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A3(V1−xMnxO4)2 (A = Ba and Sr), Ba2M1−xMnxO4+x/2 (M = Ti and Si), Ba5(BO3)(MnO4)2Cl, and Ba5(BO3)(PO4)(MnO4)Cl compounds, featuring a tetrahedral Mn5+O4 chromophore, exhibit a turquoise/green color. The investigation indicates that the tetrahedral Mn5+O4 is stabilized in hosts containing barium and to a lesser extent strontium and also identifies the role of chemical composition and electronic structure factors that likely promote the stabilization of tetrahedral Mn5+O4.
An experimental investigation of the stabilization of the turquoise-colored chromophore Mn5+O4 in various oxide hosts, viz., A3(VO4)2 (A = Ba, Sr, Ca), YVO4, and Ba2MO4 (M = Ti, Si), has been carried out. The results reveal that substitution of Mn5+O4 occurs in Ba3(VO4)2 forming the entire solid solution series Ba3(V1–xMnxO4)2 (0 < x ≤ 1.0), while with the corresponding strontium derivative, only up to about 10% of Mn5+O4 substitution is possible. Ca3(VO4)2 and YVO4 do not stabilize Mn5+O4 at all. With Ba2MO4 (M = Ti, Si), we could prepare only partially substituted materials, Ba2M1–xMn5+xO4+x/2 for x up to 0.15, that are turquoise-colored. We rationalize the results that a large stabilization of the O 2p-valence band states occurs in the presence of the electropositive barium that renders the Mn5+ oxidation state accessible in oxoanion compounds containing PO43–, VO43–, etc. By way of proof-of-concept, we synthesized new turquoise-colored Mn5+O4 materials, Ba5(BO3)(MnO4)2Cl and Ba5(BO3)(PO4)(MnO4)Cl, based on the apatite—Ba5(PO4)3Cl—structure.
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