High-pressure synthesis, crystal structure, and magnetic properties of hexagonal Ba3CuOs2O9

A new polymorph of the triple perovskite Ba3CuOs2O9, which usually exists in the orthorhombic phase, was synthesized under high-pressure and high-temperature conditions at 6 GPa and 1100 degrees C. Under the synthetic condition, Ba3CuOs2O9 crystallizes into a hexagonal structure (P6(3)/mmc) with a = 5.75178(1) angstrom and c = 14.1832(1) angstrom, and undergoes a 1.36% increment in density, compared to that of the orthorhombic phase. Although Ba3CuOs2O9 maintains its 6 H perovskite-type structure, the distribution of Cu and Os atoms are dramatically altered; (Cu)(4a)(Os,Os)(8f) transits to (Os)(2a)(Cu,Os)(4f) ordering over the corner- and face-sharing sites, respectively. The hexagonal Ba3CuOs2O9 exhibits a ferrimagnetic transition at 290 K, which is in stark contrast to the antiferromagnetic transition at 47 K exhibited by the orthorhombic Ba3CuOs2O9. The enhanced transition temperature is most likely due to the strongly antiferromagnetic Os5+-O-Os5+ bonds and the moderately antiferromagnetic Os5+-O-Cu2+ bonds, the angles of which are both approximately 180 degrees. The 290 K ferrimagnetic transition temperature is the highest reported for triple-perovskite osmium oxides. Besides, the coercive field is greater than 70 kOe at 5 K, which is remarkable among the coercive fields of magnetic oxides.