Synthesis of Phase-Pure Thermochromic VO₂ (M1)

A highly reproducible, simple, and inexpensive synthesis method for obtaining phase-pure thermochromic monoclinic VO₂ (M1) is presented. Vanadium(III) oxide and ammonium metavanadate were used as starting materials and no additional reducing agents are required. Heating a mixture of these two components under an argon atmosphere at 750 °C for 2-4 h provides the direct formation of VO₂ (M1) without detectable impurity phases. The formation reaction of VO₂ (M1) was studied using in situ powder X-ray diffraction (PXRD), where a pressed pellet of the precursor material was heated during the continuous collection of PXRD data on a two-dimensional detector. The formation takes place via at least two crystalline intermediate phases where the first forms at 170-185 °C (likely an ammonium and oxygen deficient (NH₄)_1-δVO_3-δ phase), and the second at 230 °C (likely a more disordered phase due to the increased background intensity). We assume that the solid-state reaction between the unknown but likely disordered vanadate phase and vanadium(III) oxide starts at 395 °C in concert with the appearance of several other unknown crystalline phases. At 610-750 °C, phase-pure rutile VO₂ (P4₂/mnm) is obtained, which upon cooling converts to monoclinic VO₂ (M1). The product composition, microstructure, and homogeneity are characterized by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The synthesized VO₂ (M1) has a sharp reversible insulator-to-metal transition at 71.3 °C during heating and 59.5 °C during cooling, as characterized using differential scanning calorimetry, and resistivity and magnetic property measurements.