Inorganic pigments with transition metal chromophores at trigonal bipyramidal coordination: Y(In,Mn)O3 blues and beyond

Abstract

The discovery of a brilliant-blue color through Mn3+ substitution in trigonal-bipyramidal (TBP) sites of hexagonal YInO3 has led to a search for other inorganic pigments with chromophore ions at TBP coordination. Introducing transition metal cations Fe3+, Cu2+/Ti4+ and Mn3+/Zn2+/Ti4+ into the TBP sites in YInO3 has produced various colors from orange, green to purple. These indium-containing samples are successfully prepared via several synthetic routes including solid state reactions, modified Pechini method and microwave radiation. All pigments show excellent near infrared reflectance comparable to that of commercial TiO2. Cheaper hexagonal YMO3 host materials with TBP coordination such as citrate-prepared hexagonal YAlO3 (M = Al) have failed to produce the anticipated vivid color due to the destruction of TBP sites caused by carbonate incorporation. We conclude that the resulting intense colors of pigments are due to the crystal field splitting associated with the TBP coordination and the short apical M–O bonds (M = Mn, Fe, Cu). Other colored compounds containing TBP coordination are briefly reviewed. Blue and/or bluish-purple colors are found when Mn3+, Co2+ or Cu2+ are substituted into TBP sites of metal oxides that are not hexagonal YMO3-based.