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

The mechanism of the heat-induced charge transfer observed in Prussian-blue compounds K₁-₂ₗCo₁+ₗFe (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 Fe-d to Co-d 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 (x0), 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.