## 3 Results and Discussion  
### Structural Analysis  
From the initial XRD analysis (not presented herein), it is possible to conclude that at 400 and 500 \({}^{\circ}\)C, solids do not exhibit any crystalline phase, except the Nb\({}_{2}\)O\({}_{5}\):Mn 10.0 wt.% (500 \({}^{\circ}\)C) sample, in which the phase was not identified. At 600 \({}^{\circ}\)C, solids obtained at 1.0 and 2.5 wt.% crystallize in a hexagonal structure. The XRD patterns peak analysis verifies that the samples obtained by the Pechini method annealed at 700 \({}^{\circ}\)C have an orthorhombic crystalline structure, which is particularly interesting due to its potential application in photocatalytic processes. For this reason, 700 \({}^{\circ}\)C temperature will be the temperature of interest for other analyses.  
In the present study, we have adopted the Rietveld refinement technique to study the crystalline phase formation as well as the lattice parameters (Table 1). Figure 1 illustrates the Rietveld refined XRD pattern of pure Nb\({}_{2}\)O\({}_{5}\) sample, annealed at 700 \({}^{\circ}\)C. All reflections are addressed to the orthorhombic structure. Figure 2 shows the Rietveld refined XRD patterns of Mn-doped Nb\({}_{2}\)O\({}_{5}\) at 1.0 wt.% (Figure 2a) and at 10.0 wt.% (Figure 2b) samples. Data analysis confirms the presence of two phases. The main phase corresponding to Nb\({}_{2}\)O\({}_{5}\) The Inorganic Crystal Structure Database (ICSD) number 1840 with 55 Pbam space group and orthorhombic structure and the secondary phase corresponding to MnNb\({}_{2}\)O\({}_{6}\) ISCD 31944 with 60 Pbcn space group and orthorhombic structure.  
There is no clear relation between structural parameters and the unit cell volume values. Based on the ionic sizes and assuming that the valence state of Mn is 2+, an increase in lattice parameters is expected as the Mn concentration is increased (the ionic radius of Mn\({}^{2+}\) [0.80 A] is larger than the ionic radius of Nb\({}^{5+}\) [0.70 A]). Instead of this, the structural parameter a increases as the Mn ion is added, but as long as Mn content increases, the parameter a decreases. On the other hand, the parameter b is lower for 1.0 wt.% Mn content, but if Mn content increases, the parameter b also increases, except at 5.0 wt.% Mn content. The parameter c had similar behavior. Hence, there are no substantial changes in the unit cell volume due to distortions of the crystal lattice caused by the Mn inclusion. Perhaps, the Mn ions could have entered the Nb\({}_{2}\)O\({}_{5}\) matrix and substituted some interstitial sites for 1.0 wt.% manganese content.  
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Sample** & **a (Å)** & **b (Å)** & **c (Å)** & **V (Å\({}^{3}\))** & \(\chi^{2}\)** & **R(F\({}^{2}\))** \\ \hline Nb\({}_{2}\)O\({}_{5}\) & 6.183(5) & 29.287(2) & 3.932(3) & 712.04(12) & 1.042 & 0.113 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 1.0 wt.\% & 6.189(2) & 29.227(11) & 3.926(2) & 710.22(81) & 1.950 & 0.113 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 2.5 wt.\% & 6.184(3) & 29.256(12) & 3.929(2) & 710.71(84) & 2.128 & 0.114 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 5.0 wt.\% & 6.182(5) & 29.245(24) & 3.932(3) & 710.90(17) & 1.075 & 0.138 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 10.0 wt.\% & 6.174(11) & 29.287(53) & 3.932(7) & 711.10(38) & 1.069 & 0.166 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Structural parameters obtained from the Rietveld refinement of the XRD patterns.  
Figure 1: Rietveld refinement of x-ray difraction pattern of Nb\({}_{2}\)O\({}_{5}\) annealed at 700 \({}^{\circ}\)C.  
Figure 2: Rietveld refined XRD patterns of Nb\({}_{2}\)O\({}_{5}\):Mn sample (**a**) 1.0 wt.%, (**b**) 10.0 wt.%, annealed at 700 \({}^{\circ}\)C.