Growth And Properties Investigation Of KTN Series Crystals

Potassium tantalate niobate(KTa_1-xNb_xO₃;KTN)crystal is an outstanding quality multi-functional crystal and the crystal notable for its electro-optic effect, photorefractive effect etc.for a long history.KTN crystal is usually growth from the melt and it has perfect thermal,chemical and mechanical stabilizations.Based on the above merits,KTN crystal is widely used in nonlinear-optic,optic storage, optical communication and photoelectric fields.KTN crystal is solid solution crystal of KTaO₃(KT)and KNbO₃(KN)crystals. The crystal can exist in both paraelectrie(cubic)and ferroelectric(tetragonal or orthorhombic)phases depending on the composition difference of crystal at room temperature.The Currie point(Tc)and physical properties of the crystal can all be adjust by the ration of Ta an Nb.A KTN crystal with Tc near room temperature can be grown by appropriately adjusting the composition of the raw materials.By using such a KTN crystal,excellent electro-optical components that can be used at room temperature can be designed and produced.For the excellent electro-optic and photorefractive properties,KTN crystal is widely used in beam deflect,Q switch,and optical modulate area.Although,the first KTN crystal was fabricated in 1950s and its excellent EO and photorefractive properties were discovered since that time,the investigation and application of KTN crystal were very limited for it is very difficult to get big size and good quality single crystal.Recently,due to crystal growth technology improvement,big size,good quality KTN single crystal was successfully fabricated,this make the science research and industry application of KTN crystal become hot again,In this work,big size,good quality KTN single crystals were successfully grown by Czochraiski technique.The growth technique,structure, defects and other physics properties,especially the EO effect,of the crystal were system studied,the application of the crystal was also discussed.It mainly includes such aspects: 1.Crystal growthBig size,good quality KTN single crystals were successful growth from KT-KN solution system by Czochralski method,the quality of crystal was improved by properly thermal field design and growth parameter control.The composition of KTN crystal is determined by raw material ratio and growth temperature.According to phase diagram analysis and actual measurement,the segregation property of Ta and Nb in KT-KN solution system was systemic investigated.KTN crystals with vary composition x from 0 to 0.5 were successful prepared.Crystal growth is a complex physics and chemical process. Based on thermodynamics and kinetics theory,the condition factor those affect crystal qualities during crystal growth process were studied in detail.During crystal growth process,suitable temperature distribution is precondition; appropriate growth parameter control is sticking point.2.Defects research of KTN crystal,crystal growth technique improvementThe familiar defects of KTN crystal were systemic investigated,Integrality property was measured by high resolution X-ray diffraction method;chemical etching and microscope topography method were use to investigate the inclusion,dislocation,striations.The reason that the defects formed was discussed combine with the crystal growth process.All investigation illustrate that the center part is the defect concentration area.Striation is the main defect that affects the application of KTN crystal.In order to get striation free KTN crystal,a big crucible that contains a great deal of raw material was adopted to grow relatively small crystal.The atomic microscope measurement indicates that the homogenous property we grown are quite well.3.The structure and phase transition of KTN crystalKTN crystal is a solid solution crystal of KT and KN.The crystal exists in three phases at room temperature:cubic,tetragonal or orthorhombic, depending on the ratio of Ta and Nb.Due to particularity structure,KTN crystal attract researcher's interest for a long history.Most KTN crystals with different composition will experience multi phase transition process, cubic-tetragonal-orthorhombic,from high temperature to low temperature.The phase transition temperature is different according to the composition of the crystal and a lot of physical property of crystal is also show regular change according to the composition.A KTN crystal with Tc near room temperature can be grown by appropriately adjusting the composition of the raw materials. By using such a KTN crystal,excellent electro-optical components that can be used at room temperature can be designed and produced.In this work,the structure and phase transition property of KTN crystal were investigated by X-ray powder diffraction and Roman spectra technique.4.Basic physical properties of KTN crystalThe liner optical,thermal and electric properties of KTN crystals were measured,the influence of these properties on growth and applications were also discussed.Refractive index,absorption and transmission properties of KTN crystal were studied;buoyancy method was used to measure the density of KTN crystal and the relation of density and composition was also discussed here.Thermal properties,such as melt point,molar enthalpy of fusion,molar entropy of fusion,thermal expansion coefficients,specific heats, thermal-conductivity and thermal diffusion coefficients of the KTN crystals were investigated systemically.These results show that KTN crystal is a promising material for optical application.5.Electro-optic effect of KTN crystal and its applicationElectro-optic(EO)effect is a typical property of KTN crystal and the crystal has the biggest EO coefficient in the known materials.Based on the EO coefficient tensor of KTN crystal,we deduce and designed the measurement method of KTN crystal.EO coefficients of KTN crystal with three different compositions were measured systemically.Based on the EO effect,a KTN beam deflector was design and the beam was successfully deflected 40mrad under relatively low voltage.6.Absorption coefficient and photocurrent properties of KTN crystal KTN crystal was first investigated as a semiconductor material.Photocurrent spectra of KTN crystal wafer have been studied employing electric fields up to25V/mm.The spectra show a shift towards lower energy at the voltage supply and the fundamental absorption according to density of states and Urbach rule determines the shape of the photocurrent spectrum.By modeling the crystal absorption with the density of states and the Urbach rule,it is shown that excitonic transitions contribute no effect to the absorption of the wafer.