Localized Spin Dimers and Structural Distortions in the Hexagonal Perovskite Ba(3)CaMo(2)O(9)

[Image: see text] 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 Ba(3)CaMo(2)O(9). Neutron diffraction experiments, combined with magnetic susceptibility measurements, show that the Mo(2)O(9) dimers retain localized character down to 5 K and adopt nonmagnetic spin-singlet ground states. This is in contrast to the recently reported Ba(3)SrMo(2)O(9) analogue, in which the Mo(2)O(9) dimers spontaneously separate into a mixture of localized and quasi-molecular ground states. Structural distortions in both Ba(3)CaMo(2)O(9) and Ba(3)SrMo(2)O(9) have been studied with the aid of distortion mode analyses to elucidate the coupling between the crystal lattice and electronic interactions in 6H Mo(5+) hexagonal perovskites.