From crystal chemistry to optical functionality in rare-earth orthochromites: a unified physical framework

Abstract

Rare-earth orthochromites (RCrO₃) constitute a class of distorted perovskite oxides exhibiting rich structural, electronic, magnetic, and optical properties. Despite extensive experimental investigations, the literature remains fragmented into independent structural, spectroscopic, and application-oriented studies, lacking a coherent physical framework capable of rationalizing their functional behavior. In this review, we develop a unified conceptual framework linking crystal chemistry, electronic structure, disorder physics, and optical response across the RCrO₃ series. We demonstrate that key optical characteristics—such as the band gap, absorption edge shape, and optoelectronic functionality—are governed by a hierarchical sequence of control parameters: tolerance factor → octahedral tilting → bandwidth modulation → localized states → optical transport. By combining crystal-field theory, semiconductor band concepts, and disorder-induced absorption mechanisms, we establish physically consistent scaling relations that explain the systematic evolution of optical properties with rare-earth ionic size. Disorder-induced localized states are described within the framework of disordered semiconductor physics, combining the Urbach rule and Mott–Davis formalism to account for sub-gap absorption and band-edge broadening. Importantly, the proposed framework provides trend-level predictive capability within structurally distorted RCrO₃ compounds where electronic states are primarily governed by Cr–O hybridization and lattice geometry. Its applicability is therefore limited in systems exhibiting extreme disorder, strong doping, or nanoscale effects, where additional mechanisms may dominate. Within these bounds, the framework reconciles apparent discrepancies in reported band-gap values and offers physically grounded design guidelines for ultraviolet photodetectors, optical sensing, photocatalysis, and bio-photonic applications. The present work positions rare-earth orthochromites as a model platform for structure-controlled functional oxides, while clearly defining the conditions under which such predictive understanding remains valid.