Exploring structural oddities in tin cluster compounds RuMSn₆O₈ (M=Mn, Fe, Co) with quantum mechanical methods

RuFeSn₆O₈ is among the very few known substances where Fe²⁺ is explicitly located in a tetrahedral surrounding. This causes the iron d_eg valence band to be filled with three electrons, which should result in strong lattice distortions. Surprisingly, all phases RuMSn₆O₈ (M = Mn, Fe, Co) are stable paramagnetic semiconductors. However, results from ⁵⁷Fe Mössbauer spectroscopy and scattering experiments do not allow a final answer about the crystal structure. By using DFT calculations within the GGA+U approach the electronic and magnetic properties have been described and a possible reason for the inconsistencies in the experimental structure data found in an underlying local monoclinic distortion.