Magnetic properties and structural transitions of orthorhombic fluorite-related compounds Ln3MO7 (Ln=rare earths, M=transition metals)

Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln3MO7 (Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln3MoO7 (Ln=La–Gd). They crystallize in an orthorhombic superstructure of cubic fluorite with space group P212121, in which Ln3+ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). All of these compounds show a phase transition from the space group P212121 to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility measurements from 1.8 to 400 K and specific heat measurements from 0.4 to 400 K. Gd3MoO7 shows an antiferromagnetic transition at 1.9 K. Measurements of the specific heat for Sm3MoO7 and the analysis of the magnetic specific heat indicate a “two-step” antiferromagnetic transition due to the ordering of Sm magnetic moments in different crystallographic sites, i.e., with decreasing temperature, the antiferromagnetic ordering of the 7-coordinated Sm ions occur at 2.5 K, and then the 8-coordinated Sm ions order at 0.8 K. The results of Ln3MoO7 were compared with the magnetic properties and structural transitions of Ln3MO7 (M=Nb, Ru, Sb, Ta, Re, Os, or Ir).