Growth Of Ca_xBa_1-xNb₂O₆Crystals By The Optical Floating Zone Method And Properties

Ca_xBa_1-xNb₂O₆（CBN）cryatals with the structure of the tetragonal tungstenbronze（TTB） are provided with the excellent piezoelectric, dielectric, ferroelectric,electro-optic, photorefractive properties. The Curie temperature of CBN crystals is inthe range of124～347℃with decreasing Ca content from0.366to0.224, that arehigher than the familiar Sr_xBa_1-xNb₂O₆（SBN）crystals, whose Curie temperature is inthe range of25～200℃with the variation of Sr content. In the design of devices,CBN crystals have higher temperature stability. This paper mainly includes thefollowing five aspects research:1. Cryatal growthPure phase, compact and uniform CBN polycrystalline rods with variouscomponents were prepared by using solid phase synthesis method. Transparent andpale yellow CBN crystals with six different actual components (x_Ca=0.224，0.246,0.284,0.325,0.351,0.366)were grown out by optical floating zone method in theappropriate conditions of the growth temperature, the rate of heating and cooling,feeding and growth, and rotation. The main factors affecting the quality of growncrystals in the growth procedure were investigated. To sum up, that synthesizing CBNpolycrystalline rods with high quality is the top priority, synthesizing high-qualitycrystal seed with no cracks and inclusions by using the technique of necking andexpanding shoulder must be the key, and growing out high-quality rodlike singlecrystal by using appropriate growth parameters is the guarantee.2. Quality characterization of CBN crystalsCBN crystals are transparent and pale yellow with no macro defects afterannealing in the O2. The quality of grown crystals decreases with increasing Cacontent due to segregation effect, for example, there are some cracks in the CBN-38crystal. The actual composition of CBN crystal were determined by EPMA, Thecomponents uniformity of the grown CBN-35crystal rod were analysed. The rockingcurves of CBN crystals on （001） and （100） planes were measured.3. Crystal structure analysisThe as grown crystals were single crystals checked by X-ray single crystaldiffraction instrument. The lattice structure projection maps along [001]，[010]，[100]-orientation were presented. The powder XRD patterns of the grown CBN crystals were measured. The crystals were checked to be pure phase structure, the crystal cellparameters of CBN crystals were obtained, the density of CBN crystals weremeasured by buoyancy method and the theoretical density of CBN crystals werecaculated by using the data of the crystal cell parameters.4. Spectrum of CBN crystalsThe transmittance spectra of CBN crystals were measured by usingspectrophotometer with the wavelength range of190～2500nm, and the transmittanceis above80%with increasing wavelength from about500nm with wide transparencyranges covering visible and near-infrared lights. The room temperature Micro-Ramanspectrum of （100） cut-type specimens of CBN crystals were measured and the Ramanspectrum for CBN-22crystal were recorded in different polarizations.5. Electrical properties of CBN crystalsThe relative permittivity and loss tangent of CBN （001） and （100） cut-typespecimens at room temperature and in the temperature range of25～430℃on fourfrenquencies of1kHz,10kHz,100kHz and1MHz were measured. The Curietemperature of CBN crystals were confirmed. The dielectric properties were obviousanisotropy, and showed rela_xation behavior. The ferroelectric hysteresis loops on （001）plane of CBN crystals were measured, CBN-38crystal shows good ferroelectricity,The spontaneous polarization, remanent polarization and coercive field of CBN-38crystal are27.4μC/cm²,25.5μC/cm²and14.4kV/cm respectively. The piezoelectricstrain constants d₃₃, d₁₅and d₃₁of CBN crystals were measured by resonancean-tiresonance method, with increasing Ca contant, d₃₃and absolute value of d₃₁increase, d₁₅decreases. The piezoelectric strain constant d₃₃and d₃₁of CBN-38are90.0pC/N and-54.7pC/N respectively, d₁₅of CBN-22is89.2pC/N. d₃₃of CBN-38crystals measured by ZJ-4A Berlincourt-type quasi-static d₃₃meter is90.0pC/N.