-Yb show Curie-Weiss paramagnetic behavior, and Sm₃NbO₇ and Eu₃NbO₇ show van Vleck paramagnetism. On the other hand, two magnetic anomalies were observed for both Nd₃NbO₇ (0.6 and 2.7 K) and Tb₃NbO₇ (2.0 and 3.2 K). From the results of specific heat measurements, it was found that these anomalies are due to the antiferromagnetic ordering of Ln ions in two different crystallographic sites (the 8-coordinated and 7-coordinated sites).