Enhanced Visible Photoluminescence in Niobium-Doped CaCu₃Ti₄O₁₂ Perovskite Synthesized by a Soft Chemical Route

CaCu₃Ti₄O₁₂ (CCTO) perovskite oxides have been extensively studied due to their unusual dielectric, electrical and optical properties. In recent years, increasing attention has been directed toward their photoluminescent behavior, particularly in the visible region, motivated by potential applications in light-emitting devices. This review summarizes and critically discusses the current state of knowledge on photoluminescence in CCTO-based materials, with emphasis on the role of structural order–disorder phenomena, defect chemistry and donor doping, especially with niobium (Nb⁵⁺). The relationships between synthesis methods, thermal treatment, local coordination environments and electronic structure are examined. Special attention is given to the formation of localized electronic states associated with mixed TiO₆/TiO₅ coordination and oxygen vacancies, which are key factors governing visible light emission. The review highlights how controlled structural disorder and donor doping can be used as effective strategies to enhance photoluminescence at relatively low processing temperatures, reinforcing the technological relevance of CCTO-based perovskites for visible light-emitting applications.