## 3 Results and Discussion
### Structural and Microstructural Characterization
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Sb Content (x)** & \(\rho\) **(g/cm\({}^{3}\))** & \(d_{33}\) **(pC/N)** & \(\epsilon^{*}\) & **tan\(\delta\)** & \(P_{r}\) (\(\mu\)C/cm\({}^{2}\))** & \(E_{c}\) **(kV/cm)** \\ \hline
**0.00** & 4.57 & 80 & 933.8 & 0.28 & 14.3 & 17.1 \\ \hline
**0.02** & 4.56 & 230 & 935.1 & 0.04 & 21.3 & 21.7 \\ \hline
**0.04** & 4.55 & 260 & 1211.2 & 0.03 & 23.3 & 22.6 \\ \hline \end{tabular}
\end{table} 
Table 1: Real permittivity (\(\varepsilon\)), tan(\(\delta\)), remnant polarization (\(P_{r}\)), and coercive field (\(E_{c}\)) of KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\)) (\(x\) = 0.00 to 0.04) sintered samples measured at RT. Dielectric values were determined at 10 kHz. Figure 5: (**a**) Dielectric permittivity and dielectric loss (tan \(\delta\)) vs. temperature, at 10 kHz, of KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\)) ceramics with different Sb contents sintered at 1125 \({}^{\circ}\)C. (**b**) Curie temperature value (Tc) of the KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\))ceramics decreases with Sb addition. From a functional perspective, the stabilization of an O-T coexistence phase boundary close to room temperature has been considered to improve piezoelectric properties. Under this scenario, additional information concerning the effects of the Sb\({}^{5+}\) substitution on the KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\))ceramic can be achieved from the ferroelectric characteristics. For that, the samples were examined in a high external electric field, and all consistently displayed ferroelectric behavior because of spontaneous polarization. Hysteresis loops of KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\))samples (at RT and \(E_{p}\) = 30 kV/cm) are shown in Figure 6. All cycles are modified by the Sb content and present ferroelectric characteristics. Figure 6b and Table 1 show the values of remnant polarization (\(P_{r}\)) and coercive field (\(E_{c}\)) extracted from the hysteresis loops to more clearly analyze the effect of adding Sb\({}^{+}\)[5]. It can be seen that \(P_{r}\) and \(E_{c}\) (remnant polarization and coercive field, respectively) increase with Sb content from \(E_{c}\) = 17.1 kV/cm and \(P_{r}\) = 14.3 uC/cm\({}^{2}\), for \(x\) = 0.00 to \(E_{c}\) = 21.7 kV/cm and \(P_{r}\) = 21.3 uC/cm\({}^{2}\) for \(x\) = 0.02, and to \(E_{c}\) = 22.6 kV/cm and \(P_{r}\) =23.3 uC/cm\({}^{2}\) for \(x\) = 0.04. The stabilization of the coexistence of O-T phases seems to be related to the evolution of \(P_{r}\) (Figure 6b) and the dielectric properties (Figure 5). In addition, the piezoelectric constant (\(d_{33}\)) is proportional to the product \(\epsilon_{r}P_{r}\), which makes the improvement of these properties desirable. The evolution of the \(d_{33}\) piezoelectric constant versus Sb\({}^{5+}\) content in the KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\))system is shown in Figure 6c. The piezoelectric constant increases with Sb\({}^{5+}\) doping due to the stabilization of the orthorhombic and tetragonal phases close to RT (creating a limit of multiphasic coexistence where the tetragonal phase is predominant). The same reason should be responsible for improving the ferroelectric properties at \(x\geq\) 0.02. In fact, it is known that these phases present different directions of polarization (six possible for the tetragonal phase and 12 for the orthorhombic phase). Consequently, when the coexistence of both phases is stabilized, 18 direction possibilities for polarization are obtained. The similar values obtained for samples with \(x\) = 0.02 and 0.04 could be attributed to the contribution of different factors such as grain size, final  
Figure 6: Ferroelectric loop of the KNL\(-\)(N\({}_{0.9-x}\)TS\({}_{x}\))ceramics (**a**). \(P_{r}\) and \(E_{c}\) (**b**), and \(d_{33}\) (**c**) evolution with antimony addition. microstructure, dielectric permittivity, and density values as was observed by [PERSON] et al. [29]. From this perspective, the main difference between both compositions is the grain size.