Evolution of the structural, magnetic and electronic properties of the triple perovskite Ba${}_3$CoIr${}_2$O${}_9$

Abstract

We report a comprehensive investigation of the triple perovskite iridate Ba${}_3$CoIr${}_2$O${}_9$. Stabilizing in the hexagonal $P{6}_3/mmc$ 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 Ir${}^{5+}$ 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 towards the stabilization of a highly correlated ground state in the Ba${}_3$CoIr${}_2$O${}_9$ system.