Melilite-Type Blue Chromophore Based on Mn3+ in Trigonal-Bipyramidal Coordination Induced by Interstitial Oxygen.

Abstract: Recently, Mn3+ in the trigonal-bipyramidal coordination has been attracted attention as a new route for developing earth-abundant, environmentally benign, and stable inorganic blue chromophore. However, Mn3+-based blue chromophores have been found only in very limited structures so far. Here, we report new crystal structure showing deep blue body color, melilite-type Sr2(Mg,Mn)Ge2O7+δ, which is based on Sr2MnGe2O7 known as consisting of Mn(2+)O4 and GeO4 tetrahedrons for over 30 years. From the analysis with x-ray absorption spectroscopy, synchrotron x-ray diffraction, and ab initio total energy calculation, it was found that the Mn ion is mainly trivalent and the additional interstitial oxygen is included for the compensation of the excess positive charge. As the origin of the Mn(3+)O5 formation, tendency for improving the structural coherency between BO4-B‘O4 and AO8 layers in the melilite A2BB‘2O7 structure is suggested. This finding implies that the layered-melilite structure with the various isomorphism could be a useful frame for controlling the coordination or the valence states of tran-sition metal ions.