Control of Charge-Transfer-Induced Spin Transition Temperature on Cobalt−Iron Prussian Blue Analogues

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Abstract
The electronic and spin states of a series of Co−Fe Prussian blue analogues containing Na+ ion in the lattice, NaxCoyFe(CN)6·zH2O, strongly depended on the atomic composition ratio of Co to Fe (Co/Fe) and temperature. Compounds of Co/Fe = 1.5 and 1.15 consisted mostly of the FeIII(t2g5eg0, LS, S = 1/2)−CN−CoII(t2g5eg2, HS, S = 3/2) site and the FeII(t2g6eg0, LS, S = 0)−CN−CoIII(t2g6eg0, LS, S = 0) site, respectively, over the entire temperature region from 5 to 350 K. Conversely, compounds of Co/Fe = 1.37, 1.32, and 1.26 showed a change in their electronic and spin states depending on the temperature. These compounds consisted mainly of the FeIII−CN−CoII site (HT phase) around room temperature but turned to the state consisting mainly of the FeII−CN−CoIII site (LT phase) at low temperatures. This charge-transfer-induced spin transition (CTIST) phenomenon occurred reversibly with a large thermal hysteresis of about 40 K. The CTIST temperature (T1/2 = (T1/2↓ + T1/2↑)/2) increased from 200 to 280 K with decreasing Co/Fe from 1.37 to 1.26. Furthermore, by light illumination at 5 K, the LT phase of compounds of Co/Fe = 1.37, 1.32, and 1.26 was converted to the HT phase, and the relaxation temperature from this photoproduced HT phase also strongly depended on the Co/Fe ratio; 145 K for Co/Fe = 1.37, 125 K for Co/Fe = 1.32, and 110 K for Co/Fe = 1.26. All these phenomena are explained by a simple model using potential energy curves of the LT and HT phases. The energy difference of two phases is determined by the ligand field strength around CoII ions, which can be controlled by Co/Fe.