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., 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 Zn_3-x Co_x O₁₂ (0<x≤2.0) and Cd₃ Te₂ Zn_3-x Co_x 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.

Keywords: dyes/pigments; garnets; solid-state structures; structure elucidation; tetrahedral geometry.