**2.7 Potassium niobate (KNbO3)**

KN (KNbO3) is also exhibited perovskite structure. It shows interesting ferroelectric properties at low temperature. It shows three phase transitions simple displacive type at low temperature with different symmetry and PE phase at

high-temperature [107]. Oxygen behaves as rigid body in the octahedral and vibrates about Nb atoms [108]. Liberation of oxygen octahedral leads to the irregular anisotropy exhibited by oxygen atom due to mean square displacements. Its structure shows two subshell obtained from the splitting of oxygen octahedron. Six niobium atoms are from the third nearest sub shell. 24 oxygen atoms form a fourth adjacent octahedral shell consists of four sub shell of six atoms. Fifth shell is made of 12 niobium atoms. Neutron diffraction study predicts the structural change with temperature KNbO3. It shows three phase transition from cubic to tetragonal, tetragonal–orthorhombic and orthorhombic–rhombohedral at *T* ≈ 418°C, 225°C and 10°C respectively [109, 110], Transverse optic mode exhibited by KNbO3 is softened with lessening temperature obtain from Raman, IR and inelastic neutron scattering [111–113]. Soften mode frequency obtain from dielectric measurement is good agreement with values calculated Cochran from <sup>2</sup> ∝ (*T*−*T*0 with *T*<sup>0</sup> ≈370°C Curie–Weiss temperature [114]. KNbO3 Curie–Weiss constant was found to be about 2.8×105 K. Displacive model is used to calculate this shows good agreement with the theoretical value. Large electromechanical coupling factor with zero temperature coefficients at room temperature exhibited by KN crystal is used for piezoelectric application [115, 116]. Surface acoustic wave (SAW) filter prepared using KNbO3 find its application in mobile phones and television receivers [115, 116]. The crystal symmetry of KN crystal shows 49.5° rotation about the y axis by the x-cut [117, 118]. In high quality fiber shape these crystals show small lattice defects [119]. Different melting temperature hinders to grow high quality and large size KN crystal [115]. Both Bridgman (BM) technique and Top-seeded solution growth (TSSG) method is used, however the Bridgman (BM) technique is the easier one to grow bulk shape KN crystals [120–124]. Phase diagram suggests line compounds are formed when these crystals are developed from high-temperature solutions [125]. A peritectic transformation is shown when it grows from molten stoichiometric composition. KN in nanorod form are used capacitor and nano (NG) [126].
