## 3 Results and Discussion
### Structural and Microstructural Characterization
In addition, in Figure 2b a slight shift at higher frequencies can be observed due to the incorporation of a higher concentration of Sb, which could be related to a distortion of the system towards more tetragonal symmetries, decreasing the bond distance between the B\({}^{5+}\) cations and their coordinated oxygen in the octahedral position. Therefore, the binding force constant would increase and the band \(\upsilon_{1}\) would shift to higher wavenumbers. For illustrative purposes, in Figure 2c, we show the shift of the A\({}_{\rm 1g}\) Raman mode, which reflects the increase in T phase at the expense of O due to the progressive entry of Sb\({}^{5+}\) ions. These results are in agreement with those observed with XRD where the change in the crystalline structure is detected (Figure 1). More important, as previously established, the evolution of A\({}_{\rm 1g}\) Raman mode shift is closely related to the piezoelectric coefficients (that is, functional properties); consequently, the functional properties of the system would be expected to be affected [27]. A field emission scanning electron microscope (FE-SEM) was used to study the effect of Sb content on the KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\))system. For all compositions, grains show a cubic-like shape, which is characteristic of KNN-based ceramics (Figure 3a-c), and irregularly arranged large and small grains were observed. The sequence of grain size distributions (GSDs) is shown to the right of each SEM image, while the evolution of the average grain size (AGS) determined from GSDs can be observed in Figure 3c. Furthermore, it is shown that as Sb addition rises, the average grain size decreases from 1.9 \(\upmu\)m to 1.3 \(\upmu\)m for \(x=0.00\) to \(x=0.04\), respectively. In addition, for the lowest Sb concentration (\(x=0.02\)), a non-homogeneous microstructure is observed. In fact, due to the inefficient additive distribution in the whole sample, grains containing antimony in their compositions show a restriction in their normal grain growth, whereas grains without or with low antimony concentration present grain sizes similar to the pure sample. Finally, for the highest antimony concentration (\(x=0.04\)), the improvement in the additive distribution in the whole sample produces a more homogeneous microstructure with a reduction in the average grain size. It is known that control of the composition is essential to obtain the optimal piezoelectric properties in the KNN system [23]. We have used EDS analysis for the semi-quantitative determination of the composition in the resultant KNN\(-\)based ceramic. EDS analysis was carried out on all the specimens in order to identify the composition of the sample. From the analysis, besides the KNL\(-\)NTS\(-\)based grains, the formation of a glassy phase during the sintering process is observed. Figure 4 illustrates the EDS analysis of the main grains (2 and 4) and of the glassy phase (1 and 3), while a table is also attached showing the compositional analysis (Atom % of Na, K, Nb, Sb and Ta ions) revealing a composition very close to KNL\(-\)NTS [the composition calculated by EDS analysis at points 2 and 4 is approx. (K\({}_{0.37}\)Na\({}_{0.40}\)Li\({}_{0.04}\))(Nb\({}_{0.85}\)Ta\({}_{0.11}\)Sb\({}_{0.04}\))O\({}_{3}\)]. Nevertheless, in the glassy phases, a lower Na/K ratio and a higher Ta concentration than the expected main phase is observed [approx. composition at points 1 and 3 is close to (K\({}_{0.33}\)Na\({}_{0.25}\)Li\({}_{0.04}\))(Nb\({}_{0.78}\)Ta\({}_{0.10}\)Sb\({}_{0.04}\))O\({}_{3}\)]. This suggests that this vitreous phase presents a loss of alkaline elements (specially Na) by evaporation at the sintering temperature [27]. Finally, it can be seen that the oxygen content was determined by the stoichiometry relation, although sometimes the O content in the sample was difficult to quantify accurately.