Stabilization of a Tetrahedral (Mn(5+)O4) Chromophore in Ternary Barium Oxides as a Strategy toward Development of New Turquoise/Green-Colored Pigments.

An experimental investigation of the stabilization of the turquoise-colored chromophore Mn(5+)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 Mn(5+)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 Mn(5+)O4 substitution is possible. Ca3(VO4)2 and YVO4 do not stabilize Mn(5+)O4 at all. With Ba2MO4 (M = Ti, Si), we could prepare only partially substituted materials, Ba2M1-xMn(5+)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 Mn(5+) oxidation state accessible in oxoanion compounds containing PO4(3-), VO4(3-), etc. By way of proof-of-concept, we synthesized new turquoise-colored Mn(5+)O4 materials, Ba5(BO3)(MnO4)2Cl and Ba5(BO3)(PO4)(MnO4)Cl, based on the apatite-Ba5(PO4)3Cl-structure.