As known, sintering temperature is

As known, sintering temperature is a very sensitive factor for producing KNN based ceramics because of the uncontrollable evaporation of alkali elements, which may lead to the formation of unexpected secondary phase.[34] The XRD profiles of the ceramics sintered at various temperatures are shown in Fig. 6(a). The results suggest that the structures of KNN ceramics were kept in the orthorhombic perovskite structure within a wide sintering temperature range from 1055°C to 1130°C, except the appearance of K4Nb6O17 phase for CSAS samples. Beyond that, there are no obvious phase transformation in this temperature range for both CS and CSAS samples. With higher sintering temperature of 1145°C (above the solidus temperature for KNN ceramics), a secondary phase out of the perovskite background can be clearly identified for CS and CSAS samples. The secondary phase match well with a partially filled tetragonal tungsten bronze (TTB) structure (K6Nb10.88O30, JCPDS 87-1856) as shown in Fig. 6(b). The compositional precipitation may be attributed to the volatilization of alkaline elements during sintering.[34, 35] The appearance of the TTB phase would play a negative role in densification, and can be detrimental for the electrical properties of KNN ceramics.