Synthesis of Phase-Pure Thermochromic VO₂ (M1)

Abstract

A highly\nreproducible, simple, and inexpensive synthesis method\nfor obtaining phase-pure thermochromic monoclinic VO₂ (M1)\nis presented. Vanadium­(III) oxide and ammonium metavanadate were used\nas starting materials and no additional reducing agents are required.\nHeating a mixture of these two components under an argon atmosphere\nat 750 °C for 2–4 h provides the direct formation of VO₂ (M1) without detectable impurity phases. The formation reaction\nof VO₂ (M1) was studied using in situ powder X-ray diffraction\n(PXRD), where a pressed pellet of the precursor material was heated\nduring the continuous collection of PXRD data on a two-dimensional\ndetector. The formation takes place via at least two crystalline intermediate\nphases where the first forms at 170–185 °C (likely an\nammonium and oxygen deficient (NH₄)_1−δVO_3−δ phase), and the second at 230 °C\n(likely a more disordered phase due to the increased background intensity).\nWe assume that the solid-state reaction between the unknown but likely\ndisordered vanadate phase and vanadium­(III) oxide starts at 395 °C\nin concert with the appearance of several other unknown crystalline\nphases. At 610–750 °C, phase-pure rutile VO₂ (<i>P</i>4₂/<i>mnm</i>) is obtained,\nwhich upon cooling converts to monoclinic VO₂ (M1). The\nproduct composition, microstructure, and homogeneity are characterized\nby Raman spectroscopy, scanning electron microscopy, transmission\nelectron microscopy, and energy-dispersive X-ray spectroscopy. The\nsynthesized VO₂ (M1) has a sharp reversible insulator-to-metal\ntransition at 71.3 °C during heating and 59.5 °C during\ncooling, as characterized using differential scanning calorimetry,\nand resistivity and magnetic property measurements.