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

Abstract We explored garnet‐structured oxide materials containing 3d transition‐metal ions (e.g., Co 2+ , Ni 2+ , Cu 2+ , and Fe 3+ ) for the development of new inorganic colored materials. For this purpose, we synthesized new garnets, Ca 3 Sb 2 Ga 2 ZnO 12 ( I ) and Ca 3 Sb 2 Fe 2 ZnO 12 ( II ), that were isostructural with Ca 3 Te 2 Zn 3 O 12 . Substitution of Co 2+ , Ni 2+ , and Cu 2+ at the tetrahedral Zn 2+ 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 Fe 3+ ‐containing oxides showed band‐gap narrowing (owing to strong sp–d exchange interactions between Zn 2+ and the transition‐metal ion), and this tuned the color of these materials uniquely. We also characterized the color and optical absorption properties of Ca 3 Te 2 Zn 3− x Co x O 12 (0< x ≤2.0) and Cd 3 Te 2 Zn 3− x Co x O 12 (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.