Ab initiocalculations on the mechanism of charge transfer in Co-Fe Prussian-blue compounds

Abstract

The mechanism of the heat-induced charge transfer observed in Prussian-blue compounds ${\mathrm{K}}_{1\ensuremath{-}2x}{\mathrm{Co}}_{1+x}\mathrm{Fe}(\mathrm{CN}{)}_{6}$ is studied theoretically in connection with photoinduced magnetism. In the case of $x=0,$ our first-principles band calculation confirms that the band-gap excitation corresponds to charge transfer from $\mathrm{Fe}\ensuremath{-}d\ensuremath{\varepsilon}$ to $\mathrm{Co}\ensuremath{-}d\ensuremath{\gamma}$ orbitals. By increasing the lattice constant (specifically, by elongating the Co-N bond in the crystal), the band gap decreases systematically due to the variation of the crystal field at each Co site. In the nonstoichiometric case $(x\ensuremath{\ne}0),$ we demonstrate on the basis of ab initio cluster calculations that a small increase in the Co-N distance is sufficient to cause charge transfer between Fe and Co atoms located near Fe vacancies accompanied by water molecules. This mechanism by electron-lattice coupling combined with disorder explains most of experimental findings on heat-induced changes and is suggested to be responsible also for the photoinduced magnetic transition.