Contrasting structural stability and phase transition of α- and β-Ag2WO4 under pressure

Abstract

Ag₂WO₄ has attracted wide interest due to its excellent visible light photocatalysis. Besides special property, Ag₂WO₄ exhibits rich polymorphism and crystallizes in three polymorphs. In the present work, orthorhombic α-Ag₂WO₄ and hexagonal β-Ag₂WO₄ were successfully synthesized. Structural stability and structural evolution were in situ investigated for two types Ag₂WO₄ up to 34.5 GPa using synchrotron X-ray diffraction, Raman spectroscopy, and first principle calculation. Both theoretical and experimental results showed that α-Ag₂WO₄ is thermodynamically stable, and no phase transition presents under pressure. In contrast, for β-Ag₂WO₄, one pressure-induced phase transition presents at 7.6 GPa and completely finishes around 12.8 GPa. This high pressure phase is monoclinic structure, being one new structure for Ag₂WO₄. This monoclinic phase keeps stable to 34.5 GPa, but it will convert back to the α phase rather than the initial β phase during decompression, meaning this phase transition is irreversible. Moreover, theoretical result predicts γ-Ag₂WO₄ will transform to β-Ag₂WO₄ around 5.0 GPa, resulting one phase transition sequence of γ→β→monoclinic phase→α for γ-Ag₂WO₄ during a decompression-decompression cycle. These phase transitions were first revealed for polymorphism Ag₂WO₄, which is helpful for its application in high pressure condition.