Optical properties and structural phase transitions of W-doped VO2(R) under pressure

We investigated the optical properties and structural phase transitions of W-doped VO2(R) nanoparticles under pressure based on in situ synchrotron X-ray diffraction (XRD) and infrared (IR) spectroscopy. The structural transition sequence follows VO2(R)–VO2(CaCl2-type)–VO2(Mx) and VO2(Mx)–VO2(Mx′) within metallic phases, in compression and decompression processes, respectively, demonstrating that the structural transition can be decoupled from the metal-insulator transition (MIT). VO2(R) and VO2(CaCl2-type) exhibit expected behavior of increased metallicity under pressure; surprisingly, VO2(Mx) shows gradually decreased metallicity with increasing pressure and VO2(Mx′) is still metallic under ambient conditions. We find that the reduced metallicity of VO2(Mx) is attributed to W-doping induced local structure distortion in the high-pressure region, while the metallic properties of VO2(Mx′) are associated with the enhancement of electron concentration due to the presence of W donors, which shifted the Fermi level toward the conduction band. The present results demonstrate that the structural transition is not the key factor in driving the metal-insulator transition, and provide an effective method for inducing MIT in VO2(Mx′).