How does the spin-state of Co ions affect the insulator-metal transition in BiACoO (A = Ca, Sr, Ba)?

The misfit layered BiACoO (A = Ca, Sr, Ba) compounds experience an insulator to metal transition as A's ionic radius increases. This feature is contradictory to the conventional wisdom that larger lattice constant favors insulating rather than metallic state, and is also difficult to be reconciled using the Anderson weak localization theory. In this paper, we show from the first-principles calculation that an insulator-metal transition takes place from a nonmagnetic low-spin state of Co ions to a hexagonally arranged intermediate-spin low-spin mixed-state in CoO plane when ionic radius increases from Ca to Ba. The predicted low-spin state of BiCaCoO and BiSrCoO and intermediate-spin low-spin mixed-state of BiBaCoO are consistent not only with their measured transport properties, but also with the magnetic-field suppressed specific-heat peak observed at the transition temperature. In agreement with experiments, strong electronic correlation is required to stabilize the low-spin insulator and intermediate-spin low-spin metal.