Synthesis and characterization of metastable β-Ag₂WO₄: an experimental and theoretical approach

Abstract

Metastable silver tungstate (β-Ag2WO4) has attracted much attention lately because of its many potential applications. However, the synthesis of metastable phases of inorganic compounds is challenging because of the ease of transformation to the stable phase. We have overcome this challenge and have successfully synthesized β-Ag2WO4 microcrystals using a dropwise precipitation (DP) method in aqueous media at low temperature. The microcrystals were characterized by X-ray diffraction (XRD), including powder X-ray diffraction structural determination, field-emission scanning electron microscopy (FE-SEM), and micro-Raman/ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy. To complement the experimental data, we present first-principles quantum-mechanical density functional theory (DFT) calculations. Using XRD data, Raman/UV-vis data, and the determined optical band gap, together with geometric optimization calculations, we confirmed the structure of this compound. β-Ag2WO4 has a hexagonal structure with a P63/m space group. The building blocks of the lattice comprise two types of W-O clusters, [WO4] and [WO5], coordinated to four and five O atoms, respectively, and two types of Ag-O clusters, [AgO6], and [AgO5], linked to six and five O atoms, respectively. This type of fundamental study, combining multiple experimental methods and first-principles calculations, helps to obtain a basic understanding of the local structure and bonding in the material.

Uncovering the metastable γ-Ag2WO4phase: a joint experimental and theoretical study

“…This emission peak in the blue region is typically related to distorted [WO 6 ] octahedral in the aand b-Ag 2 WO 4 crystals. 9,15,35,45 However, in our samples, it can be related to distorted [WO 4 ] tetrahedral. For the a-Ag 2 WO 4 phase, another diffused emission in the red region, from 620 to 640 nm, can be attributed to the [AgO y ] clusters.…”

“…For example, a metastable b-Ag 2 WO 4 phase, which coexisted with a weak amount of a-phase, was obtained using a co-precipitation method at pH 4; 31 in addition, the synthesis of pure b-Ag 2 WO 4 at low temperatures has also been reported. 30 Recently, our research group reported the synthesis of b-Ag 2 WO 4 microcrystals by simple 34 and drop-wise precipitation 35 methods. Moreover, Gupta et al presented a new strategy for synthesizing hexagonal metastable b-Ag 2 WO 4 at room temperature through the use of aliovalent Eu 3+ dopants (to induced the orthorhombic (a-Ag 2 WO 4 ) to hexagonal (b-Ag 2 WO 4 ) phase transition) along with the coprecipitation method.…”

Dielectric Behavior of α-Ag2WO4 and its Huge Dielectric Loss Tangent

“…Orthorhombic is the most stable, and its chemical stability is retained up to ~347 ºC, while β-and γ-Ag 2 WO 4 are metastable, being easily transformed to α-Ag 2 WO 4 under heating at 187°C and 257 ºC, respectively 30 . Regardless of the crystallographic phase, the lattice crystal of Ag 2 WO 4 comprises a system of building-block units formed by a series of allotropic forms of [WO 6 ] and [AgO y ] clusters 15,[22][23][24]29 Since the physical and chemical properties are heavily dependent on structural features, those properties of each Ag 2 WO 4 structure will depend on the order-disorder relationship between these clusters, since a symmetry break induced by strain, stress, and distortions within the Ag 2 WO 4 crystal lattice may create new and distinct structures, and thereby give rise to new and different properties 15,23,33 . According to the published literature, these structural variations are a consequence of the synthesis method and experimental conditions (temperature, processing time, solvent, heating source, pH, template, electron beam exposure, etc.)…”

“…According to the published literature, these structural variations are a consequence of the synthesis method and experimental conditions (temperature, processing time, solvent, heating source, pH, template, electron beam exposure, etc.) 10,11,16,29,33 . The preparation of α-Ag 2 WO 4 has been successfully carried out by several methods which include solid state reaction [34][35][36] , co-precipitation 22,37-39 , laser ablation in liquid (LAL) 18 , hydrothermal 19,[21][22][23][24][25][26][27]29,[40][41][42] , sonochemical 22,28 and microemulsion 22 methods.…”

Laser and electron beam-induced formation of Ag/Cr structures on Ag2CrO4

“…Our research group investigates Ag-containing semiconductors that have attracted significant attention owing to their rich history in technical applications, e.g., in catalysis, 44 photocatalysis, 45 for gas sensors, 46 and antimicrobial agents. 45 In this context, a variety of Ag-containing oxides such as Ag 2 WO 4 in their different polymorph forms a-Ag 2 WO 4 , 20 b-Ag 2 WO 4 , 21,47 and g-Ag 2 WO 4 48 have been studied. Various Ag-based compounds such as b-Ag 2 MoO 4 , 49 Ag 3 PO 4 , 50,51 b-AgVO 3 , 52 and Ag 2 CrO 4 53 have shown the ability to form Ag NPs under electron beam irradiation.…”