Electronic phase diagram of valence-controlled cyanide:Na0.84−δCo[Fe(CN)6]0.71⋅3.8H2O(0≤δ≤0.61)

Abstract

Electronic phase diagram has been derived for the Prussian-Blue-type cyano-bridged transition-metal compound, ${\text{Na}}_{0.84\ensuremath{-}\ensuremath{\delta}}\text{Co}{[\text{Fe}{(\text{CN})}_{6}]}_{0.71}\ensuremath{\cdot}3.8{\text{H}}_{2}\text{O}$ $(0.0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.61)$, as a function of the hole concentration $\ensuremath{\delta}$ of the $d$-electron system. The mother compound $(\ensuremath{\delta}=0)$ takes the ${\text{Co}}^{2+}$ $({t}_{2g}^{5}{e}_{g}^{2}:S=3/2)$ and ${\text{Fe}}^{2+}$ $({t}_{2g}^{6}:S=0)$ configuration and is paramagnetic down to zero temperature. At room temperature, the holes are selectively introduced on the Fe site. A slight hole doping $(\ensuremath{\delta}=0.13)$ causes the charge-transfer (CT) transition, that is, cooperative electron transfer from the ${\text{Co}}^{2+}$ site to the ${\text{Fe}}^{3+}$ site, with a decrease in temperature below ${T}_{\text{CT}}\ensuremath{\approx}250\text{ }\text{K}$. With a further increase in $\ensuremath{\delta}$, ${T}_{\text{CT}}$ slightly decreases from $\ensuremath{\approx}230\text{ }\text{K}$ at $\ensuremath{\delta}=0.24$ to $\ensuremath{\sim}210\text{ }\text{K}$ at $\ensuremath{\delta}=0.61$. 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 ${\text{Fe}}^{3+}$ $({t}_{2g}^{5}:S=1/2)$ species mediate the ferromagnetic exchange coupling between the adjacent ${\text{Co}}^{2+}$ spins. The ferromagnetic transition appears at $\ensuremath{\delta}=0.39$, and the transition temperature ${T}_{C}$ increase from 7 K at $\ensuremath{\delta}=0.39$ to 13 K at $\ensuremath{\delta}=0.61$. 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.