| Laser is an important new light source that discovered in the middle of the 20th Century.Lasers have been widely used in military and civil fields because of their good monochromaticity,strong directionality and high coherence.The laser beam can be obtained directly from the laser source,produced by the laser working medium;however,the laser wavelengths obtained in this way have a limited range.At present,one of the most effective methods to obtain new laser is frequency conversion by the nonlinear optical(NLO)crystal to transform the nonlinear frequency of the laser.Therefore,the design and synthesis of novel nonlinear optical crystals has been a hot topic in the field of lasers.Recently,a kind of compounds that contain both the transition metal cations(Ti4+,V5+,Nb5+,Mo6+,W6+,etc.)and the lone-pair electrons cations(Se4+,Sb3+,Te4+,I5+,Pb2+,etc.),attracted the attention of scientists.Due to the influence of the second order Jahn-Teller(SOJT)effect,a lot of compounds compounds containing transition metal cations and lone-pair electrons cations are with non-centrosymmetric(NCS)structures,and most of them have a strong frequency doubling effect.In recent years,the authors' research group has devoted huge efforts to the research of this kind of crystals,and has obtained plentiful research results.It was this group that in 2008,large-sized ?-BaTeMo2O9 crystal has been grown for the first time,and the NLO properties,piezoelectric properties,electro-optic properties and Raman laser properties of the crystal have been systematically studied.The results of the study show that the ?-BaTeMo2O9 crystal is an excellent multifunctional crystal.In 2011,the same group reported the growth of centimeter-sized Cs2TeMo3O12(CTM)crystals.The study found that the direction of distortion of all TeO3 polyhedra in the structure are basically the same,which leads the crystal to generate a net dipole moment in the axial direction.However,due to the high viscosity of the flux system and the substantial anisotropy of the thermal conductivity of the crystal,the quality of the grown crystal was poor and the crystal defects were significant,which limits the fabrication and application of such devices.Through literature investigation,it was found that adding a certain amount of WO3 in the growth process of KTiOP04(KTP)crystal can significantly reduce the viscosity of the flux system.Therefore,the Cs2TeW3O12(CTW)crystals,which are isostructural to the CTM crystals,have attracted the authors' attention.In this paper,high quality CTW crystals were grown by the flux method;the nonlinear optical properties and piezoelectric properties of CTW crystal were investigated,and the relationship between the crystal structure and physical properties were also discussed in details.The main contents and conclusions are as follows:?.Exploration the defects and growth mechanism of CTM crystalsThe macro-defects including stress cracking,inclusions,negative crystals and added-crystals in CTM crystals were discussed.Dislocations in crystals were investigated by chemical etching.The results show that the corrosion rate,the morphology and the orientation of the etch pits of(2110)face are the same on both sides,indicating that the polarity does not exist in the a-axis.However,the corrosion rate of both sides of(0002)face is obviously different,which shows that the crystal is polar in the c-axis.And that the hexagonal etch pits morphology also verified the presence of a 6-fold axis along the c-axis.In addition,the growth mechanism of the(0002)face of the crystal has been investigated by atomic force microscopy.It is found that the growth mechanism of the(0002)face belongs to two-dimensional nucleation.?.Growth and growth parameters optimization of the CTW crystalBased on the detailed investigation of the Cs2O-TeO2-WO3 ternary diagram,the suitable crystalline phase region for the growth of CTW crystal was determined.Large-sized CTW single crystals were grown by top seeded solution growth(TSSG)method.The effects of growth parameters on the crystal quality and morphology were studied.The experimental results show that the quality of crystal with cooling rate between 0.5-1.0 ?/d is significantly better than the one with cooling rate between 1.0-1.5 ?/d.The crystal grown at 15 rpm has a more natural surface than the crystal grown at 30 rpm.Comparing the quality of the crystals grown in the three different flux ratio,when the flux ratio was CTW:TeO2=1:2,the quality of the grown crystals was of significantly higher quality than the other two ratio for the growth of crystals.?.Crystal structure and basic properties of the CTW crystal 1.The crystal structure of CTW was analyzed.CTW crystal belongs to P63 spacegroup,the cell parameters is a=b=7.4045 A,c=12.2053 A,V=577.00(4)A3,Z=2.The crystal form a two dimensional layered structure,and due to the effect of the second order Jahn-Teller effect,TeO3 and WO6 polyhedron was distorted.WO6 polyhedra connect each other by the common vertex O atoms to form the W-O layer,with TeO3 tetrahedron covering one side of the W-O layer,and Cs+ filling between the layer to maintain the charge balance.2.By using the buoyancy method,the room temperature density of crystal was measured to be 6.467 g/cm3.The hardness of the crystal was measured by a micro-hardness tester,and it is found that the hardness of CTW crystal is moderate,which is beneficial to the processing and polishing of the crystal.3.The specific heat,thermal expansion,thermal diffusivity and thermal conductivity of the crystal were measured.The specific heat of the crystal is 0.3016 J/(gK)at room temperature,and increases slowly with the increase of temperature.According to the thermal expansion curves,the average linear thermal expansion coefficients are ??=?11=1.003×10-6 K-1,?c=?33=26.27×10-6 K-1.The thermal conductivity are 2.33 W/(mK)and 0.80 W/(mK).The rocking curve of the crystal is measured,and the full-width at half-maximum is 33" which shows that the lattice of the crystal is good and the crystal quality is high.In addition,the chemical stability of the crystal was measured at room temperature.The results show that CTW is non-hygroscopic and resistant to HNO3 solution,but can slowly dissolve in saturated NaOH solution.?.Optical properties and piezoelectric properties of CTW crystal1.Optical properties.The transmission spectrum of the crystal was measured,and its high transmittance range was 430?5310 nm.The refractive indexes were measured and the refractive index dispersion equation was calculated.The NLO optical coefficients of the crystal were measured by the rotating Mark fringe method,and the two NLO coefficients were obtained as follows:d32= 6.2 pm/V,d33= 4.3 pm/V.The effective NLO coefficient is 4.0 pm/V at 1064 nm.2.Piezoelectric properties.The piezoelectric properties of CTW crystal at room temperature were measured by resonance and impedance analysis.The dielectric,elastic,piezoelectric and electromechanical coupling coefficients were measured at room temperature.The dielectric constant of CTW crystal e11 and e33 are 29.4 and 10.1,respectively,showing a large anisotropy.The longitudinal piezoelectric coefficient(d33)of CTW crystal reached 19.1 pC/N,which is 8 times larger than that of the d11 of ?-SiO2,approximately equal to the d22 of LiNbO3,and slightly smaller than that of CTM crystal(d33=20.3 pC/N).?.Relationship between crystal structure and propertiesAccording to the bond length and bond angle of the octahedron,the distortion of WO6 was calculated.Based on the coordinates of each atom in the crystal cell and the corresponding bond length,the valence of Cs+,Te4+,W6+,O2-were calculated,respectively.The dipole moments of each polyhedron and crystal cell were calculated by the dipole moment formula.The relationship between the nonlinear optical properties and piezoelectric properties of crystals and the dipole moments of crystals were analyzed. |
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