Cu-Doping-Modified Order State, Ferroelectric Property, Multiscale Inhomogeneities, and Local Structure in the Relaxor KTa1–xNbxO3 Crystal

Cu doping in the KTa1–xNbxO3 crystal was investigated comprehensively from the dielectric measurement, global ferroelectric property, electro-optic (EO) modulation, and local structure. The orientation of the domain structure can be stabilized by Cu doping and is immune to thermal disturbance in the ferroelectric phase, and the field-driven activity of polar nanoregions (PNRs) can be decreased in the paraelectric phase. The coercive field was reduced and the reversal process of the domain structure was greatly changed, which could be attributed to the enhanced heterogeneity and the driving force provided by the defect dipole, respectively. The frequency dependence of the EO modulation depth for KTN and KTN:Cu crystals implied that Cu doping can strengthen the multiscale inhomogeneity and polarization of PNRs. Based on the first-principles calculation, the octahedral distortions of KTN and KTN:Cu were calculated, and the results implied that Cu doping can result in a greater octahedron distortion, thus enhancing the local heterogeneity and resulting in stronger local polarization. The study will provide guidance for the growth of crystals to realize the optimization and design of functional materials.