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, Ca3Sb2Ga2ZnO12 (I) and Ca3Sb2Fe2ZnO12 (II), that were isostructural with Ca3Te2Zn3O12. 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 Ca3Te2Zn3-xCoxO12 (0< x <= 2.0) and Cd3Te2Zn3-xCoxO12 (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.