Ba3MIr2 O9 hexagonal perovskites in the light of spin-orbit coupling and local structural distortions

Spin-orbit coupling (SOC) is found to be crucial for understanding the magnetic and electronic properties of 5d transition metal oxides. In 5d systems, with Ir5+ ions, where ideally a nonmagnetic J=0 ground state is expected to be stabilized in the presence of strong SOC, often spontaneous moments are generated due to hopping induced superexchange. This effect is more pronounced when the Ir atoms are close by, as in systems with Ir2O9 dimers in 6HBa3MIr2O9 compounds where magnetism is an outcome of complex Ir-O-Ir exchange paths, and is strongly influenced by the presence of local distortions. We find that subtle variations in the local structure of Ba3MIr2O9 (M = Mg, Sr, and Ca) lead to markedly different magnetic properties. While SOC plays a pivotal role in explaining the insulating ground states of these systems, it is seen that Ba3MgIr2O9, having a P63/mmc symmetry, does not order down to low temperature despite having antiferromagnetic exchange interactions, while Ba3CaIr2O9 shows weak dimer-like features and stabilizes in C2/c′ magnetic configuration with no net moment, and Ba3SrIr2O9 possesses a ground state corresponding to the magnetic space group C2′/c′ and exhibits ferromagnet-like features.