Universal metastability of the low-spin state in Co 2+ systems: Non-Mott type pressure-induced spin-state transition in CoCl 2

We have investigated the pressure-induced spin-state transition in Co2+ systems in terms of a competition between Hund's exchange energy (J) and crystal-field splitting (ΔCF). First, we show the universal metastability of the low-spin state in octahedrally coordinated Co2+ systems. Then we present the strategy to search for a Co2+ system, for which the mechanism of spin-state and metal-insulator transitions is governed not by Mott physics but by J versus ΔCF physics. Using CoCl2 as a prototypical Co2+ system, we have demonstrated the pressure-induced spin-state transition from high-spin to low-spin, which is accompanied with insulator-to-metal and antiferromagnetic to half-metallic ferromagnetic transitions. Combined with the metastable character of Co2+ and the high compressibility nature of CoCl2, a transition pressure as low as 27 GPa can be identified on the basis of J versus ΔCF physics.