## 3 Results and Discussion
### Optical Characterization  
The vibrational modes of Mn-doped Nb\({}_{2}\)O\({}_{5}\) samples subjected to a temperature of 700 \({}^{\circ}\)C were studied by IR spectroscopy within the wavenumber range of 3000-500 cm\({}^{-1}\). In the spectrum presented in Figure 5, neither band was associated to the presence of organic components nor were found pronounced shoulders related to the amorphous phase of niobium pentoxide. The shoulder around 766 cm\({}^{-1}\) for the sample at 1.0 wt.% Mn content is characteristic of the orthorhombic structure of niobium pentoxide. The absorption bands observed at 699 cm\({}^{-1}\) for 5.0 w% may correspond to the coupling mode of vibration of Mn-O stretching in octahedral sites of MnNb\({}_{2}\)O\({}_{6}\). For the sample at 10.0 wt.%, the band observed at 1658 cm\({}^{-1}\) is attributed to vibrations of the surface adsorbed O-H molecules [13]. In general, IR spectroscopy confirmed the MnNb\({}_{2}\)O\({}_{6}\) phase presence in 5.0 and 10.0 wt.% manganese content. \begin{table}
\begin{tabular}{l l l l} \hline
**Sample** & \(\mathbf{S}_{BET}(\mathbf{m}^{2}\mathbf{g}^{-1})\) & **Pore Volume (cm\({}^{3}\) g\({}^{-1}\))** & **Pore Size (Å)** \\ \hline Nb\({}_{2}\)O\({}_{5}\):Mn 1.0 wt.\% & 1.9124 & 0.0231 & 526.660 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 2.5 wt.\% & 3.0618 & 0.0206 & 533.622 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 5.0 wt.\% & 6.9633 & 0.0511 & 416.871 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 10.0 wt.\% & 19.7553 & 0.1335 & 262.577 \\ \hline \end{tabular}
\end{table} 
Table 2: Results of \(N_{2}\) physisorption analysis on samples subjected to 700 \({}^{\circ}\)C The specific surface area (\(S_{BET}\)) for Nb\({}_{2}\)O\({}_{5}\) corresponds to 1.19 (m\({}^{2}\) g\({}^{-1}\)). Figure 4: Adsorption isotherms of Mn-doped Nb\({}_{2}\)O\({}_{5}\) (**a**) 1.0 and (**b**) 2.5 wt.% samples, annealing at 700 \({}^{\circ}\)C.  
In Figure 6, the Raman spectra recorded in the 120-1300 cm\({}^{-1}\) range of Mn-doped Nb\({}_{2}\)O\({}_{5}\) submitted at 700 \({}^{\circ}\)C are displayed. The spectra in Figure 6a-d presents well-defined peaks where the position of these Raman peaks is narrow, which indicates crystallinity in the materials at this calcination temperature. The Nb\({}_{2}\)O\({}_{5}\) modification changes the relative intensities of almost all lines in the Raman spectrum, but the corresponding Raman shift remains unchanged, which suggests constancy in the bond lengths between the ions in the structure [14]. Comparing the most intense mode of all the spectra in Figure 6 (around 688 cm\({}^{-1}\)), it is evident that the relative peak-intensity decreases as the Mn concentration increases, which can be associated with disordered Nb\({}_{2}\)O\({}_{5}\) crystalline lattice. The most intense lines in the spectra of the Mn-doped Nb\({}_{2}\)O\({}_{5}\) samples may be assigned to the full-symmetry vibrations. Figure 6 shows three distinct groups of intense lines ranging between 230-400 cm\({}^{-1}\), between 520-690 cm\({}^{-1}\) and 850-885 cm\({}^{-1}\), which suggests weak interactions between these groups. The lines in the 150-400 cm\({}^{-1}\) range correspond to the cations vibrations in octahedra and tetrahedra  
Figure 5: Evolution with dopant concentration of the infrared spectra of Mn-doped Nb\({}_{2}\)O\({}_{5}\) submitted at 700 \({}^{\circ}\)C.  
Figure 6: Raman spectra of Mn-doped Nb\({}_{2}\)O\({}_{5}\) samples obtained at 700 \({}^{\circ}\)C.  
for the samples at 1.0, 2.5 and 5.0 wt.%. For these three concentrations, the broad band around 688 cm\({}^{-1}\) can be attributed to the stretching modes of the NbO\({}_{6}\) polyhedra typical of the orthorhombic Nb\({}_{2}\)O\({}_{5}\) crystalline structure. The broadening of this band suggests the presence of distorted niobia polyhedra [9]. Bands in 318 and 311 cm\({}^{-1}\) are allocated to the symmetric and antisymmetric stretching mode of the Nb-O-Nb linkage, in Figure 6a,b, respectively.