The perovskite-type transition metal (Tr) oxides are important from both viewpoints of fundamental study and practical applications. In the mostly studied d³ - d⁵ oxides, the octahedral TrO₆ distortion and tilting will significantly affect the features of d electrons. A distortion-free perovskite series covering 3d to 5d oxides have therefore always been desired. The recent synthesis of 3d⁴ cubic BaFeO₃ and 4d⁴ BaRuO₃ enabled a study on d electrons from 3d to 4d, whereas the route to 5d electrons is blocked up due to the lack of a 5d⁴ cubic perovskite oxide.

Recently, SHI Youguo, an associate professor in WANG Nanlin’s group in Institute Of Physics, Chinese Academy of Sciences, through the collaboration with NIMS of Japan, reported the discovery of a cubic 5d oxide BaOsO₃ (Fig. 1), an exact compound in continuation of the 3d⁴ to 5d⁴ cubic perovskite-type oxides free from octahedral TrO₆ distortion. Unlike that cubic BaFeO₃ and BaRuO₃ show ferromagnetic transitions, the cubic BaOsO₃ remains paramagnetic over the measured temperature range of 2 - 400 K.

However, compared with the Pauli paramagnetism in the first-principle calculations, the temperature independent term estimated from the measured magnetic susceptibility is much larger, indicating a significantly enhanced paramagnetism. Moreover, the density of states (DOS) at the Fermi level (E_F) estimated from specific heat is also much larger than calculations. It looks like that cubic BaOsO₃ is a ferromagnetic rather than a paramagnetic metal. A possible explanation is that, 5d⁴ BaOsO₃ may not satisfy the Stoner criterion due to the significantly extended 5d valence orbitals that can reduced the DOS at E_F as compared with those of 3d⁴ BaFeO₃ and 4d⁴ BaRuO₃, or caused by the remarkably reduced electronic correlation strength. 

The magnetism evolution from 3d to 5d oxides has a tight relation with the structure, for example, the tolerance factor (left figure of Fig.2). Estimated from the effective magnetic moments from magnetic susceptibilities, the spin-orbital coupling in 3d⁴ BaFeO₃ is negligible, whereas it plays a necessary role in 4d⁴ BaRuO₃. In 5d⁴ BaOsO₃, the spin-orbital coupling is enhanced, but it is hard to split the t_2g band to have a gap as that occurs in Sr₂IrO₄ which has an antiferromagnetic Mott insulating ground state. The evolution of ferromagnetism with respect to some key physical parameters is indicated by the right figure. It is presumably that the spin-orbital coupling strength in cubic BaOsO₃ is between that of 4d⁴ BaRuO₃ and Sr₂IrO₄ and the BaOsO₃ is on the phase boundary of a metal-insulator transition. However, to sufficiently explain the unusual properties of 5d⁴ BaOsO₃, besides spin-orbital coupling, other physical parameters such as the on-site Coulomb repulsion and t_2g band width should be taken into account.

This work was published in Journal of the American Chemical Society (135, 16507, 2013). The research was partially funded by the National Basic Research Program of China (973 program), and Chinese Academy of Sciences.

Appendix:
http://pubs.acs.org/doi/pdfplus/10.1021/ja4074408