**2.2. Reaction Mechanism to form Na2Nb2O6.H2O and NaNbO3**

The NbO7 decahedra and NbO6 octahedra resulted from the breaking up of the Nb2O5. The NbO7 and NbO6 are connected by corner forming Nb6O198– hexaniobate *Lindqvist* ion, which can provide different niobate compositions. These compositions are related to the different synthesis conditions used to obtain the materials.

In solution, the Nb6O198– ions incorporate Na+ ions forming Na2Nb2O6.nH2O microfibers, followed by structural rearrangement, releasing water molecules forming NaNbO3 structures [20,21,22].

The reaction mechanism between Nb2O5 and NaOH to form NaNbO3 can be described by the equations below:

$$+3\text{ Nb}\_2\text{O}\_5 + 8\text{ OH}^- \rightarrow \text{Nb}\_6\text{O}\_{19}{}^{8-} + 4\text{ H}\_2\text{O}\tag{1}$$

$$\text{Nb}\_6\text{O}\_{19}\text{^{8-}} + 34\text{OH}^- \rightarrow 6\text{ NbO}\_6\text{^{7-}} + 17\text{H}\_2\text{O} \tag{2}$$

$$2\text{ NbO}\_6^{7-} + 2\text{ Na}^+ + 7\text{ H}\_2\text{O} \rightarrow \text{Na}\_2\text{Nb}\_2\text{O}\_6\text{H}\_2\text{O} \cdot 12\text{ OH}^-\tag{3}$$

$$2\text{ Na}\_2\text{Nb}\_2\text{O}\_6\text{H}\_2\text{O} \to 2\text{ NaNbO}\_3 + \text{H}\_2\text{O}\tag{4}$$
