Electronic phase diagram has been derived for the Prussian-Blue-type cyano-bridged transition-metal compound, Na 0.84−␦ Co͓Fe͑CN͒ 6 ͔ 0.71 · 3.8H 2 O ͑0.0Յ ␦ Յ 0.61͒, as a function of the hole concentration ␦ of the d-electron system. The mother compound ͑␦ =0͒ takes the Co 2+ ͑t 2g 5 e g 2 : S =3/ 2͒ and Fe 2+ ͑t 2g 6 : S =0͒ configuration and is paramagnetic down to zero temperature. At room temperature, the holes are selectively introduced on the Fe site. A slight hole doping ͑␦ = 0.13͒ causes the charge-transfer ͑CT͒ transition, that is, cooperative electron transfer from the Co 2+ site to the Fe 3+ site, with a decrease in temperature below T CT Ϸ 250 K. With a further increase in ␦, T CT slightly decreases from Ϸ230 K at ␦ = 0.24 to ϳ210 K at ␦ = 0.61. Accordingly, the nature of the transition changes from the second-order type to the first-order type. In all the concentration ranges, the high-temperature ͑HT͒ phase is metastable even at low temperature. In this metastable phase, the Fe 3+ ͑t 2g 5 : S =1/ 2͒ species mediate the ferromagnetic exchange coupling between the adjacent Co 2+ spins. The ferromagnetic transition appears at ␦ = 0.39, and the transition temperature T C increase from 7 K at ␦ = 0.39 to 13 K at ␦ = 0.61. Based on these experimental data, we will discuss the significant roles of the coupling between the charge, spin, and lattice degrees of freedom in the transition-metal cyanides.

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