Magnetic Structure and Origin of Insulating Behavior in the Ba 2 CuOsO 6 System, and the Role of A-Site Ionic Size in Its Bandgap Opening: Density Functional Theory Approaches.

The magnetic structure and the origin of band gap opening for Ba 2 CuOsO 6 were investigated by exploring the spin exchange interactions and employing the spin-orbit coupling effect. It revealed that the double-perovskite Ba 2 CuOsO 6 , composed of the 3d (Cu 2+ ) and 5d (Os 6+ ) transition metal magnetic ions is magnetic insulator. The magnetic susceptibilities of Ba 2 CuOsO 6 obey the Curie-Weiss law, with an estimated Weiss temperature of -13.3 K, indicating AFM ordering. From the density functional theory approach, it is demonstrated that the spin exchange interaction between Cu ions plays a major role in exhibiting an antiferromagnetic behavior in the Ba 2 CuOsO 6 system. An important factor to understand regarding the insulating behavior on Ba 2 CuOsO 6 is the structural distortion shape of OsO 6 octahedron, which should be closely connected with the ionic size of the A-site ion. Since the d-block of Os 6+ (d 2 ) ions of Ba 2 CuOsO 6 is split into four states (xy < xz, yz < x 2 -y 2 < z 2 ), the crucial key is separation of doubly degenerated xz and yz levels to describe the magnetic insulating states of Ba 2 CuOsO 6 . By orbital symmetry breaking, caused by the spin-orbit coupling, the t 2g level of Os 6+ (d 2 ) ions is separated into three sublevels. Two electrons of Os 6+ (d 2 ) ions occupy two levels of the three spin-orbit-coupled levels. Since Ba 2 CuOsO 6 is a strongly correlated system, and the Os atom belongs to the heavy element group, one speculates that it is necessary to take into account both electron correlation and the spin-orbit coupling effect in describing the magnetic insulating states of Ba 2 CuOsO 6 .