Melilite-type blue chromophores based on Mn³⁺ in a trigonal-bipyramidal coordination induced by interstitial oxygen

A new, earth-abundant, environmentally benign, and stable inorganic blue chromophore series Sr₂(Mg_1-xMn_x)Ge ₂O_7+δ has been prepared and characterized by X-ray absorption spectroscopy, synchrotron X-ray diffraction, and ab initio total energy calculation. As the content of Mn in the compound was increased, the color changed from white (for x = 0.0) to sky blue, deep blue, and finally black body color (for x = 1.0). Sr₂MgGe₂O₇ has the melilite-type structure comprising Mg(2+)O₄ and GeO₄ tetrahedra. On the other hand, it was found that the Mn ion in Sr ₂(Mg_1-xMn_x)Ge₂O_7+δ is mainly trivalent and that the additional oxygen atom infiltrates into the lattice to compensate for the excess positive charge. The additional oxygen atom occupies an interstitial site, leading to the formation of Mn(3+)O₅ trigonal bipyramid and Ge(4+)O₅ square pyramid. As the driving force for the Mn(3+)O₅ formation, the tendency for improving the lattice coherency between BO₄-B′O₄ and AO₈ layers in the melilite A₂BB′₂O₇ is suggested.