Ternary rare earth rhenium oxides Ln 3 ReO 7 (Ln = Y, ErLu) have been prepared. Their X-ray diffraction measurements and the Rietveld analysis show that these compounds have an orthorhombic superstructure of fluorite-type with space group C222 1 (for Ln = Y, Er, Tm) or a defect-fluorite structure Fm 3m (for Ln = Yb, Lu). The zero-field-cooling and field-cooling magnetic susceptibility for Y 3 ReO 7 diverse when the temperature is decreased through 10 K. At the same temperature, specific heat anomaly is also observed. The results of the specific heat measurements for Tm 3 ReO 7 and Yb 3 ReO 7 indicate the existence of magnetic anomaly below 7 and 2.2 K, respectively. On the other hand, Er 3 ReO 7 and Lu 3 ReO 7 are paramagnetic down to 1.8 K.

Solid solutions Pr 3 (Ru 1-x Ta x)O 7 (0 ≤ x ≤ 1.0) and (Pr 1-x Y x) 3 RuO 7 (0 ≤ x ≤ 0.7) were obtained as a single phase compound. They crystallize in an orthorhombic superstructure derived from that of the cubic fluorite with space group Cmcm. The results of the Rietveld analysis for X-ray diffraction profiles of Pr 3 (Ru 1-x Ta x)O 7 showed that Ru and Ta atoms are randomly situated at the six-coordinate 4b site. For (Pr 1-x Y x) 3 RuO 7 , with increasing the concentration of Y ions (x value), the smaller Y ions occupy selectively the seven-coordinate 8g site rather than the eight-coordinate 4a site. Through magnetic susceptibility measurements for Pr 3 (Ru 1-x Ta x)O 7 , the antiferromagnetic transition temperatures decrease linearly with increasing x value, and at x = 0.75 no magnetic ordering was found down to 1.8 K, indicating the magnetic interaction is not one-dimensional, but three-dimensional. On the other hand, the antiferromagnetic transition temperature for (Pr 1-x Y x) 3 RuO 7 decreases with increasing x value, but above x ≥ 0.50 it becomes constant (~12 K). This result indicates that Pr 3+ ions at the seven-coordinate site greatly contribute to the antiferromagnetic interactions observed in (Pr 1-x Y x) 3 RuO 7. Density functional calculations of Pr 3 RuO 7 demonstrate that the electronic structure gives insulating character and that oxygen 2p orbitals hybridize strongly with Ru 4d orbitals in the valence band (VB). Near the top of VB, the Pr 4f orbitals at the seven-coordinated site also show a weak hybridization with the O(1) 2p orbitals. The Ru-O(1)-Pr superexchange pathway take part in three-dimensional magnetic interaction and play an important role in an enhancement of long-range magnetic ordering.

Ternary oxides Ce 3 NbO 7 and Ce 3 TaO 7 were successfully synthesized by the solid state reaction under flowing hydrogen atmosphere. The structures were determined by the powder X-ray diffraction. Both the compounds were crystallized in the orthorhombic space groups Pnma (for Ce 3 NbO 7) and Cmcm (for Ce 3 TaO 7). Both the structures have similar features: two kinds of infinite chains formed by corner-sharing NbO 6 (TaO 6) octahedra and edge-sharing Ce(1)O 8 cubes, the slabs consisting of alternate chains, and 7-coordinated Ce(2) ions existing between the slabs. In the structure of Ce 3 NbO 7 , the NbO 6 octahedra running along the a-axis are titled towards the 0 0 1 direction, while in the Ce 3 TaO 7 structure, the TaO 6 octahedra running along the c-axis are titled towards the 0 1 0 direction. Magnetic susceptibility measurements for Ce 3 NbO 7 and Ce 3 TaO 7 show that both compounds are paramagnetic down to 1.8 K, and confirm that the Ce ion is in the trivalent state. From specific heat (C p) measurements, a rapid increase of C p /T has been observed below 3 K for both the compounds, indicating the onset of magnetic ordering between Ce 3+ ions at further lower temperatures.