Localized Spin Dimers and Structural Distortions in the Hexagonal Perovskite Ba3CaMo2O9

Extended solid-state materials based on the hexagonal perovskite framework are typified by close competition between localized magnetic interactions and quasi-molecular electronic states. Here, we report the structural and magnetic properties of the new six-layer hexagonal perovskite Ba₃CaMo₂O₉. Neutron diffraction experiments, combined with magnetic susceptibility measurements, show that the Mo₂O₉ dimers retain localized character down to 5 K and adopt nonmagnetic spin-singlet ground states. This is in contrast to the recently reported Ba₃SrMo₂O₉ analogue, in which the Mo₂O₉ dimers spontaneously separate into a mixture of localized and quasi-molecular ground states. Structural distortions in both Ba₃CaMo₂O₉ and Ba₃SrMo₂O₉ have been studied with the aid of distortion mode analyses to elucidate the coupling between the crystal lattice and electronic interactions in 6H Mo⁵⁺ hexagonal perovskites.