The crystal structures and magnetic properties of rare earth tantalates RE3TaO7 (RE = rare earths) are reported. Their crystal structures were grouped into three types: La3NbO7-type, Y3TaO7-type, and defect-fluorite-type. For the Ho3TaO7 compound, two different phases (Y3TaO7-type and defect-fluorite-type) were prepared. At around room temperature, Nd3TaO7 was found to transform from the Y3TaO7-type phase to La3NbO7-type phase with increasing temperature. Temperature dependences of the magnetic susceptibilities and specific heats indicated that the Nd and Tb compounds undergo a ‘two-step’ antiferromagnetic transition with separate ordering of ions on different crystallographic sites below 2.6 and 3.6 K, respectively. The Dy3+ ion was found to be in an antiferromagnetic state below 2.3 K. The Ho3TaO7 with the Y3TaO7-type structure showed an antiferromagnetic-like transition, while no magnetic ordering was observed down to 1.8 K for the defect-fluorite-type Ho3TaO7.

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Ternary lanthanide iridium oxides Ln 3 IrO 7 Ln Pr, Nd, Sm, and Eu were prepared and their structures were determined by X-ray diffraction measurements. At room temperature, Pr 3 IrO 7 crystallizes in an orthorhombic superstructure of cubic fluorite with the space group Cmcm. The results of the differential thermal analysis DTA and specific heat measurements show the existence of a phase transition for Ln 3 IrO 7 Ln Pr, Nd, Sm, and Eu at 262, 342, 420, and 485 K, respectively. At low temperatures, Ln 3 IrO 7 crystallizes in a monoclinic structure with the space group P2 1 n.

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