Mössbauer spectra of mixed-valence dimeric iron clusters in charge-ordered crystals

The theory of charge ordering and phase transitions in molecular crystals consisting of dimeric mixed-valence iron clusters is developed. It is supposed that the cluster contains high-spin Fe²⁺ and Fe³⁺ ions. The model employed involves intracluster Heisenberg-type exchange interaction between iron ions, "extra" electron transfer (double exchange), as well as dipole-dipole intercluster interaction. It is shown that depending on the parameters of the abovementioned interactions three types of order parameter (the mean value of the crystal dipole moment) temperature dependence are possible. These types of dependences lead to one-, two- and three-phase transitions. The mechanism of temperature transitions of the type of "localization-delocalization" in the Mössbauer spectra of mixed valence crystals is discussed. It is shown that at low temperatures spectra with averaged parameters may exist.