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Silver tungstate[editar | editar código-fonte]
The structural refinement using the Rietveld method 27 was performed to confirm the orthorhombic structure for α-Ag2WO4 crystals.28 According to van den Berg and Juffermans,29 Ag2WO4 microcrystals can exhibit polymorphism; this oxide can have three structure types: α-orthorhombic with a space group of Pn2n, β-hexagonal with a space group of P63/m, and γ-cubic with a space group of Fd3m, depending on the pH range (acid or alkaline).
Figure 1 illustrates a schematic representation for orthorhombic α-Ag2WO4 unit cells modeled from Rietveld refinement data.
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Legenda1
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Legenda2
This unit cell was modeled through the Visualization for Electronic and Structural Analysis (VESTA) program, version 3.0.9, for Windows39 using lattice parameters and atomic positions obtained from the Rietveld refinement data shown in Table 1 for α-Ag2WO4 microcrystals obtained by the CH method. α-Ag2WO4 crystals belong to an orthorhombic structure with a space group of Pn2n, a point-group symmetry of C2v 10, and two molecular formulas per unit cell (Z = 2).25 Figure 6 shows that bonds between O−Ag−O and O−W−O atoms were projected out of the unit cell. In these unit cells, all of the W (W1, W2, and W3) atoms are coordinated only to six O atoms, which form distorted [WO6] clusters with an octahedral configuration, a symmetry group Oh, and octahedron polyhedra (6 vertices, 8 faces, and 12 edges).40 The [WO6] clusters are distorted in the α-Ag2WO4 crystal lattice. These differences in O−W−O bond angles can lead to different levels of order−disorder and/or distortions in the α-Ag2WO4 crystal lattice. In addition, in these unit cells, Ag atoms can exhibit four types of coordination. The Ag1 and Ag2 atoms are coordinated by seven O atoms, forming the deltahedral configuration. Deltahedral [AgO7] clusters are irregular pentagonal dipyramid polyhedra with symmetry group D5h, which forms 7 vertices, 10 faces, and 15 edges.41 Ag3 atoms are bonded to six O atoms, which form distorted octahedral [AgO6] clusters and polyhedra of the type distorted octahedra with symmetry group Oh with 6 vertices, 8 faces, and 12 edges.40 The Ag4 and Ag5 atoms are coordinated by four O atoms, forming tetrahedral coordination. These tetrahedral [AgO4] clusters are polyhedra of distorted