| Tracing back to more than one thousand years far before the establishment of modern sciences,ancient China has a quite long history in crystal growth,involving for example brine vermilion refining.Since the beginning of the 1960s,the invention of lasers has promoted the demand for functional crystal research in modern science and technology,leading to a continuous boom in new crystal discoveries and single crystal growth.Crystal is an important basic material for investigating intrinsic properties and for practical applications.Especially functional crystals are various energy conversion media such as light,heat,electricity,force,sound,magnetism,etc.,and are becoming one of indispensable materials for the development of modern science and technology.According to the main effects and practical application characteristics,functional crystals can be divided into:nonlinear optical crystals,piezoelectric crystals,electro-optic crystals,pyroelectric crystals and scintillation crystals,and so on.Among above them,the nonlinear optical(NLO)crystal can be divided into a second-order nonlinear crystal and a Raman laser crystal,which can be used as a laser medium for nonlinear optical frequency conversion and,can expand the laser wavelength and realize laser modulation,and have important applications in the civil and military fields.Governments of all countries have put NLO crystals in the priority position of development.And as an important strategic measure,it has been included in their high-tech development plans and given high attention and support.Crystals that can produce nonlinear effects are called nonlinear optical crystals,and an effective idea of obtaining NLO crystals is to synthesize a compound that is structurally non-centrosymmetric.Since the late 1980s,researchers have synthesized a large number of excellent second-order nonlinear optical crystals.In recent years,the strategy that the introduction of octahedral coordination of d0 transition metal cations(Nb5+,Mo6+,W6+,etc.)and main group cations containing unbonded lone pairs of electrons(Te4+,I5+,Se4+,Sn2+,etc.)are of great interest in the search for the novel second-order NLO crystals.The primary cause is that the above two kinds of ions are prone to the second-order Jahn-Teller(SOJT)distortions and thus produce asymmetric oxide coordination environments.In particular,the non-centrosymmetric quaternary compounds containing W6+/Mo6+ and Te4+ with lone pairs of electrons possess a wider transmission range and a large band gap corresponding to a larger laser damage threshold compared to the ternary compounds,which is beneficial to the application in mid-infrared laser field.Under the guidance of this idea,based on a molybdenum/tungstate system containing heavy metal oxide groups,aiming to expand the mid-infrared transmission characteristics of single crystals by our research group,a systematic research on the synthesis,structure,growth,phase transition,crystal physical properties and optoelectronic device application represented by ?-BaTeMo2O9(?-BTM)were performed for the first time and obtained a series of original achievements.All the results indicating that they are photoelectric functional materials with excellent properties.Unfortunately,there is a color unevenness in the ?-BTM crystal,causing a serious absorption of light at 532 nm,which has an adverse effect on the laser frequency conversion output.However,considering the BaTeW2O9(?-BTW)have the same structure as that of ?-BTM,?-BTW may also have excellent nonlinear optical properties;on the other hand,WO3 system has higher thermal stability and lower viscosity that is conducive to the growth of high-quality crystals,which attracted more and more attention.In addition,both CdTeMoO6(CdTM)and Cs2TeW3O12(CTW)crystal not only have a strong second harmonic generation(SHG),but also have high symmetry(tetragonal and hexagonal crystal system)which facilitates later crystal processing and device fabrication,and thus this also aroused our great interest.In this study,the single crystal growth of tetragonal CdTM is firstly studied and the problem of layered growth habit is solved from the perspective of growth kinetics,and its basic properties,nonlinear optical and electrical properties are also comprehensively studied.Secondly,the optimized growth of CTW crystal was performed,and the Raman properties were studied for the first time.The relationship between structure and properties of CTW was analyzed by first-principle calculations.Finally,the ?/?-BTW phase transition mechanism was systematically studied for the first time.The bulk single crystals of ?-BTW were cultivated and its physical properties were systematically studied.The major research achievements are as follows:(i)Crystal growth,structure and characterization of CdTeMoO6In this thesis,the CdO-TeO2-MoO3 system was explored systematically,and the large-size CdTM bulk single crystals were obtained by the flux method for the first time.The effects of growth kinetics on crystal growth were studied comprehensively,and the relationship between crystal growth morphology and crystal rotation rate and temperature lowering rate was determined,and the thicker crystals could be obtained with lower cooling rates.Based on the optimized conditions,the single crystals with a size of 30 mm × 12 mm × 9 mm was finally obtained,from which the problem of difficulty in growing crystals due to layered habits is solved.The high-resolution X-ray diffraction rocking curve shows that the full width at half maxim(FWHM)of the(003)and(220)crystal planes of CdTM are 39.28" and 63.46" respectively,indicating that CdTM has complete structure and high crystal quality,which is sufficient to meet the requirements of the measurements.Based on high-quality crystal,the structure of CdTM was solved and analyzed,which belongs to a tetragonal system,P-421m(No.113)space group.The spatial structure is a typical two-dimensional layered structure,and the layers are arranged in a layer-to-layer superposition along the crystallographic c-axis.This two-dimensional layered structure is attributed to the layered growth of CdTM crystals.(ii)Optical properties and theoretical calculations of CdTeMoO6The optical properties including the UV diffuse reflectance spectrum,transmission spectrum,polarized Raman spectrum and refractive index of CdTM crystal were systematically studied.The results show that the CdTM crystal has a wide transmission range of 345 nm?5.4 ?m with a high transmittance in the wavelength range of 400 nm?5.0 ?m.The refractive index of the crystal was carefully tested by a prism coupler and the results show that CdTM belongs to a negative uniaxial optical crystal with large birefringence.Meanwhile,the refractive index dispersion equation is obtained through the Sellmeier equations.The birefringence of CdTM is 0.2868?0.2219(514 nm-1.5467?m),indicating that CdTM crystals can be used as a potential prism device.From the refractive indices of CdTM,the phase-matching angles can be calculated based on the Sellmeier equations.And the results indicated that the type-? PM can be realized in CdTM over a broad wavelength range.It is obvious that PM can be realized in CdTM in a wide transmission range.At 1064 nm,the PM angle is shown to be 34.6°,The Maker Fringe method was used to test the NLO coefficients for CdTM.The NLO coefficient of CdTM relative to d36(KDP)was measured through fitting the calculated Maker Fringes.Considering the absolute value of d36(KDP)=0.39 pm/V,the corresponding NLO coefficient of d36=d14 for CdTM is calculated to be 8.5 pm/V,which is much larger than that of the KDP crystal.Considering its wide mid-infrared transmission range,CdTM crystals have potential applications in the field of second-order mid-infrared nonlinear lasers.In addition,the first-principle calculations results based on the CdTM crystal structure shows that it is an indirect band gap with a forbidden band of 3.24 eV.The Raman performance of CdTM crystals is comprehensively studied.The results show that the spontaneous Raman spectra along X and Z-axes with different configurations show the strongest Raman shifts at 921.6-1 and 944.1 cm-1.Note that the Raman shifts of CdTM in different polarization configurations exhibit obvious anisotropy,which may be related to the vibrations of anionic groups in the structure of CdTM crystal.To further evaluate the Raman performance of the CTW,comparison measurements of polarized Raman scattering spectra for both CdTM with X(YY)X configuration and the Raman YVO4 crystal with X(ZZ)X configuration were also performed.Clearly,CdTW has a remarkable Raman gain coefficient,about 1.5 × YVO4,indicating that the CdTM crystal has an extremely attractive application prospect in the field of Raman laser.(iii)Electric-elastic properties of CdTeMoO6The samples for piezoelectric applications were accurately designed and the room temperature piezoelectric characteristics of the CdTM crystal were fully characterized by the resonance method for the first time.The dielectric constants of the CdTM crystal,?11 and ?33,were 35.58 and 10.45,respectively,which showed a relatively large anisotropy.The piezoelectric coefficients d14 and d36 reached 20.14 pC N-1 and 7.58 pC N-1,respectively,which is much larger than that of ?-SiO2(d11 =-2.31 pC/N,d14=-0.73 pC/N).And the corresponding electromechanical coupling coefficients k14 and k36 were up to 22.78%and 19.48%,respectively,which is greater than that of La3GasSiO14(k12=16%).Our observations indicate that CdTM crystals are good potential candidates for piezoelectric applications.In addition,to better comprehend the relationship between the asymmetrical coordination environments and piezoelectric properties,the local dipole moments for the MoO4,TeO4 and CdO4 polyhedra were calculated by using a bond-valence approach based on previous studies.As a result,the vector summation over the TeO4 geometry gave a net dipole moment of 11.9907 Debye,apparently larger than the corresponding average value(8.67 Debye).the dipole moment of the MoO4 was found to be 6.9996 Debye,slightly less than the corresponding average value.These above results reasonably reveal the origin of its excellent piezoelectric performance.(?)Optimized growth,properties and theoretical calculations of Cs2TeW3O12From the perspective of crystal growth kinetics,by optimizing the growth parameters,especially the optimization of flux(viscosity)and cooling rate,the CTW single crystal with the largest size of 75 mm× 35 mm × 8 mm was successfully grown,and the relationship between the crystal morphology and cooling rate was established.Both the high-resolution rocking curve and the Laue diffraction test results show that the crystals grown are of good quality and is sufficient to meet the test requirements.Based on high quality single-crystal,the transmission spectra along three different principal axes of CTW were carefully tested and compared.In addition,we also calculated the band structure and analyzed the density of states using the first principle calculations.The results show that the direct bandgap value of CTW is 2.65 eV,and WO6 and TeO3 polyhedra are attributed to its optical properties.In addition,the Raman performance of CTW crystals is comprehensively studied.The results show that the strongest Raman shift peaks at 910.34 cm-1 and 148.28 cm-1 are obtained under different geometric configurations along the crystallographic X-axis and Z-axis,respectively.Clearly that the Raman shifts of CTW in different polarization configurations exhibit obvious anisotropy,which may be related to the vibrations of anionic groups in the structure of CTW crystal.To further evaluate the Raman performance of the CTW,comparison measurements of polarized Raman scattering spectra for both CTW with X(ZZ)X configuration and the Raman YVO4 crystal with X(ZZ)X configuration were also performed.Note that CTW has a remarkable Raman gain coefficient,about 2 x YVO4.Considering that CTW single crystal also has wide transmission,large refractive index and excellent thermal stability,indicating that the CTW crystal has an extremely attractive application prospect in the field of Raman laser.(v)Phase transformation,controlled crystallization,and linear and nonlinear optical properties of ?-BaTeW2O9The mechanism of ?/?-BTW phase transition was systematically studied for the first time,and the two-phase relationship of ?/?-BTW,namely high temperature orthogonal phase a-BTW and low temperature monoclinic phase ?-BTW,was clarified.The results show that the temperature of synthetic pure phase ?-BTW should not exceed 550 ?.As the temperature increases,it is easy to cause the appearance of a-BTW phase.When it reaches or exceeds 600 ?,it is easy to obtain pure a-BTW phase.Both in-situ XRD and thermal analysis test results show that there is a phase transition from ?-BTW to a-BTW at about 608 ?,and the phase transition process is irreversible.In addition,the powder SHG test shows that it has a strong SHG effect,which is about 1.5 times that of the KTP crystal,and can achieve class I phase matching,showing excellent second-order nonlinear optical performance.Secondly,we carried out a detailed exploration of the BaO-TeO2-WO3 system and,the crystalline phase region of the ?-BTW was determined.After several optimization experiments,the centimeter-sized ?-BTW single crystal was successfully grown.Importantly,the relationship between the two-phase of BTW and the cooling rate was established,in which we can regulate the crystal growth of the two phases according to different cooling rates.Both the high-resolution rocking curve and the Laue diffraction test results confirm that the ?-BTW crystal is sufficient to implement the measurements of intrinsic physical properties.?-BTW crystallizes in an acentric space group P21(No.4,a = 5.499(6)A,b 7.469(9)A,c = 8.936(10)A,and Z = 2),with a two-dimensional layered structure consisting of WO6 octahedra linked to TeO4 polyhedra,analogous to those of ?-BTM,both of which are polar crystal materials.Finally,the thermal and optical properties of the ?-BTW crystal were systematically investigated.The results show that the ?-BTW crystal is an incongruent melting compound,which undergoes a phase transition at 608 ? and decomposes at 784 ?,and the specific heat result is 0.36?0.41 J/g-K in the temperature range of 30 ??350 0C.The UV absorption edge of ?-BTW was located near 325 nm,which is more reliable than that obtained from the diffuse reflectance test of the powder samples.The?-BTW crystal possessed high mid-IR transparency up to 5.4 ?m,and the IR absorption edge was extended to 5.7 ?m covering a critical atmospheric transparent window(3-5?m).It is gratifying to note that the broad transmission range(325 nm?5.70 ?m)of ?-BTW is better than other previously reported molybdenum tellurite compounds.In summary,the ?-BTW crystal is a potentially excellent mid-infrared laser material.In addition,in order to further explore the intrinsic relationship between the microstructure of the ?-BTW crystal and the nonlinear optical properties,we analyzed the magnitude of distortion for WO6 using the valence bond theory.The obtained Ad values for W(1)O6 and W(2)O6 octahedra in ?-BTW are 1.074 and 1.066,respectively,both of which are obviously larger than the 0.8 reported by previous studies,and correspond to a strong distortion.With respect to the dipole moments,the vector summation over the TeO4 geometry showed a net dipole moment of 10.74 Debye,which was apparently larger than the corresponding average value(8.57 Debye).Thus,it is reasonable that the strong SHG efficiency is attributable to the cooperative effect of WO6 and TeO4 polyhedra,which is also in good agreement with the previous findings. |
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