Editors’ Choice—Equilibrium Boundaries of VO₂ Phase Stability: Determination by Solid Electrolyte EMF Measurements and Effect on Optical Changes during Metal-Insulator Transition

Abstract

Vanadium dioxide is widely known for its metal-insulator transition (MIT), in which drastic changes in resistivity and IR-transparency occur. This makes VO2 thin films promising materials for high-frequency optoelectronic devices. To get the most MIT sharpness, thin films should not contain impurities of hyper-oxygen or hypo-oxygen phases arising during VO2 synthesis. To ascertain the conditions of single-phase VO2 existence, the equilibrium boundaries of VO2 with neighboring phases were determined using the electromotive force method (EMF) with a solid electrolyte ZrO2(Y2O3). Our data for the high-oxygen boundary of VO2 existence in equilibrium with the V6O13 phase agree with the only data known in the literature. We established that VO2 is, in equilibrium with the V9O17 phase at the low-oxygen boundary, which forms V8O15 under further reduction. The temperature of the peritectoid decomposition of V9O17 is established, and the corresponding corrections to the phase diagram of the vanadium-oxygen system are introduced. The Gibbs energies for V9O17, V8O15, and V6O13 formation reactions are calculated. It is also shown that the IR reflectance of VO2 films brought to equilibrium at the high-oxygen boundary is much greater than that of films equilibrated at the low-oxygen boundary.