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

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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 P2_[1]2_[1]2_[1], in which Ln^[3+] 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 P2_[1]2_[1]2_[1] 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 K 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).