Study On The Crystal Structure And The Electric Polarization Of Ti⁴⁺ And V⁵⁺ Doped NaNbO₃ Perovskite Oxides

Sodium niobate（NaNbO₃）perovskite oxide is a kind of classical lead-free antiferroelectrics.It is expected to improve its piezoelectric and ferroelectric properties by precisely adjusting its crystal structure.Traditional dielectric ceramic modification strategies focus on finding morphotropic phase boundary and constructing corresponding component solid solutions.In order to enhance the noncentric lattice distortion of Nb O₆octahedron in perovskite,Ti⁴⁺and V⁵⁺ions with second-order Jahn-Teller activity were partially substituted for Nb⁵⁺ions at B site to strengthen the cooperative lattice distortion at B site and enhance the intrinsic polarization of the system,so as to improve the piezoelectric and ferroelectric properties of the materials.In this paper,a series of Ti⁴⁺,V⁵⁺doped Na Nb_1-xM_xO_3-δ（0≤x≤0.2,M=Ti,V）perovskite oxide were synthesized by solid-state method.X-ray powder diffraction and Raman spectroscopy confirmed that the sample was transformed from the undoped orthorhombic Pbma（No.57）phase to the doped orthorhombic P2₁ma（No.26）phase.According to the results of X-ray diffraction full spectrum fitting and Rietveld structure refinement analysis,the Nb-O4 bond length and Nb-O2/O4-Nb bond angle are more sensitive to Ti or V doping,which is the main crystallographic reason for the cooperative distortion of B-site Nb O₆ octahedral network.X-ray photoelectron spectroscopy further shows that the covalency of Nb-O chemical bond is enhanced after doping.The results of UV-Vis diffuse reflectance spectroscopy and Reflected electron energy loss spectroscopy show that the band gap decreases after doping,and the change of band structure is realized by reducing the conduction band edge position of Nb-4d and O-2p hybrid.Based on the above experimental results,Ti⁴⁺or V⁵⁺doping enhances the covalent orbital hybridization between Ti/V-3d state and O-2p and Nb-4d state,promotes the charge transfer between the valence orbitals,and finally leads to the change of energy band structure and crystal structure.Furthermore,the dielectric,ferroelectric and piezoelectric properties of the target compound ceramics show that Ti⁴⁺and V⁵⁺ions doping can effectively improve the dielectric permittivity,ferroelectric polarization and d₃₃ piezoelectric coefficient of the materials,and the optimal doping concentration(Ti⁴⁺:5 at%,V⁵⁺:15 at%)of the two systems is determined.The results show that the enhancement of the electric polarization properties of the samples is mainly due to the cooperative Second Order Jahn-Teller distortion induced by Ti⁴⁺or V⁵⁺doping,which increases the intrinsic polarization of the compounds.Higher doping concentration can not further improve the electric polarization performance of the material.The concentration distribution of oxygen ion vacancy,composition and charge fluctuation near the grain boundary caused by the substitution of different valence charge is the possible reason for the decrease of the electric polarization performance of the material.In conclusion,by introducing Ti⁴⁺or V⁵⁺ions with Second Order Jahn-Teller activity into the B-site of NaNbO₃ perovskite oxide,the controllable B-site cooperative lattice distortion was achieved,and the electric polarization performance of NaNbO₃ ceramics was significantly improved.