Structural Phase Transition And Electro-optic Properties Of Potassium Sodium Tantalate Niobate Single Crystals

Potassium Sodium Niobate Tantalate?KNNT?lead-free single crystal has attracted a lot of attention due to its excellent electro-optic property.However,optical research on KNN based materials are mainly focused on transperant ceramics because of the difficulty in growing high quality single crystals with large-size and good quality.The ceramic formed with randomly distributed grains would lower the transmittance,alignment,monochromaticity of laser.Therefore,it's unfavorable for the optical research.In this dissertation,the growth method of KNNT single crystal was discussed.The dielectric,optical energy band structure,structural phase transition,and electro-optic properties were studied systematicallyA series of Potassium Sodium Niobate Tantalate single crystals with high quality were grown using top-seeded solution growth method.By defect controlling mothods,large size as 12 mm width/13 mm height and good quality?compositional difference<±0.01?of KNNT single crystal has been achieved.Phase structure and lattice parameters were studied using XRD for KNNT single crystals with different compositons.Grown KNNT single crystals already meet the requirement of optical research and application.Primary properties,such as dielectric,transmittance,absorption,and energy band properties have been studied.Dielectric properties were investigated,and the phase transition temperatures were found linear to the Tantalate content.KNNT shows high transmittance and low absorption,and be better after poling.Combined optical interband transition properties of KNNT78/22 single crystal with Raman spectra,vibration modes corresponding to the different phases were identified.A new approach for phase transition detection was carried out.Raman spectra were measured to studied the structure of KNNT65/35 single crystal.Based on the Lorenz fitting result,12 Raman peaks were identified in the room temperature Raman spectrum,and linked to different vibration modes.Phase transition behavior was studied by analyzing the shift,intensity,and degeneration of Raman peaks,and confirmed by dielectric and XRD measurements.Polarized Raman spectra indicate the KNNT single crystal achieved a single domain state with some micro/nano ferroelectric domains.The spontaneous polarization would enhance the Raman intensity parallel to it and has no influence on the perpendicular ones.EO measurements were carried out on the KNNT65/35 single crystals.Refractive indices were measured using Brewster's angle method,and ellipsometry was used for the dispersion study.A EO effect theory of mm2 crystals and its piezoelectric correction was carried out.Mach-Zehnder interferometer and single-beam PSA compensater were built to measure both longitude and shear linear EO coefficients for KNNT65/35 single crystal.The?₃₃ was found to be 270 pm/V,7.5 times larger than LiNbO₃.The orientation distribution of EO coefficients was calculated based on phenomenology of crystal symmetry.An ultrahigh?₃₃ of 693 pm/V was found after rotating the crystal around[011]_c axis for 50.85°.It's 21 times larger than LiNbO₃,and attribute to the large shear EO coefficient.