Uncovering the metastable gamma-Ag2WO4 phase: a joint experimental and theoretical study

The synthesis of metastable solid phases is a pivotal starting point for innovative materials research. Here, we report the synthesis by means of a precipitation method of the metastable cubic gamma-Ag2WO4 phase under ambient conditions. Different experimental techniques such as X-ray diffraction (XRD) with Rietveld refinement, field emission scanning electron microscopy (FE-SEM), micro-Raman/ultravioletvisible (UV-vis) diffuse reflectance, photoluminescence spectroscopies, and differential scanning calorimetry (DSC) were employed. To complement the experimental data, the geometry, morphology, vibrational and electronic structure of gamma-Ag2WO4 were characterized and evaluated using first-principles quantum mechanical calculations at the density functional theory (DFT) level. A theoretical model based on the Wulff construction was introduced to explain the possible crystal morphologies of the faceted crystals, which were obtained by tuning the surface chemistry of the crystals, and are related to their relative stability. Both the experimental and theoretical data revealed the presence of (100), (110), and (111) facets with low surface energies in the gamma-Ag2WO4 crystals. When the surface energy value of the (111) surface was lowered, the experimental morphologies of the as-synthesized samples were similar to the theoretically obtained shapes. From the analysis of the experimental and calculated Raman spectra, PL emissions, and the density of states, we can assume that gamma-Ag2WO4 is primarily formed by both distorted clusters: [WO4] tetrahedra and, distorted in a lesser extent, [AgO6] octahedra.