Evolution of the structural, magnetic, and electronic properties of the triple perovskite Ba3CoIr2O9

We report a comprehensive investigation of the triple perovskite iridate Ba3CoIr2O9. Stabilizing in the hexagonal P6(3)/mmc symmetry at room temperature, this system transforms to a monoclinic C2/c symmetry at the magnetic phase transition. On further reduction in temperature, the system partially distorts to an even lower symmetry (P2/c), with both these structurally disparate phases coexisting down to the lowest measured temperatures. The magnetic structure as determined from neutron diffraction data indicates a weakly canted antiferromagnetic structure, which is also supported by first-principles calculations. Theory indicates that the Ir5+ carries a finite magnetic moment, which is also consistent with the neutron data. This suggests that the putative J = 0 state is avoided. Measurements of heat capacity, electrical resistance noise, and dielectric susceptibility all point toward the stabilization of a highly correlated ground state in the Ba3CoIr2O9 system.