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

Abstract A new polymorph of the triple perovskite Ba 3 CuOs 2 O 9 , which usually exists in the orthorhombic phase, was synthesized under high-pressure and high-temperature conditions at 6 GPa and 1100 °C. Under the synthetic condition, Ba 3 CuOs 2 O 9 crystallizes into a hexagonal structure (P 6 3 /mmc) with a = 5.75178(1) Å and c = 14.1832(1) Å, and undergoes a 1.36% increment in density, compared to that of the orthorhombic phase. Although Ba 3 CuOs 2 O 9 maintains its 6 H perovskite-type structure, the distribution of Cu and Os atoms are dramatically altered; (Cu) 4 a (Os,Os) 8 f transits to (Os) 2 a (Cu,Os) 4 f ordering over the corner- and face-sharing sites, respectively. The hexagonal Ba 3 CuOs 2 O 9 exhibits a ferrimagnetic transition at 290 K, which is in stark contrast to the antiferromagnetic transition at 47 K exhibited by the orthorhombic Ba 3 CuOs 2 O 9. 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°. 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. Graphical abstract fx1 Highlights • A new polymorph of Ba 3 CuOs 2 O 9 is synthesized under high pressure. • The new polymorph has a hexagonal structure and a 1.36% density increment. • The lattice maintains its 6 H perovskite-type structure. • It exhibits a ferrimagnetic transition near room temperature, i.e. at 290 K. • Coercive field is greater than 70 kOe at 5 K. [ABSTRACT FROM AUTHOR]