α-Ag 2-2x Zn x WO 4 (0 ≤ x ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties.

A theoretical study was elaborated to support the experimental results of the Zn-doped α-Ag ₂ WO ₄ . Theses α-Ag _2-2x Zn _x WO ₄ (0 ≤ x ≤ 0.25) solid solutions were obtained by coprecipitation method. X-ray diffraction data indicated that all α-Ag _2-2x Zn _x WO ₄ (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 Zn ²⁺ 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 _2-2x Zn _x WO ₄ (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 Zn ²⁺ 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 _2-2x Zn _x WO ₄ models. The electronic properties of α-Ag ₂ WO ₄ and α-Ag _2-2x Zn _x WO ₄ microcrystals were linked to distortion effects and oxygen vacancies (V _O ^x ) present in the clusters, respectively. Finally, photoluminescence properties of α-Ag ₂ WO ₄ and α-Ag _2-2x Zn _x WO ₄ microcrystals were explained by means of distortional effects and oxygen vacancies (V _O ^x ) in [AgO _y ] (y = 2, 4, 6, and 7) and [WO ₆ ] clusters, respectively, causing a red shift. Calculations revealed that the substitution for Ag ⁺ with Zn ²⁺ occurred randomly in the α-Ag ₂ WO ₄ lattice, and it was more favorable on the Ag4 site, where the local coordination of Ag ⁺ cations was four.