α-Ag_{2–2<i>x</i>}Zn_<i>x</i>WO₄ (0 ≤ <i>x</i> ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties

Abstract

A theoretical study was elaborated to support the experimental results of the Zn-doped α-Ag₂WO₄. Theses α-Ag_2-2xZn_xWO₄ (0 ≤ x ≤ 0.25) solid solutions were obtained by coprecipitation method. X-ray diffraction data indicated that all α-Ag_2-2xZn_xWO₄ (0 ≤ x ≤ 0.25) microcrystals presented an orthorhombic structure. The experimental values of the micro-Raman frequencies were in reasonable agreement with both previously reported and calculated results. Microscopy images showed that the replacement of Ag⁺ by Zn²⁺ promoted a reduction in the average crystal size and modifications in the morphology, from rod-like with hexagonal shape to roll-like with a curved surface. A theoretical methodology based on the surfaces calculations and Wulff constructions was applied to study the particle shapes transformations and the surface energy variations in α-Ag_2-2xZn_xWO₄ (0 ≤ x ≤ 0.25) system. The decrease in the band gap value (from 3.18 to 3.08 eV) and the red shift in photoluminescence with the Zn²⁺ addition were associated with intermediary energy levels between the valence and conduction bands. First-principles calculations with density functional theory associated with B3LYP hybrid functional were conducted. The calculated band structures revealed an indirect band gap for the α-Ag_2-2xZn_xWO₄ models. The electronic properties of α-Ag₂WO₄ and α-Ag_2-2xZn_xWO₄ microcrystals were linked to distortion effects and oxygen vacancies (V_O^x) present in the clusters, respectively. Finally, photoluminescence properties of α-Ag₂WO₄ and α-Ag_2-2xZn_xWO₄ microcrystals were explained by means of distortional effects and oxygen vacancies (V_O^x) in [AgO_y] (y = 2, 4, 6, and 7) and [WO₆] clusters, respectively, causing a red shift. Calculations revealed that the substitution for Ag⁺ with Zn²⁺ occurred randomly in the α-Ag₂WO₄ lattice, and it was more favorable on the Ag4 site, where the local coordination of Ag⁺ cations was four.