Valence and structural transitions in the mixed Ru-Ir perovskites Ba2PrRu1−xIrxO6

The valence state of Pr in the B-site ordered double perovskites Ba₂PrRu_1−xIr_xO₆ is shown to be sensitive to both the precise Ru:Ir content and temperature. Pr L_III XANES measurements show that at room temperature the Pr is trivalent in the Ru-rich compounds with x<0.25. At higher Ir contents the Pr is tetravalent. High-resolution powder synchrotron X-ray and neutron diffraction methods have been used to study the composition and temperature dependence of the crystal structures of these oxides. The Ru and Ir are statistically distributed on one of the two available B-sites. The oxides undergo an apparently first-order monoclinic P2₁/n to tetragonal P4/mnc phase transition in response to the change on the Pr and Ru/Ir valence. High temperatures and Ru contents favor the lower symmetry monoclinic structure, that is arises from the presence of the larger Pr^III cations. The variations in the observed metal-oxygen bond distances are consistent with a simultaneous change in the valence of the Ru/Ir accompanying the Pr^III-Pr^IV valence transition.