Temperature-dependent infrared spectrum of V_1-xMo_xO₂ single crystals across the metal-to-insulator phase transition

Vanadium dioxide (VO₂) is a promising material for photonic applications due to its reversible phase transition from an insulating monoclinic phase to a metallic rutile phase near room temperature, enabling devices such as optical switches and modulators. By doping VO₂ with chemical substituents, the metal-insulator transition temperature can be adjusted, expanding its applicability to lower or higher energy regimes. In this study, we investigate the optical properties of V_1-xMo_xO₂ single crystals (x = 0.17 and 0.20) across their metal-insulator transitions. MoO₂ is nonmagnetic, metallic, with one extra electron per metal site, and has a monoclinic crystal structure, isostructural with VO₂ at room temperature. V_1-xMo_xO₂ alloy will help resolve the structural and electronic connections among the different crystal phases. We measured a broadband reflection spectrum from visible to long-wave mid-infrared at various temperatures. Using Kramers-Kronig analysis, we derived an optical conductivity of V_1-xMo_xO₂. To evaluate the metal-to-insulator or metal-to-semiconductor transition, we applied an extended Drude model to fit the optical conductivity.