NbO 2 F : An oxyfluoride phase with wide band gap and electrochromic properties

It was demonstrated that the oxyfluoride ${\mathrm{NbO}}_2\mathrm{F}$ with an ${\mathrm{ReO}}_3$ -type structure is a promising electrochromic material with wide band gap. Measurement of diffuse reflectance spectra revealed the optical band gap of ${\mathrm{NbO}}_2\mathrm{F}$ (∼3.1 eV) to be wider than that of ${\mathrm{WO}}_3$ (∼2.6 eV). It was rendered electron conductive by heating at 770 K in the presence of Pt powder in ${\mathrm{H}}_2.$ Electrical conductivity for the blue-colored sintered sample of ${\mathrm{NbO}}_2\mathrm{F}$ obtained was ${\text{6×10}}^{\text{−3}}\hspace{0.17em}\mathrm{S} {\mathrm{cm}}^{\text{−1}}$ at 300 K, and a temperature dependence showed the semiconductive behaviors. A reversible electrochromism between pale-blue and deep-blue-color was found in ${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{Na}}_2{\mathrm{SO}}_4$ aqueous solution.