Figure 1 shows the densities of KNNLST ceramics with 4 wt% LiF content as a function of sintering temperature. With increasing of sintering temperature, the density increases and reaches the maximum value (4.26 g/cm³) at 930˚C sintering temperature, then decreases. Meanwhile, the sintering temperature of undoped KNNLST ceramics was as high as 1100˚C, the density of 4.24 g/cm³ (Table 1). Thus, the addition of LiF improved the sinterability, reduced the sintering temperature of 170˚C compared with pure samples and increasing density of the ceramic samples. This is probable due to the formation of liquid phase of LiF. As is well known, LiF has a low melting point of 845˚C, therefore it may form liquid phase during sintering process, which could probably promote the densification behavior of KNN-based piezoceramics at reduced temperatures [17] .

Figure 2 shows the planar electromechanical coupling factor k_p, k_t and dielectric constant ε of KNNLST + 4 w% LiF ceramics as a function of sintering temperature. The k_p, k_t and ε show similar tendency, initially they increase with the sintering temperature, reach the maximum values at 930˚C sintering temperature, then decrease when sintering temperature exceeds 930˚C. As indicated in Table 1, for the KNNLST + 4 w% LiF ceramics sintered at 930˚C showed the good electrical properties, the electromechanical coupling factor k_p, k_t are 0.27, 0.40, respectively, which were relatively higher than that pure KNNLTS ceramics. These results may be related to the suppressed volatilization of alkali elements at the low temperature sintering and the increase of density as discussed above.

From the results above, the sintering temperature for KNNLST doped LiF ceramics was selected as 930˚C.

Figure 3(a) shows the XRD patterns at room temperature with the 2q range from 20˚ to 70˚ of KNNLST + x wt% LiF ceramic samples sintered at 930˚C. It can be seen that all samples exhibit dominant phase structure of perovskite ABO₃ type, beside a small amount of second phase LiSbO₃ (PDF card # 024-0598) and K₆Nb_10.8O₃₀ (PDF card # 070-505) have been found in pure KNNLST and KNNLST doped 2 w% LiF samples as shown in Figure 3(a). The appearance of second phase may be due to the composition inhomogeneity of ceramics [17] . Crystal structure of the samples is modified significantly by LiF additions as shown in Figure 3(b).

Figure 3(b) shows the enlarged XRD patterns of the ceramics in the ranges of 2θ from 45˚ to 47˚. It is well known that KNN-based ceramics usually show orthorhombic symmetry at room temperature [14] . According to research results of the works [17] [22] [23] , when the ceramics is of orthorhombic phase, the relative intensity of (002)/(020) peaks is approximately 2 and the (002) peak appears at a smaller Bragg angle, but the I(002)/I(020) decreases to 0.5 for a tetragonal phase. Therefore, from X-ray diffraction diagram in Figure 3(b) shows that likely the dominate phase in the pure KNNLST ceramics is orthorhombic, whereas the addition of LiF changes it to tetragonal symmetry. With x equals 4, the ceramics shows the coexistence of two phases, and then with x = 6, the tetragonal phase became dominant. This result suggests that may be Li⁺ ions are substituted for A-site of perovskite structure ABO₃ which leads to the change of crystal structure from orthorhombic to tetragonal. So it could be said that the composition with x = 4 is near to the morphotropic phase boundary of KNNLST doped LiF ceramics [16] .

Figure 4 shows the SEM images of the KNNLST + x wt% LiF ceramics sintered at 930˚C: (a) x = 0, (b) x = 2, (c) x = 4 and (d) x = 6. It can be seen that the microstructure of the pure KNNLST ceramics with square or

rectangular shaped grains discrete distributions, porous, not sintered materials as shown in Figure 4(a). However, the microstructure of samples becomes denser and average grain size increases from 0.4 to 2 mm (shown in Table 2) as the LiF content is increased (Figures 4(b)-(d)). A homogeneous microstructure developed for the sample with 4 wt% LiF added (Figure 2(c)). Figure 4(d) also shows that further increasing LiF content to 6 wt% gives rise to some large abnormal grains, porous. Such with the 4 wt% LiF added sample, the highly dense and homogeneous microstructure was obtained, which may expect improved properties of ceramics. The grain growth with LiF addition can be explained by liquid phase sintering.

Figure 5 shows temperature dependence of dielectric constant ε and dielectric loss tand measured at 1 kHz of KNNLST + x wt% LiF ceramics sintered at 930˚C. As seen in Figure 5, KNNLST doped LiF ceramics undergo two phase transition, that are the orthorhombic-tetragonal phase transition (T_O-T) and tetragonal-cubic phase transition (T_C) during the measured temperature range [16] [17] [22] [23] . Figure 6 shows the orthorhombic-tetragonal phase transition temperature (T_O-T) and Curie temperature (T_C) of KNNLST ceramics sintered at 930˚C change as a function of LiF content. The pure KNNLST ceramics possess T_O-T and T_C temperature of 67˚C and 193˚C, respectively. With the addition of LiF, the T_O-T temperature of KNNLST ceramics decreases consistent with the analysis of XRD patterns that the phase structure of ceramics changes toward tetragonal symmetry when LiF content increases, while T_C temperature increases due to diffusion of Li⁺ into A-site of KNN lattice, showing similar tendency to the works [17] [23] . Figure 5 also shows that the dielectric loss little changes as the temperature rises from room temperature to 150˚C and increases rapidly in the vicinity Tc.

Some physical properties of KNNLST+ x wt% LiF ceramics sintered at 930˚C measured at room temperature

also listed in Table 2. It is seen that with increasing of LiF content, the density, dielectric constant (e), planar electromechanical coupling factor (k_p), (k_t), piezoelectric constant (d₃₁) are increased. The largest values for density of 4.26 g/cm³, k_p of 0.27, k_t of 0.40, d₃₁ of 41 pC/N, e of 744 were obtained at 4 wt% LiF content, then decrease. These are probably related to characteristics of the density, the increasing grain size and morphotropic phase boundary effect [16] .

Figure 7 shows the shape of ferroelectric hysteresis loops of the samples KNNLST+ x wt% LiF measured at room temperature. From the shape of these loops, the remanent polarization P_r and the coercive field E_C of ceramics were determined, as shown in Figure 8. With increasing of LiF content, a sharp increases in P_r was observed for samples until x = 4, reaches the highest value (11.96 µC/cm²) at this content, and then decreases; while the coercive field E_C strong decreases from 8.06 to 5.38 kV/cm during the measured LiF content range. These results are in good agreement with the studied dielectric and piezoelectric properties of the ceramic samples.