Study On The Optoelectronic Properties Of KNN-Ca(Bi_0.5M_0.5)O₃（M=Nb,Ta） Fluorescent Transparent Ferroelectric Ceramics Modulated By Rare Earth Doping

With technological advances,the demand for multifunctional materials continues to rise.Sodium potassium niobate（KNN）ceramics are becoming a hot topic of research,not only because of their high electrical constants and high Curie temperatures,but also because of the different properties（e.g.light transmission,photoluminescence,photochromism,photoelectricity,etc.）that can be achieved by doping with the right second element.In this paper,the phase structure of KNN ceramics is transformed from an orthogonal phase to a highly symmetrical tetragonal or pseudo-cubic phase by the introduction of a suitable second group element,thus achieving the transparency in KNN ceramics.By further introducing rare earth ions,fluorescent transparent ceramics with multiple properties such as upconversion luminescence,transparency,ferroelectricity,and dielectric properties were prepared.The details of the study are as follows:（1）K_0.5Na_0.5Nb O₃-Ca(Bi_0.5Ta_0.5)O₃transparent ferroelectric ceramics were obtained by the conventional solid-phase method.The phase structure,surface morphology,optical transparency,ferroelectricity and dielectric properties of the ceramics were investigated.Compared with the standard KNN standard card（PDF#52-1583）,Ca(Bi_0.5Ta_0.5)O₃is well dispersed in K_0.5Na_0.5Nb O₃,which is a pure ABO₃structure.The phase structure transforms into a pseudo cubic phase,with an average grain size reaching the nanoscale.When x=0.06,KNN-x CBT ceramics achieve the highest transmittance,reaching 67%at780 nm,79%at 1100 nm,and the band gap reaches 3.17 e V.In addition,electrical performance tests have shown that KNN-x CBT ceramics have smaller P_rand E_c,which is a typical relaxor ferroelectric characteristic.The maximum energy storage density is 0.37J/cm³,the energy storage efficiency is greater than 72%,and the maximum dielectric constant is 456.（2）K_0.5Na_0.5Nb O₃-Ca(Bi_0.5Nb_0.5)O₃ceramics were prepared by the conventional solid-phase method.The effects of CBN content on the microstructure,transmittance,ferroelectricity and dielectric properties of ceramics were investigated.The KNN-x CBN ceramic has a layered stacked structure compared to the angular and grainy surface morphology of the KNN-x CBT system.The average grain size of the KNN-x CBN system is smaller,with a minimum value of 79 nm.With increasing CBN content,the transparency of the ceramics increases and then decreases,with a maximum value achieved at x=0.06,reaching 63%in the visible region（780 nm）and 74%in the near infrared region（1100 nm）,while the forbidden band width E_greaches a maximum value of 3.16 e V.A comparison of the electrical properties shows that the KNN-x CBN system has a significantly higher P_mvalue and achieves better electrical properties.The maximum energy storage density is1.143 J/cm³and the maximum dielectric constant is 887,which is much higher than the KNN-x CBT system.（3）Investigation of the up-conversion luminescence properties of Er³⁺（x=0.1,0.2,0.3,0.4）doped 0.94KNN-0.06CBN ceramics and influence from different Er³⁺contents on the optoelectronic properties of KNN-0.06CBN ceramics.As the Er³⁺content increases,the（200）diffraction peak gradually splits into（002）and（200）diffraction peaks,and the phase structure of the ceramics changes from a pseudo-cubic phase to a tetragonal phase.The ceramic samples have a uniform grain distribution,high density,fewer pores,and the average grain size is less than 0.1μm,reaching a minimum value of 73 nm at x=0.1.In addition,the introduction of Er³⁺,increased the transmittance of the KNN-0.06CBN ceramic,reaching 77%in the near infrared region（1100 nm）and a forbidden band width E_gof 3.18 e V when the Er³⁺doping concentration was 0.1wt%.The best combined energy storage characteristics are reached at x=0.4,with W_recandηof 0.53 J/cm³and 46.72%,respectively.The maximum dielectric constant of 1030 is reached for KNN-CBN:xwt%Er³⁺ceramics,indicating that a suitable amount of Er³⁺can increase the dielectric constant of KNN-0.06CBN ceramics.In addition,the introduction of rare earth Er³⁺also achieves up-conversion luminescence,with two strong green fluorescence peaks at 530 nm and 550 nm and a weak red fluorescence peak at 660 nm in the spectrum.（4）KNN-0.06CBN:Ho/Er ceramics were prepared.The ceramics with different Ho contents were analyzed for microscopic characterization and optoelectronic properties.A high transmittance（1100 nm）of 69%was obtained at x=0.1 and the forbidden band width E_greached 3.15 e V.The best comprehensive ferroelectric performance is achieved at x=0.2,the energy storage density is 0.37 J/cm³,and the energy storage efficiency is 76%.In addition,there are two green fluorescence peaks（531 nm,549 nm）and one red fluorescence peak（672 nm）.The peak intensity of fluorescence peak is much higher than KNN-0.06CBN:x%Er³⁺system.