Color Tuning in Garnet Oxides: The Role of Tetrahedral Coordination Geometry for 3 d Metal Ions and Ligand-Metal Charge Transfer (Band-Gap Manipulation).

We explored garnet-structured oxide materials containing 3d transition-metal ions (e.g., Co2+ , Ni2+ , Cu2+ , and Fe3+ ) for the development of new inorganic colored materials. For this purpose, we synthesized new garnets, Ca3 Sb2 Ga2 ZnO12 (I) and Ca3 Sb2 Fe2 ZnO12 (II), that were isostructural with Ca3 Te2 Zn3 O12 . Substitution of Co2+ , Ni2+ , and Cu2+ at the tetrahedral Zn2+ sites in I and II gave rise to brilliantly colored materials (different shades of blue, green, turquoise, and red). The materials were characterized by optical absorption spectroscopy and CIE chromaticity diagrams. The Fe3+ -containing oxides showed band-gap narrowing (owing to strong sp-d exchange interactions between Zn2+ and the transition-metal ion), and this tuned the color of these materials uniquely. We also characterized the color and optical absorption properties of Ca3 Te2 Zn3-x Cox O12 (0<x≤2.0) and Cd3 Te2 Zn3-x Cox O12 (0<x≤1.0), which display brilliant blue and green-blue colors, respectively. The present work brings out the role of the distorted tetrahedral coordination geometry of transition-metal ions and ligand-metal charge transfer (which is manifested as narrowing of the band gap) in producing brilliantly colored garnet-based materials.