Research On The Growth, Properties And Application Of Trigallium Phosphorus Heptoxide

Crystals are solid materials of long-range order. And the primary units of the crystal are periodic arranged, which determines its unique physical properties. Therefore, crystals are special materials which can realize energy interaction and transformation of electricity, magnetism, light and heat. Thus, crystal material are one kind of indispensable materials in modern science and technology, and they are substantial base of optical-electric, laser and microelectronics technologies. Now, they are widely used and play an important role in many fields. Piezoelectric effect is one of the important properties in functional crystals, and it was discovered by the Curie brothers in1880. Piezoelectric effect is the phenomenon that electric charges accumulate in the corresponding surfaces of the crystal when the deformation occures under the impact of external force. The crystals with piezoelectric effect are named piezoelectric crystals and they can realize the transformation between mechanical enegry and eletric enegry. Various piezoelectric devices made by piezoelectric crystals, such as piezoelectric filter, piezoelectric resonator, piezoelectric optical deflector, surface acoustic wave component, have been successfully applied in the fields of military, aerospace and communication, et al., and piezoelectric crystals have been regarded as an important basic materials in modern electronic technology.In1935, the aluminium phosphate was researched firstly, and it was the beginning of research on the phosphate crystals as piezoelectric crystal. Its structure is the same as quartz crystal which has been used widely, and the temperature of phase transformation is close to that of quartz crystal. The performances of aluminium phosphate are superior to that of quartz, and it is substitute material of quartz crystal. Gallium orthophosphate is the kin crystal of aluminium phosphate, and it not only possesses almost all the advantages of quartz, but also has higher thermal stability, higher piezoelectric effect and larger electromechanical coupling constants than quartz. There are only small changes of the physical properties in the temperature range25℃to933℃, which makes it a very potential piezoelectric crystal material, and its piezoelectric devices can take place of quartz devices in high temperature. Gallium orthophosphate is of great interest owing to its excellent properties and becomes one of research hotspots in piezoelectric crystals. But there are some problems in the growth of high-quality and large size GaPO4crystal, and its industrial growth can not be achieved. It is too difficult to realize the commercialization and application of GaPO4crystal. GaPO4crystal is the first noncentrosymmetric structure crystal that discovered in Ga2O3-P2O5binary system. And the second noncentrosymmetric structure crystal, trigallium phosphorus heptoxide(Ga3PO7), was discovered by Sophie Boudin et al. in1998. Ga3PO7was synthesized by the hydrothermal synthesis method at high temperature (773K) and pressure (210x106Pa), using NH4H2PO4and Ga2O3as solvend and H2O as solvent. It crystallizes in a noncentrosymmetric trigonal crystal system with space group R3m, which belongs to one of the ten polar crystal systems. Due to its special structure, it may possess the basic properties that exist in the piezoelectric crystals. Ga3PO7is a novel piezoelectric crystal. In2007our group had grown the Ga3PO7crystal by the flux method firstly, and then characterized its thermal and optical properties. In2008Z. X. Cheng et al. calculated the structural parameters, electronic structures and optical properties. The results showed that Ga3PO7has potential application in optoelectric decvices. Except our work on the growth and physical properties of Ga3PO7, little work was reported since Ga3PO7was synthesized. In this work, we researched this noncentrosymmetric crystal including the growth, properties and application in depth.In this work, spontaneous nucleate method was used to explore the flux systems, and the flux suitable for the growth of single crystal Ga3PO7was determined. Large-size Ga3PO7crystals have been grown by the top-seeded solution-growth method, and the growth parameters are optimized. The structure, morphology and defects of Ga3PO crystal were studied. The Raman spectra technique was used to observe the growing interface. The basic properties of this crystal were measured and researched systematically, especially the elastic, dielectric, and piezoelectric properties. The resonators with different frequency were designed and evaluated. The main research contents and results are as follows:1. The exploration of flux systems for the growth of Ga3PO7crystal.The experiments of Ga3PO7crystal growth were carried out by the spontaneous nucleate method with the fluxes X2Mo3O10(X=Li, Na, K), X2W3O10(X=K), X2Mo2O7(X=Li), X2W2O7(X=Li) and X2MoO4(X=Li) respectively. The results indicate that the fluxes X2Mo3010(X=Li, Na, K) are suitable for the growth of Ga3PO7crystal. By comparing the flux systems X2Mo30io(X=Li, Na, K), the flux K2Mo3O10 had the minimum volatilization of MoO3and the maximum crystallization of Ga3PC>7. The spontaneous nucleate Ga3PO7has regular shape, well developed faces and the crystallinity was good. Therefore, the flux K2M03O10is the most suitable for the growth of large-sized Ga3PO7crystal.2. The growth of centimeter-sized Ga3PO7crystal by the top-seeded method.The single Ga3PO7crystals were grown using different orientated seeds from K2CO3-MoO3flux system by the top-seeded method. The orientations of different seeds were a random direction, a-direction, a random direction but perpendicular to c-direction and the direction perpendicular to the (101) face, respectively. All the as-grown Ga3PO7crystals were oriented and the{101},{021} and{012} faces were present in all the crystals. The c-axis polar growth of Ga3PO7crystal was analysed by the anion coordination polyhedron growth unit theory mode.The growth parameters were studied and optimized during the process of Ga3PO=7crystal growth. In the same growth period, cooling rate and thermal field, the spontaneous nucleate experiments were carried out with different solute concentrations. The results show that when the concentration is20wt%the crystallization of Ga3PO7is maximum. Considering the crystal size, crystal quality, the late orientation and utilization ratio, it can be concluded that the direction perpendicular to the (101) face is best. According to the experience and experinments of crystal growth, the applicable temperature field, proper cooling rate and the seed fixed form are determined.3. The characterizations of structure, morphology and detects for Ga3PO7crystal.The crystal structure was determined using a X-ray single crystal diffractometer (four-circle diffractometer). The result shows that Ga3PO7crystallizes in trigonal crystal system with the space group R3m, the unit cell parameters are a=b=7.897(3)A, c=6.757(6)A, Z=3. Its structure comprise PO4tetrahedra and GaO5trigonal bipyramids which are distorted in vary degrees. The dipole moments of both units were calculated. The component analysis of Ga3PO7crystal is using the x-ray energy spectrometer, and the result shows that the atomic number ratio(Ga:P:O) is26.54:8.94:64.52which is approximate with the theoretical value.According to the exposed{101},{021} and{012} faces, the spontaneous nucleate crystal shape and the crystal symmetry, we determined the ideal shape of crystal. The surface of as-grown crystal was investigated by optical microscopy and atomic force microscope. The grain boundary, inclusion, crack and irregular surface are observed. Combined with the crystal growth process, we analyze the reasons of the defect formation and put forward the corresponding solutions to improve the crystal integrity.(001),(110) and (101) faces are etched, the etch pits are observed by optical microscopy. The etch pits on (001) and (110) accord with three fold axis and symmetry plane respectively.4. The Raman spectrum of Ga3PO7crystal.The phonon modes of the Ga3PO7single crystal at room temperature were investigated by the Raman scattering technique. For x(zz)x, x(yz)x, x{yy)x z(yy)z and y(xx)y polarizations, the raman spectrum were observed corresponding to A1(TO), E(TO), A1(TO)+E(TO), A1(LO)+E(TO) and A,(TO)+E(LO) symmetries. A tentative mode assignment is given based on correlations with other PO4-based compounds. The TO-LO splitting is observed only for the v1, v3and v4internal PO4modes. We undoubtedly observed7A1(TO)+7E(TO) out of7A1(TO)+10E(TO) vibrations predicted.Using the high micro-Raman spectroscopy, Ga3PO7crystal, GaPO4crystal and their high temperature solutions were studied. When the temperature rose, the wavenumber in Ga3PO7or GaPO4raman spectrum shift to low frequency. The internal modes of the PO4were observed in boundary layer between the seed and the solution. It is confirmed that the PO4units is existing stably in the boundary layer and the corresponding growth mechanism is discussed.5. The measurements of basic properties for Ga3PO7crystal.The density of Ga3PO7crystal was measured to be4.8413g/cm3at30℃using Archimedes’method, and its temperature dependence was calculated. When the load was25GF, the vickers hardness of a direction and that of c direction were1107and920kgf/mm2respectively, which certified that the GasPO7crystal has enough mechanical strength. The measurement of the DTA/TG proved that the crystal would decompose into GaPO4and Ga2O3up to1348.0℃. The specific heat, thermal diffusion and thermal expansion of Ga3PO7crystal were tested. All the thermal testing results show Ga3PO7has good thermal stability. The transmittances of different wafers are above80%in the wavelength range200nm-2800nm. The room temperature mid-IR spectra of the as-grown crystal indicates that the peaks at1141.59and1095.17cm-1correspond to PO4asymmetric stretching vibration modes, the peak at964.25cm-1is assigned to the symmetrical stretching band of the PO4group, and the peaks at702.94and657.49cm-1are both attributed to the asymmetric bending deformation vibration.The crystal is stable and nonhygroscopic, and it cannot be dissolved in nitric acid and muriatic acid at room temperature. But it can be dissolved in boiling phosphate acid, therefore the phosphate acid is the corrosive agent of Ga3PO7crystal.6. The electro-elastic properties and the piezoelectric device design of Ga3PO7crystal.All the10crystal samples, which are used to measure the dielectric, piezoelectric and elastic constants of Ga3PO7crystal, have been designed by coordination rotation method. At room temperature, the dielectric, piezoelectric and elastic constants of Ga3PO7crystal were measured. The results are as follows:The temperature dependence of all the electro-elastic constants, including dielectric constants, piezoelectric constants, elastic constants and coupling coefficients, have been measured in the temperature range-50～120℃. By comparing the first-, second-and third-order temperature coefficients of the elastic constants, it can be confirmed that the elastic constant s11is the most stable in the whole temperature range.The400K and2M resonators were designed, and the temperature dependence of the resonators were measured in the temperature range-50～120℃. The results indicate that yx-cut plate is the most stable and potential in piezoelectric application.This work researched the growth technology and growth process parameters in depth. The optimization results are as follows:K2Mo3O10flux is the best solvent, solute concentration is20%wt, the seed direction is perpendicular to the (101) face, and the growth temperature range is from975℃to850℃. Ga3PO7crystal is an noncentrosymmetric piezoelectric crystal in Ga2O3-P2O5binary system. It crystallizes in trigonal crystal system with space group R3m, a=b=7.8890(1)A, c=6.7314(2)A, Z=3. At30℃the density of Ga3PO7crystal is4.8413g·cm-3. When the load is25GF, the hardness of a direction and that of c direction are1107and920kgf/mm2respectively. The room temperature raman spectrum are tested, and the assignments for Ga3PO7crystal are as following:the vi modes at959and1001cm-1, the v2mode at484cm-1, the v3modes at1095,1127,1139and1208cm-1, and the v4modes at678,706,716and773cm-1. By using the high micro-Raman spectroscopy, the internal modes of the PO4were observed in the boundary layer between the seed and the solution. It is confirmed that the PO4unit is existing stably in the boundary layer. The thermal properties of Ga3PO7crystal are measured and the results are as follow:at20℃the specific heat is0.491J/(g-K), in the temperature range35℃～494℃, the thermal expansion coefficients of a and c directions are2.484×10-6K-1and3.794×10-6K-1respectively, at30℃the thermal conductivities of a and c directions are5.144W/(m·K) and5.446W/(m·K) respectively, when the temperature is up to1348.0℃, the Ga3PO7crystal decompose into Ga2O3and GaPO4before melting. The optical properties are also measured. The transmittances of different wafers are above80%, which indicate that the Ga3PO7crystal has high transmittance. The room temperature mid-IR spectra of the as-grown Ga3PO7crystal indicates that the peaks at1141.59and1095.17cm-1correspond to PO4asymmetric stretching vibration modes, the peak at964.25cm-1is assigned to the symmetrical stretching band of the PO4group, and the peaks at702.94and657.49cm-1are both attributed to the asymmetric bending deformation vibration. The frequency doubling effect of the Ga3PO7crystal powder is too low to apply to nonlinear optical field. The electro-elastic properties of crystal are researched. The relative dielectric constants en and ε33are11.08and7.73, the piezoelectric constants d22and d15are4.3and7.7pC/N, and the elastic constants S11、S12、S13、S14、S33、S44and S66are5.84、-0.35、0.84、0.70、4.98、9.92and12.38pm2/N, respectively. By comparing the first-, second-and third-order temperature coefficients of the elastic constants, it can be confirmed that the elastic constant s11is the most stable in the whole temperature range. And yx-cut plate is the most stable and potential in piezoelectric application.