Phase evolution and lattice dynamics in Ag2WO4 from 15 K to 530 K: Insights from powder X-ray diffraction and Raman spectroscopy

Abstract

Silver tungstate (Ag₂WO₄) is a multifunctional and polymorphic material, recognized for its metastable phases and a wide range of applications due to its highly desirable properties. The synthesis of its phases, along with the resulting structure and morphology, is highly sensitive to the synthesis method, processing conditions, and the presence of surfactants, allowing the surface properties to be optimized. Ag₂WO₄ polymorphism involves three distinct crystalline phases: alpha (orthorhombic), beta (hexagonal), and gamma (cubic), each exhibiting different micrometric-scale morphologies. Additionally, the beta and gamma phases are metastable and transform into the alpha phase with increasing temperature. In this investigation, we synthesized γ-Ag₂WO₄ crystals using a precipitation method at low temperatures. The resulting precipitate was washed with acetone and dried at 50 °C. We characterized the physical properties using Powder X-Ray Diffraction and Raman Spectroscopy across varying temperature ranges. The synthesized samples exhibited a dominant cubic phase with a minor beta phase (∼5 %) at room temperature. Powder X-Ray Diffraction and Raman Spectroscopy revealed three Ag₂WO₄ phases (alpha, beta, gamma) between 15 K and 530 K, with phase transitions occurring at 360 K, 420 K, and 480 K. The gamma phase predominated up to 340 K, while the alpha phase became dominant above 400 K, as the beta and gamma phases gradually disappeared, especially above 470 K. The γ→α phase transition can be classified as reconstructive.