Evolution of the structural, magnetic and electronic properties of the triple perovskite Ba$_{3}$CoIr$_{2}$O$_{9}$

TL;DR

This study investigates the structural, magnetic, and electronic evolution of Ba$_{3}$CoIr$_{2}$O$_{9}$, revealing complex phase transitions, magnetic ordering, and a correlated ground state through experimental and theoretical methods.

Contribution

It provides a detailed analysis of phase transitions, magnetic structure, and electronic properties of Ba$_{3}$CoIr$_{2}$O$_{9}$, highlighting the avoidance of a $J=0$ state and the stabilization of a correlated ground state.

Findings

Structural phase transitions from hexagonal to monoclinic and lower symmetries.

Observation of weakly canted antiferromagnetic order.

Evidence of a highly correlated ground state.

Abstract

We report a comprehensive investigation of the triple perovskite iridate Ba$_{3}$CoIr$_{2}$O$_{9}$. 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 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.