## 3 Results and Discussion  
### Structural and Morphological Analysis  
The structural analysis of the samples was carried out using XRD and Raman spectroscopy, as shown in Figure 1. The XRD patterns were indexed to the crystalline phases of Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) and FeWO\({}_{4}\). Both samples correspond to monoclinic structures, with no secondary phases detected, according to PDF cards 100606 (Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\)) and 26843 (FeWO\({}_{4}\)). The diffraction peaks for both materials were sharp and well-defined, indicating a high degree of crystallinity in the synthesized samples. Figure 1: (**A**) XRD patterns and Raman spectra of the as-synthesized catalysts (**B**) Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) and (**C**) FeWO\({}_{4}\). The crystal structure of Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) corresponds to a monoclinic system with a \(P2_{1}/a\) space group, featuring a unit cell composed of alternating [FeO\({}_{6}\)] octahedra (green) and [MoO\({}_{4}\)] tetrahedra (black). The crystal structure of FeWO\({}_{4}\) adopts a monoclinic system with a \(P2/c\) space group, with its unit cell containing interconnected [FeO\({}_{6}\)] (green) and [WO\({}_{6}\)] (pink) octahedra, forming a robust three-dimensional framework. Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) crystallizes in the monoclinic \(P2_{1}/a\) space group and is composed of [FeO\({}_{6}\)] octahedra (green clusters in Figure 1A) and [MoO\({}_{4}\)] tetrahedra (black clusters in Figure 1A). In contrast, FeWO\({}_{4}\) crystallizes in the \(P2/c\) space group and contains [FeO\({}_{6}\)] and [WO\({}_{6}\)] (pink-colored clusters in Figure 1A) clusters. The diffraction peaks corresponding to the (011) and (110) planes are commonly used to assess preferential crystal growth, as the enhanced intensity of a specific plane often indicates oriented crystallization and higher crystallinity. In the case of FeWO\({}_{4}\), the XRD pattern shows that the intensities of the (011) and (110) reflections are comparable, suggesting the absence of preferential growth along a specific crystallographic direction [21]. These structural differences may influence the photocatalytic behavior of the materials, as the arrangement and type of polyhedral units affect charge separation and mobility. As shown in Figure 1B, the Raman spectrum of Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) crystals revealed four distinct spectral regions. These correspond to the symmetric stretching modes of the distorted Mo = O bonds in the [MoO\({}_{4}\)] clusters (1100-900 cm\({}^{-1}\)), the antisymmetric stretching modes of the [MoO\({}_{4}\)] clusters (900-700 cm\({}^{-1}\)), the bending modes of the [MoO\({}_{4}\)] clusters (500-300 cm\({}^{-1}\)), and the lattice modes (below 300 cm\({}^{-1}\)) [14; 22]. In the Raman spectrum of FeWO\({}_{4}\) crystals (Figure 1C), five characteristic spectral regions were identified: the symmetric A\({}_{\rm g}\) modes of terminal WO\({}_{2}\) groups (1100-900 cm\({}^{-1}\)), the antisymmetric stretching modes of [WO\({}_{6}\)] clusters (900-750 cm\({}^{-1}\)), the antisymmetric bridging vibrations related to tungstate chains (750-650 cm\({}^{-1}\)), the bending modes of [WO\({}_{6}\)] clusters (500-200 cm\({}^{-1}\)), and the lattice modes (below 200 cm\({}^{-1}\)) [23; 24]. Notably, the vibrational modes associated with [WO\({}_{6}\)] units exhibit reduced intensity, which is consistent with the lower degree of crystallinity observed in the FeWO\({}_{4}\) sample, as also indicated by the XRD results. The optical properties of the synthesized materials were investigated using UV-vis diffuse reflectance spectroscopy. The E\({}_{\rm gap}\) values were estimated using the Kubelka-Munk function, followed by extrapolation of the linear portion of the Tauc plot to the energy axis, as shown in Figure 2A,B. The calculated E\({}_{\rm gap}\) values were approximately 2.14 eV for Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) and 2.00 eV for FeWO\({}_{4}\), in good agreement with those reported in the literature [25; 26; 27; 28].