Pressure induced transformation and subsequent amorphization of monoclinic Nb₂O₅ and its effect on optical properties

Pressure-induced phase transitions of monoclinic H-Nb₂O₅ have been studied by in situ synchrotron x-ray diffraction, pair distribution function (PDF) analysis, and Raman and optical transmission spectroscopy. The initial monoclinic phase is found to transform into an orthorhombic phase at ~9 GPa and then change to an amorphous form above 21.4 GPa. The PDF data reveal that the amorphization is associated with disruptions of the long-range order of the NbO₆ octahedra and the NbO₇ pentagonal bipyramids, whereas the local edge-shares of octahedra and the local linkages of pentagonal bipyramids are largely preserved in their nearest neighbors. Upon compression, the transmittance of the sample in a region from visible to near infrared (450-1000 nm) starts to increase above 8.0 GPa and displays a dramatic enhancement above 22.2 GPa, indicating that the amorphous form has a high transmittance. The pressure-induced amorphous form is found to be recoverable under pressure release, and maintain high optical transmittance property at ambient conditions. The recoverable pressure induced amorphous material promises for applications in multifunctional materials.