Metastable low-spin character of Co$^{2+}$ and the control of spin state transition

We have studied different spin states of the octahedrally coordinated Co$^{2+}$ systems. For every tested systems, we found metastable character of low-spin phase and, interestingly, the energy differences between the high-spin and low-spin phases are similar regardless of the anion (X) type, Co$^{+2}$-X bond lengths and CoX$_{6}$ octahedron distortion. For CoCl$_{2}$ as a model system, we studied pressure-induced high-spin to low-spin state transition, which is governed by $J$/$\Delta_{CF}$ value ($J$: exchange parameter, $\Delta_{CF}$: crystal-field parameter). CoCl$_{2}$ shows sudden collapse of volume and spin moment at the point of spin state transition together with the insulator-to-metal transition. Unlike the other transition-metal oxides, which shows pressure-driven Mott-type transition, physics of CoCl$_{2}$ is determined mainly by $J$ and $\Delta_{CF}$, not by $U$ and $W$.