In situ Raman investigation of the phase transition of NaVO₂F₂ under variable temperature conditions

In this study, the phase transition of NaVO₂F₂ was measured at different temperatures via in situ Raman spectroscopy. The NaVO₂F₂ compounds were synthesized by a hydrothermal method and were identified to be monoclinic with the P2₁/c space group at room temperature by XRD. Accordingly, the variations of Raman shifts and intensities of the characteristic peaks for NaVO₂F₂ associated with temperature were obtained and investigated. It was confirmed that NaVO₂F₂ had three types of phase transitions, which occurred in the temperature region from 78 K to 573 K. Further, the results indicate that transition from a low-temperature phase (I) to another low-temperature phase (II), low-temperature phase (II) to P2₁/c phase and P2₁/c phase to P2₁/m phase occurred near the three temperature points of 93 K, 233 K, and 453 K, respectively, during the heating process. Therefore, a novel characterization method was provided for further research on the phase transition theory and performance of vanadate compounds.