Temperature-induced phase transitions in metastable β-Ag2WO4: a Raman scattering study

Abstract Temperature-dependent Raman study was performed in order to obtain information on structural changes induced by the temperature in the β-Ag2WO4 crystal. The stability of the hexagonal structure (P63/m) phase of the Ag2WO4 crystal was accessed, and our results indicated that this material undergoes a sequence of three phase transformations, which were observed at 413-433, 483–493 and at approximately 613 K. In these transitions, the changes that occurred in the Raman spectra were characterized by the appearance and disappearance of new Raman bands. The results lead us to assume the following sequence of phase transitions: beta to phase II, phase II to phase III, and phase III to alpha phase. After temperature decreases from the alpha phase at the highest temperature to room temperature, another phase transition occurred, but the final phase is not the β-Ag2WO4 phase. Such phase transition is irreversible and relates to tilting and/or rotations of the WO4 tetrahedra, which leads to a disorder at the WO4 sites. The Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) were also performed to evaluate the Raman spectra data of the β-Ag2WO4 to identify the correlations with the phase transformations that occurred during the increasing and decreasing of temperatures.