Growth And Characterization Of Terbium Molybdate Crystal

The tribalent metal tungstates and molybdates include a number of crystals with interesting fluorescent, laser, piezoelectric, ferroelectric and ferroelastic properties. The chemical formula of Ln₂(MO₄)₃ was used to denote the crystal family, where Ln is a rare earth lanthanum element or other trivalent element and M is W or Mo. There has been many reports on research of the Gd₂(MoO₄)₃ crystal, however, there was only few report on the studies of the growth and basic properties on Tb₂(MoO₄)₃ crystal which is isomorphous with Gd₂(MoO₄)₃ crystal according to our knowledge. In this dissertation, the growth, structure, defects , themal, optical and ferroelectric properties of Tb₂(MoO₄)₃ crystal are systemically studied, the growth of Co³⁺, Cr³⁺ doped Tb₂(MoO₄)₃ crystals were also explored. The outline of this dissertation is as follows:1.Crystal GrowthTb₂(MoO₄)₃ polycrystalline material was synthesized by means of a solid-state reaction with 4N pure Tb₄O₇ and MoO₃. Crack free Tb₂(MoO₄)₃ single crystals with high quality and large dimensions were successfully grown along c-axis direction by using Czochralski method.Since crystal growth is a complex process, the factors which are related to the quality of crystals during the growth were analyzed, based on the theories of thermodynamics and dynamics. High quality polycrystalline material and a suitable field temperature are the precondition to grow an excellent crystal; the optimal procedure is the key for crystal growth; using a good crystal seed can ensure the quality of crystals. We also presented the corresponding solutions related to these exoteric factors.2. Crystal Composition, Structure and DefectsThe concentrations of Tb and Mo elements in the as-grown Tb₂(MoO₄)₃ single crystal were measured by X-ray fluorescence analysis method. The results indicate that as-grown Tb2(MoO₄)₃ single crystal is closed to stoichimetric Tb₂(MoO₄)₃.The structure of as-grown Tb₂(MoO₄)₃ single crystal was studied by X-ray powder diffraction method. The X-ray powder diffraction confirmed that as-grown Tb₂(MoO₄)₃ single crystal belongs to the orthorhombic crystal system, Pba2 space group with unit cell dimensionsα=β=γ=90°, a=10.3404(?), b=10.3798(?), c=10.6467(?). Defects in the as-grown Tb₂(MoO₄)₃ single crystal have been investigated for the first time by using chemical etching method, SEM topography and high-resolution X-ray diffraction. The main defects in the as-grown Tb₂(MoO₄)₃ single crystals are cracks, dislocations and inclusions. These defects seriously reduced the perfection of crystals. The formation mechanism of the defects has been discussed and this will be great helpful for growing high quality single crystals. In order to avoid or decrease these defects, we take some corresponding steps according to the above results.3. Basic PropertiesThe density, hardness and thermal properties of the as-grown Tb₂(MoO₄)₃ single crystal were measured, and the influences of these properties on crystal growth and applications were also discussed.The density of Tb₂(MoO₄)₃ crystal was measured to be 4.611 g·cm^-3 by using the buoyancy method at room temperature.The microhardness of Tb₂(MoO₄)₃ single crystal was measured for the first time.The results indicate that the single crystal is soft and more easily processed.The thermal properties of Tb₂(MoO₄)₃single crystal were systemically studied. The melting point, enthalpy of fusion, entropy of fusion were measured by the method of differential scanning calorimetry, which are 1167.75℃, 13186.2 J/mol and 9.15J·K^-1·mol^-1 respectively. The specific heat values are 0.3944 J·g^-1·K^-1-0.4246J·g^-1·K^-1 from 54℃to 162℃. The thermal expansion of Tb₂(MoO₄)₃ single crystalwas measured by using a thermal dilatometer from 28℃to 496℃, and the average linear thermal expansion coefficient for all three crystallographic directions were calculated in the temperature range from room temperature to the Curie temperature, which areα_a=3.74×10^-5/K,α_b=2.74×10^-5/K andα_c=-1.58×10^-5/K respectively. The thermal diffusion coefficient of Tb₂(MoO₄)₃ single crystal was measured for the first time by the laser flash method in the temperature range from 30 to 270℃, and then the thermal conductivities were calculated. At 30℃, the thermal diffusion coefficientsare 0.604mm²Â·s^-1, 0.631 mm²Â·s^-1and 0.644 mm²Â·s^-1 along [100], [010] and [001] directions. The calculated thermal conductivities of Tb₂(MoO₄)₃ single crystal are 1.152 W·m^-1·K^-1,1.1715 W·m^-1·K^-1 and 1.2065 W·m^-1·K^-1 along [100], [010] and [001] directions at 30℃. The influences of these thermal properties on crystal growth and applications were also discussed.4. Optical and Ferroelectric PropertiesThe transmission spectrum of Tb₂(MoO₄)₃ single crystal were measured at room temperature and the result shows that Tb₂(MoO₄)₃ single crystal has a wide spectral transparency range and there is an absorption peak at 486nm.A prism-coupler was used to measure the refractive indices of Tb₂(MoO₄)₃ single crystal at the wavelength of 632.8nm, and its value are n₁=n_a=1.8479, n₂=n_b=1.8483 and n₃=n_c=1.8990 respectively. The results indicate that Tb₂(MoO₄)₃ single crystal is optically biaxial, positive crystal. Its conoscopic diagram can also be used to confirmed the validity of such conclusion.The temperature dependence of capacity along [0 0 1] direction for Tb₂(MoO₄)₃ crystal has been studied, which shows the Curie temperature is 162.3℃. And then we calculated the dielectric constant along [0 0 1] direction,ε₃₃ at different frequencieswith increased temperature. A visible dielectric anomaly can be observed at low frequencies.The domain boundaries were observed by using chemical etching method. The domain structure of Tb₂(MoO₄)₃ single crystal is 90°domain wall.The polarization-electric field (P-E) hysteresis loop for a Tb₂(MoO₄)₃ plated sample was measured at the temperature of 100℃and the frequency of 2 Hz. The result shows that loop is unsaturated and there is certain internal bias field in the sample.5. Exploration of the Co³⁺, Cr³⁺ doped Tb₂(MoO₄)₃ Crystal GrowthWe explored the process of magnetic Co³⁺, Cr³⁺ ions doped Tb₂(MoO₄)₃ crystal growth, however, the results were unsatisfied.