Workshop "NMR, µSR, Mössbauer spectroscopies in the study of Fe-based and other unconventional high-Tc superconductors"
Spin state alteration of the divalent iron in a distorted tetrahedral coordination – a crystal field guide for the iron based chalcogenides superconductors
Yurii Pashkevich, National Academy of Science of Ukraine
The spin state diagram of the divalent iron in [FeX4]6- coordination complex, which is the basic structural unit of iron chalcogenides FeX (X = S, Se, Te) and M2Fe4Se5 (M = K, Rb, Cs) has been investigated. Numerical calculations of electronic spectra and spin state diagrams for a tetrahedral coordinated divalent iron have been performed in the framework of the modified crystal field theory. The main regularities of the iron spin state formation under distortions of the tetrahedral complex have been studied. A complex topology of the iron spin state diagram relative to S = 0, 1, 2 spin states distribution under breathing-mode and Jahn-Teller distortions has been uncovered. Based on this diagram one can qualitatively explain the transport properties of the iron-superconductors containing the [FeX4]6- complexes. It shown that the phase transition like «superconductor-metal-superconductor», which occurs under pressure and which is seen in a family of iron-containing superconductors may be caused by the spin subsystem evolution like «low-spin state – intermediate-spin state – low-spin state». The origin of phase separation and dynamic spin state instabilities  in iron chalcogenides has been discussed. The DFT calculations provide similar conclusion.
 V. Gnezdilov et al., Phys. Rev. B 87, 144508 (2013)