## 3 Results and Discussion  
### XRD Analysis  
Figure 3 shows the crystal evolution of the NiSb\({}_{2}\)O\({}_{6}\) powders calcined at 700, 800, and 900 \({}^{\circ}\)C. In these diffraction patterns, it is observed that the material showed its characteristic peaks at the three calcination temperatures. However, portions of secondary phases to the compound's main phase were identified for each heat treatment. At 700 \({}^{\circ}\)C, although the peaks of the material's crystalline structure were obtained, secondary phases were identified for Sb\({}_{2}\)O\({}_{3}\) (PDF # 71-0383) and Sb\({}_{2}\)O\({}_{5}\) (PDF # 50-1376), located at the 2\(\theta\) points 28.14\({}^{\circ}\), 28.96\({}^{\circ}\), 30.11\({}^{\circ}\), and 33.78\({}^{\circ}\) corresponding to the fewer intensity peaks. At 800 \({}^{\circ}\)C, crystal planes of NiSb\({}_{2}\)O\({}_{6}\) with more intense peaks of greater purity and crystallinity were observed. These reflections were identified  
Figure 3: X-ray diffraction patterns of NiSb\({}_{2}\)O\({}_{6}\) calcined at 700, 800, and 900 \({}^{\circ}\)C in air. at 20 = 19.22\({}^{\circ}\), 21.43\({}^{\circ}\), 27.14\({}^{\circ}\), 33.50\({}^{\circ}\), 34.99\({}^{\circ}\), 38.80\({}^{\circ}\), 40.19\({}^{\circ}\), 44.73\({}^{\circ}\), 53.22\({}^{\circ}\), 56.03\({}^{\circ}\), 60.23\({}^{\circ}\), 62.77\({}^{\circ}\), 63.32\({}^{\circ}\), 67.20\({}^{\circ}\), 67.78\({}^{\circ}\), 73.89\({}^{\circ}\), 80.89\({}^{\circ}\), and 86.83\({}^{\circ}\) (PDF # 38-1083). In addition, a small phase associated with NiO (PDF # 44-1159) was located at 20 = 37.19\({}^{\circ}\). At 900 \({}^{\circ}\)C, planes attributed to NiO were again found, but with less intensity than at 800 \({}^{\circ}\)C. As expected, comparing the results of Figure 3 with PDF # 38-1083, it was found that NiSb\({}_{2}\)O\({}_{6}\) belongs to the trirutile-type family of materials [21], with a tetragonal structure (network parameters: a = 4.641 A, c = 9.223 A) and space group P4\({}_{2}\)/mnm [22]. Based on the diffractograms' peaks, the crystallite size was estimated with [PERSON]'s equation [23] as follows:  
\[\mathbf{t}=\frac{0.9\lambda}{\beta\mathbf{cos}\theta} \tag{1}\]  
where \(\lambda\) is the wavelength of the radiation (in this case, Cu = 1.5406 A), \(\beta\) is the width of the peak measured at half the maximum intensity, and \(\theta\) is the Bragg angle. For the calculation, the most intense peak corresponding to the plane (110) located at 20 = 27.14\({}^{\circ}\) was considered. From the diffractograms depicted in Figure 4a, at 700 \({}^{\circ}\)C, the crystal size was estimated at \(\sim\)36.43 \(\pm\) 1.65 nm; at 800 \({}^{\circ}\)C, it was \(\sim\)38.40 \(\pm\) 0.96 nm; and at 900 \({}^{\circ}\)C, it was \(\sim\)44.02 \(\pm\) 0.50 nm. Accordingly, it was verified that when increasing the temperature, the crystallite size increased. This is attributed to the fact that a particles' agglomeration occurred due to the increasing calcination temperature [24; 25] and to the material's residence time in the muffle at the given temperature. It is worth mentioning that the time for the thermal treatments was relatively short (5 h), thus achieving that meant the crystal size stayed in the order of nanometers [25]. Figure 4b,c show a projection of the octahedral gap formed by the Ni\({}^{2+}\) and Sb\({}^{5+}\) cations, which are coordinated with the oxygen anions O\({}^{2-}\). Additionally, Figure 4c depicts the atomic arrangement of NiSb\({}_{2}\)O\({}_{6}\)'s crystal structure seen from the plane of greatest intensity (110). Both structures were obtained with the Diamond software using PDF # 38-1083 and reference [26]. It is important to mention that the cations' arrangement forms a triplicate rutile-type structure in the c-axis and, according to the literature, this modification on the c-axis of NiSb\({}_{2}\)O\({}_{6}\) represents a trirutile-type structure [16; 17; 27].