Electronic phase diagram of valence-controlled cyanide: Na0.84−deltaCo[Fe(CN)6]0.71·3.8H2O (0<=delta<=0.61)

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Electronic phase diagram has been derived for the Prussian-Blue-type cyano-bridged transition-metal compound, Na0.84−deltaCo[Fe(CN)6]0.71·3.8H2O (0.0<=delta<=0.61), as a function of the hole concentration delta of the d-electron system. The mother compound (delta=0) takes the Co2+ (t2g5eg2:S=3/2) and Fe2+ (t2g6: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 (delta=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[approximate]250 K. With a further increase in delta, TCT slightly decreases from [approximate]230 K at delta=0.24 to ~210 K at delta=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+ (t2g5:S=1/2) species mediate the ferromagnetic exchange coupling between the adjacent Co2+ spins. The ferromagnetic transition appears at delta=0.39, and the transition temperature TC increase from 7 K at delta=0.39 to 13 K at delta=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.