Flux Growth, Structure And Properties Investigation Of Ga₃PO₇ Crystal

Since piezoelectric effect was firstly found in quartz crystal in 1880 by the Curie brothers,various piezoelectric devices made by piezoelectric crystals,such as piezoelectric oscillator,piezoelectric transducer,piezoelectric presure sensor, electroacoustic transducer and ultrasonic transducer,have been successfully applied in the fields of electric,laser,communication and aeronautical technology,and piezoelectric crystals have been regarded as an important function materials.Quartz homotypic single crystal gallium orthophosphate(GaPO₄) is a new piezoelectric material which possesses nearly all the advantages of quartz,but has a much higher thermal stability,a higher piezoelectric effect,larger electromechanical coupling constants than quartz.There are only small changes of the physical properties up to 933℃,this property makes it a very attractive material for high temperature piezoelectric devices.Because it exists the temperature-compensated cuts like quartz,it is a promising material for bulk acoustic wave(BAW) and surface acoustic wave (SAW) applications and has a wide temperature ranges.Moreover,with the rapid development of high-speed electronic and communication,it will has significant science meanings and practical values to research new piezoelectric crystals with improved properties and easy to be grown.Recently,crystalline trigallium phosphate(Ga₃PO₇) has attracted our attention due to its special crystal structure.It crystallizes in a noncentrosymmetric trigonal crystal system with space group R3m,a=b=7.885(1)(?),c =6.727(1)(?),Z=3,which belongs to the same space group with LiNbO₃ and LiTaO₃, and belongs to one of the ten polar crystal systems that have excellent optical and electromechanical properties,it is a promising candidate for piezoelectric,pyroelectric and optical applications.Single crystal Ga₃PO₇ was first synthesized by Boudin et al.in 1997 using hydrothermal synthesis at high temperature(773K) and high pressure (210×10⁶Pa).Up to now,there have been no other reports of Ga₃PO₇ single-crystal growth and properties characterization except for the above-mentioned work.In this work,we do some research on the growth of GaPO₄ crystals and Ga₃PO₇ crystals by flux method.Centimetre sized and high quality trigallium phosphate single crystals,Ga₃PO₇,have been grown for the first time by flux method.The growth process parameters,physical and chemical properties,especially the thermal,optical and electric properties of Ga₃PO₇ crystals are studied systematically,and the main research contents and results are as follows:1.Growth of GaPO₄ crystals by flux method:Firstly,a spontaneous nucleation method is used to find some proper solvents,after a lot of experiments,three different solvents which can be used to grow GaPO₄ are obtained,MoO₃—Li₂CO₃,MoO₃—K₂CO₃ and KPO₃—MoO₃—LiF.IR spectrum was carried out between 4000 and 2800 cm^-1,which is used to calculate the absorption coefficient of OH^-.We find that OH^- in the crystal can be avoided by using flux method.In order to obtain large size GaPO₄ crystals,the top-seeded solution growth(TSSG) slow-cooling method is also used.However,it is difficult to get large size single crystal,because of the volatilization of the solvents,especially the component of MoO₃,which will destroy the component of the sulution and induce the change of supersaturation on the surface of the solution.2.Growth of Ga₃PO₇ crystals by flux method:Firstly,a spontaneous nucleation method is used to find the proper solvent,after a lot of experiments,millimeter sized Ga₃PO₇ crystals have been grown by a spontaneous nucleation method using Li₂CO₃-3MoO₃ as flux.On the basis of the millimeter size crystal,the top-seeded solution growth(TSSG) slow-cooling method is used to grow large size Ga₃PO₇ crystal.The growth process parameters are systematic studied.The results show that the proper concentration of the solution is 15wt%—20wt%,the rotation speed of the seed crystal is 30rpm.The crystal shows obviously anisotropy during the growth process,the orientation of perpendicular to(101) face is much better than the c-orientation.Transparent single crystal Ga₃PO₇ with size of 23×20×15 mm³ has been grown by the top-seeded solution growth(TSSG) method from the Li₂CO-3MoO₃ flux system with the optimized process parameters.3.A series of physical and chemical properties of Ga₃PO₇ crystal are characterized, such as structure,density,hardness and stability.The crystal structure was determined using a Bruker APEX2 CCD area-detector diffractometer,the result show that Ga₃PO₇ crystallizes in the trigonal crystal system with a noncentrosymmetric space group R3m,the unit cell parameters are a= b=7.897(3)(?),c=6.757(6)(?).There are no distinct differences in our work from the above mentioned.The density of Ga₃PO₇ crystal was measured to be 4.874 g/cm³ using Archimedes' method.The hardness test of Ga₃PO₇ crystal shows that it belongs to harder materials category,it's Mohs' hardness value is about 6.5,which is slightly lower than that of quartz.The crystal is stable and nonhygroscopic in the air, and it cannot be dissolved in dilute nitric acid,muriatic acid at room temperature.It can be dissolved in boiling phosphate.4.Thermal properties of the as-grown Ga₃PO₇ crystal have been carefully studied by measuring the DTA/TG,anisotropic thermal expansion,specific heat,thermal diffusion and thermal conductivity.The TG/DTA test shows that Ga₃PO₇ is stable over the temperature range of 25 to 1364.8℃with no mass change.It will be decomposed when the temperature is above 1364.8℃,and that its melting point was not reached.Ga₃PO₇ crystal possesses low thermal expansion coefficients and good thermal conductivity.The calculated average thermal expansion coefficients along a- and c-axis areα_a=1.66×10^-6K^-1,α_c=3.53×10^-6K^-1 from 293.15 to 773.15K.The thermal conductivities along a- and c-axis are up to 6.34Wm^-1K^-1,6.18 Wm^-1K^-1 at the temperature of 303.15K.It will be beneficial for the thermal transmitting in the bulk crystal.The crystal will have a stable application under the high temperature.5.The optical properties of the as-grown Ga₃PO₇ crystal have been carefully studied, including transmittance,refractive indices and the nonlinear optical effect.The results show that the Ga₃PO₇ crystal has a wide transmission range from 215 nm to 4300 nm,and the transmittance is above 80%over the range of 300nm to 3750nm. The refractive index of Ga₃PO₇ is decreasing with the increasing of the wavelength, and it's refractive index n_o=1.7849 and n_e=1.7739 are obtained at the wavelength of 633nm.The smaller difference between the refractive indices values of n_o and n_e indicated that the Ga₃PO₇ crystal is an optically uniaxial negative crystal.The result of the powder frequency-doubling experiment shows that there is no obvious frequency-doubling effect,and the crystal can not be used in the field of nonlinear optical application.6.The free dielectric constants,piezoelectric constants and elastic constants are measured through designing reasonable experiments.The capacitances of x-cut and z-cut wafers are measured by Agilent HP 4279A LCR meter,and then the free dielectric constants are calculated.The piezoelectric constants are characterized using a ZJ-3A Berlincourt-type quasi-static d33 meter,and the elastic constants are measured by resonance method.Moreover,the pyroelectric coefficient of Ga₃PO₇ is also tested by measuring the pyroelectrie current.The results of dielectric constants, piezoelectric constants,elastic constants and pyroelectric coefficient are compared with other common piezoelectric crystals.From the comparison,we can clearly see that Ga₃PO₇ has the best temperature stability up to 1364.8℃,and the piezoelectric constants are as good as GaPO₄ and greater thanα-SiO₂.Therefore it is a very promising candidate for use as a high temperature piezoelectric material.However, because of the small pyroelectric coefficient,it cannot be used in the fields of pyroelectric detector and pyroelectric vidicon.