Electronic phase diagram of valence-controlled cyanide: Na0.84-δ Co [Fe (CN) 6] 0.71 3.8 H2 O (0≤δ≤0.61)

Electronic phase diagram has been derived for the Prussian-Blue-type cyano-bridged transition-metal compound, Na0.84-δ Co [Fe (CN) 6] 0.71 3.8 H2 O (0.0≤δ≤0.61), as a function of the hole concentration δ of the d -electron system. The mother compound (δ=0) takes the Co2+ (t 2g 5 eg2: S=3/2) and Fe2+ (t 2g 6: S=0) configuration and is paramagnetic down to zero temperature. At room temperature, the holes are selectively introduced on the Fe site. A slight hole doping (δ=0.13) causes the charge-transfer (CT) transition, that is, cooperative electron transfer from the Co2+ site to the Fe3+ site, with a decrease in temperature below TCT ≈250 K. With a further increase in δ, TCT slightly decreases from ≈230 K at δ=0.24 to ∼210 K at δ=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 (HT) phase is metastable even at low temperature. In this metastable phase, the Fe3+ (t 2g 5: S=1/2) species mediate the ferromagnetic exchange coupling between the adjacent Co2+ spins. The ferromagnetic transition appears at δ=0.39, and the transition temperature TC increase from 7 K at δ=0.39 to 13 K at δ=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.