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).