Growth And Properties Of Relaxor Ferroelectric Calcium Barium Niobate Series Crystals

Many niobate crystals with the tetragonal tungsten bronze (TTB) structure possess excellent piezoelectric, electro-optical and nonlinear optical properties. Among these crystals, SrxBa1-xNb2O6 (SBN) has been widely investigated. But their applications at high temperature have been limited because of the relatively low phase transition temperature (in the range of 25 to 120℃, depending on the composition). Recently, CBN-28 (Cao.28Bao.72Nb206) crystal has been grown by using Czochralski technique. It is found that the Curie temperature of CBN-28 is 260℃, much higher than SBN crystal (close to 200℃), which implies that CBN-28 crystal may be applied potentially over the high temperature range. There are several vacancies exists in the TTB niobate crystal. Thus, crystal quality and other proprieties can be improved by means of ions dopant or further molecular design. However, the doped CBN crystals have never been reported to our best knowledge. Therefore, it is necessary to study the doped CBN crystals. Based on these, we studied the doped and undoped CBN crystals in this dissertation and main contents are listed as following:1. Crystal GrowthThe polycrystalline materials used for the crystal growth were synthesized by the solid phase reaction method. The series of CBN crystals were grown using the Czochralski technique by selecting the appropriate crystal growth parameters, such as proper heating and cooling rate, the pulling and rotation speed. The series of CBN crystals are CBN-20, CBN-40, KCBN, NCBN, KNCBN, Nd:CBN-28 six different crystals, respectively. The growth process of the CBN crystal has been introduced. The main factors that influence the crystal growth and crystal quality also have been discussed. Among them, the synthesis of high-quality polycrystalline material and a reasonable temperature field are the prerequisite for the growth of high-quality single crystal, controlling appropriate parameters is the key to crystal growth and using good quality seed is very important to improve the crystal quality. The content of each element in Nd:CBN-28 and KNCBN crystals were determined by using the X-ray fluorescence spectroscopy. The segregation coefficients of each element were also calculated using the corresponding formula.2. Crystal Structure AnalysisThe diffraction data of the CBN crystals were collected on a CCD diffractometer at room temperature. These CBN crystals crystallize in the tetragonal tungsten bronze structure with P4bm space group and 4mm point group. The structure analysis also gives the crystal coordinates, bond lengths, bond angles and displacement parameters of all the atoms. The bond length and bond angle of NbO6 octahedral change regularly as the Ca2+ concentration increasing in three undoped CBN crystals. Na+ and Ca2+ occupy the same position in the Na-doped CBN crystal, while K+ and Ba+ occupy the same position in the K+ doped CBN crystal. Thus, it is assumed that Na+ and Ca2+ occupy the same site and K+ and Ba2+ occupy the same site in KNCBN crystal. Then the structure of KNCBN crystal was solved. In Nd:CBN-28 crystal, Nd3+ ions occupy the position of Ca+ ions, resulting in cation vacancies. The measured XRD results of all CBN crystals are consistent with the results fitted using the single crystal data. X-ray photoelectron spectroscopy (XPS) has been performed to explore the effect of Nd-doping on the local electronic environment in the CBN-28 crystal. The content of oxygen vacancies is increased after Nd-doping in CBN crystals. The relative components of CBN-28 and Nd:CBN-28 crystals can also be obtained by XPS quantitative analysis.3. Thermal PropertiesThe Curie temperature of the CBN crystals, including CBN-20, CBN-28, CBN-40, Nd:CBN-28, NCBN, KCBN, KNCBN, were determined to be 294,247,184,215,305, 250,319℃by specific heat measurement, respectively. The Curie temperatures of the undoped CBN crystals are decreased with the increase of Ca2+ concentration. The Curie point of the CBN crystal is increased when K+ or Na+ is doped. But the Curie temperature has been greatly improved with Na-doping. The Nd:CBN-28 crystal possesses lower Curie temperature compared to the undoped CBN-28 crystal.The CBN crystals exhibit similar thermal expansion characteristics. Below the phase transition temperature, the CBN crystals have positive thermal expansion along the a-axis, while the crystals have negative thermal expansion along the c-axis. But above the phase transition point, the thermal expansion are positive both alongα-and c-axis. The anisotropy in thermal expansion is obvious.The density of the CBN crystals were measured by the buoyancy method at room temperature, which is in good agreement with the theoretical value. The density vs. temperature curves among these crystals have been obtained by using formula and thermal expansion ratio. Despite of shrinking along c-axis, the volume thermal expansion is still positive, because thermal expansions along a-axis is larger than the thermal shrink along c-axis. Therefore, the density of crystal is decreased with increasing temperature.The thermal diffusion coefficients of the CBN crystals were measured by the laser flash method and then the thermal conductivities were calculated by using formula K=λpCp. Thermal conductivitis of CBN crystals tend to be similar near the Curie point whether the measurement is along c-axis or a-axis and they have obvious anisotropy deflecting the Curie point. The results show that the thermal conductivity of CBN crystals increases with increasing temperature, which indicates some glass-like characteristics in such crystals. The glass-like behavior is due to the disordered tungsten bronze structure giving rise to an effectively constant phonon mean free path l. The phonon mean free path la and lc along theα-and c-axes of Nd:CBN-28 are estimated, and they are on the order of the unit cell dimensions. 4. Spectra of CrystalsThe transmission spectra of CBN crystals were measured at room temperature. The results show that these crystals have a wide spectral transparency range, covering ultraviolet, visible and infrared bands. The transmittance of CBN crystals can reach 60%. The CBN crystals belong to the tetragonal tungsten bronze structure crystal. There are five formula in their primitive cell. Thus, there are 135 basic lattice vibration mode in CBN crystals. The space group theoretical analyses has been made on the CBN crystals. Polarized Raman spectra have been measured at room temperature by using a laser Raman scattering spectrophotometer. The four backscattering geometry configurations for measuring Raman spectra were X(ZZ)X,X(YY)X, Z(XY)Z and X(YZ)X respectively. Raman characteristic peaks arising mainly from internal vibrations of the Nb-O anionic cluster have been identified from vibration spectra. As the Ca2+ concentration increased, the crystal peaks broaden and move to the low wave number. The FWHM of Raman peak is related to the NbO6 octahedral distortion degree (△[Nb-O]). The changes of Nb-O bond length and bond angle can be related with the Nb-O vibration frequency.5. Dielectric and Ferroelectric PropertiesThe temperature dependences of capacity alongα-and c-orientation for CBN crystals have been measured. And then we calculated the dielectric constant at different frequencies with increasing temperature. The Curie temperatures also have been determined. The dielectric properties of the crystals have significant anisotropy, and also show relaxation features. Dielectric spectra at 1 MHz along the c-orientation were fitted according to the phenomenological theory. The fitting parameters are given. The relaxation properties of the CBN crystals were studied. The domain boundaries were observed by using chemical etching method with (100) and (001) wafers. The domain structure of CBN crystal is 180°domain wall. The polarization-electric field (P-E) hysteresis loops for CBN crystals were measured with the frequency of 100 Hz at room temperature. The result shows that the loop of CBN-40 crystal is saturation and with small leakage current. The spontaneous polarization, remanent polarization and coercive field have been determined to be Ps = 3.8μC/cm2, Pr= 2.9μC/cm2 and Ec= 12.5 kV/cm, respectively. The hysteresis loops of CBN-40 crystal at different temperature were also measured. An anomaly behavior can be found appearing near the Curie temperature (180℃or so), where the measured Ec reaches to the minimum while the Ps reaches to the maximum. A complete set of dielectric, piezoelectric and elastic constants of CSBN-25 has been obtained using resonance techniques and impedance analysis at room temperature. The relative dielectric constants areε11=343 andε33=502. Our results show that CSBN-25 has excellent piezoelectric properties with d33=94.3, d31=-31.1, and d15=16.6 pC/N. The elastic compliance constants are:s11=5.1, s12=-4, s13=1.4,s33=5.1, s55=13.2, and s66=19 pm2/N.6. Superprism phenomenon of Nd:CBN-28 crystalWe have shown the ability of Nd:CBN-28 as a new nonlinear crystal for SHG in a continuous and broad range of wavelengths covering the visible spectrum without angle or thermal tuning. The nonlinear properties of this system are based on the particular micro-structure of 180°sized square prism shaped domains appearing naturally in the crystal with a broad width distribution. The large wavelength dispersion obtained for the case of conical SHG allows considering our system as a "nonlinear" superprism, with variations in the SHG angle as high as 35°in the whole range SHG wavelength range (430-660nm). The CBN crystal appears as a relevant system with potential applications as a functional optical stable device that can be fabricated in a simple way and with low cost.