**3. Conclusions**

*Multifunctional Ferroelectric Materials*

about 2.8×105

**2.8 Sodium niobate (NaNbO3)**

one of useful candidate [151].

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

Curie–Weiss temperature [114]. KNbO3 Curie–Weiss constant was found to be

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].

This is also the member of perovskite family, but with anti ferroelctric properties. Six phase transition in between −200 to 650°C range affects its structural, dielectric, and optical properties. Phase transition at 200,360 and 480°C has been observed due to off center displacement of Nb ion with tilting of oxygen octahedral. Its stable cubic structure of the *Pm3m* space group has been observed at high temperature, i.e. >640°C [127, 128]. The orthorhombic structure of the phase *Pbcm* space group has been observed at room temperature is antiferroelctric one. At 360°C the antiferroelcetric orthombic structure with *Pbma* space group undergoes phase transition to antiferroelectric orthorhombic structure with a *Pnmm* space group associated with maximum dielectric constant [129–134]. NaNbO3 single crystal exhibit low-frequency relaxation processes [135]. Distinct discontinuity is observed in mean relaxation time and relaxation parameter at Curie temperature Tc. At high temperature, low frequency relaxation increases due to crystalline structure disorder. This leads formation of local dipole in the polar region [136]. Stimulating electrical and mechanical properties of Sodium niobate-based ceramics make them a useful candidate for many technological applications [136–147]. At attainable electric fields this well-known antiferroelectric shows FE properties. Cost effective lead-free nanowire based NaNbO3 piezoelectric has a high-output [148]. So it find application in hologram and optical data storage having high density [137–143]. It also used as nanocapacitors, NGs and in the memories of nanoscale [148–150]. For large-scale lead-free piezoelectric NG may be NaNbO3 nanowires is

K. Displacive model is used to calculate this shows good agreement

<sup>2</sup> ∝ (*T*−*T*0 with *T*<sup>0</sup> ≈370°C

good agreement with values calculated Cochran from

**38**

Several reports on ferroelectric materials and their use for different piezoelectric application has been studied in the last few years. As discussed in this chapter the effect of perovskite structure affect its ferroelectric properties. Doping in these perovskite structures also responsible for the enhanced of its properties by tailoring its crystal structure. These materials now investigated in the composite, nanowire and the nanorods form to make the device mechanically robust and more compact. This is a vital field for the research, as a key element in the digital world.
