【作者】 徐民；

【导师】 王继扬； 张怀金；

【作者基本信息】 山东大学 ， 材料学， 2008， 硕士

【摘要】 六、七十年代人们就认识到三价金属钨酸盐和钼酸盐包括一批集荧光、激光、压电、铁弹、铁电性质于一身的材料。Ln₂（MO₄）₃被用来表示这一材料家族，其中，Ln代表稀土元素或其他三价元素，M代表W或者Mo。在这些材料中，钼酸钆[Gd₂（MoO₄）₃]晶体研究得比较充分，而与Gd₂（MoO₄）₃晶体具有相同结构的钼酸铽[Tb₂（MoO₄）₃]晶体的生长和基本性质的研究还没有报道。因此，本论文对Tb₂（MoO₄）₃晶体的生长、结构、缺陷、基本热学性质、光学性质及其铁电性质进行了系统的研究，对掺杂Co³⁺、Cr³⁺离子Tb₂（MoO₄）₃晶体的生长进行了探索，主要包括以下几方面的工作：一、晶体生长采用纯度为99．99％的Tb₄O₇和MoO₃，通过固相反应合成出Tb₂（MoO₄）₃多晶原料，利用熔体提拉法沿c轴成功地生长出了大尺寸、无开裂、高质量的Tb₂（MoO₄）₃单晶。由于晶体生长是一个复杂的物理—化学过程，我们结合晶体生长热力学和动力学规律，系统讨论了影响晶体生长和晶体质量的主要因素。包括优质多晶原料的制备和建立合理温场的设计，控制合适的生长工艺参数，选用优质籽晶，消除外界因素的影响等。同时针对影响晶体生长不同问题的产生原因和特点，提出了相应的解决方法。二、晶体组分、结构与缺陷采用X射线荧光分析法分别测量了Tb元素和Mo元素在生长的Tb₂（MoO₄）₃单晶中的浓度。测量结果表明所生长的Tb₂（MoO₄）₃单晶接近化学计量比Tb₂（MoO₄）₃。利用X射线粉末衍射对生长的Tb₂（MoO₄）₃晶体进行了结构研究。根据X射线粉末衍射结果，确定生长的Tb₂（MoO₄）₃晶体属于正交晶系，Pba2空间群，晶胞参数为α=β=γ=90°，a=10．3404 （?），b=10．3798 （?），c=10．6467 （?）。首次采用化学腐蚀法、扫描电子显微形貌术（SEM）和高分辨X射线衍射对Tb₂（MoO₄）₃晶体的缺陷进行了观测，并结合晶体生长过程探讨了缺陷的形成机制，为生长优质晶体提供了依据。在Tb₂（MoO₄）₃晶体中存在的主要缺陷为开裂、位错和镶嵌物。这些缺陷的存在影响了晶格的完整性。我们根据晶体缺陷的研究结果，针对这些缺陷的形成机制，采取了相应措施，不断改进生长工艺条件，有效地减少或消除了晶体中的这些缺陷。三、晶体的基本性质测量了Tb₂（MoO₄）₃晶体的密度、硬度和热学性质，并讨论了这些基本性质对晶体生长和应用的影响。采用浮力法测得Tb₂（MoO₄）₃晶体的密度为4．611 g·cm^-3。我们首次测量了Tb₂（MoO₄）₃晶体的显微硬度，结果表明Tb₂（MoO₄）₃晶体的硬度较低，加工比较容易。系统地研究了Tb₂（MoO₄）₃晶体的热学性质。通过差示扫描量热法测量出Tb₂（MoO₄）₃晶体的熔点为1167．75℃，熔化热为13186．2 J／mol，熔化熵为9．15J·K^-1·mol^-1。在54℃到162℃的温度范围内Tb₂（MoO₄）₃晶体的比热为0．3944J·g^-1·K^-1到0．4246 J·g^-1·K^-1。采用热膨胀仪在28～496℃的温度范围内测量了Tb₂（MoO₄）₃晶体的热膨胀，室温到居里点热膨胀系数主值分别为α_a=3．74×10^-5／K，α_b=2．74×10^-5／K和α_c=-1．58×10^-5／K。我们首次测量了Tb₂（MoO₄）₃晶体的热扩散系数并计算了其热导率。在30℃时，Tb₂（MoO₄）₃晶体沿[100]、[010]和[001]方向的热扩散系数分别为0．604 mm²．s^-1、0．631 mm²．s^-1和0．644 mm²．s^-1，相应的热导率为1．152 W·m^-1·K^-1、1．1715 W·m^-1·K^-1和1．2065 W·m^-1·K^-1。讨论了热学性质对Tb₂（MoO₄）₃晶体的生长和应用所产生的影响。四、光学性质和铁电性质测量了Tb₂（MoO₄）₃晶体的室温透过谱，结果表明该晶体透过波段范围较宽，覆盖了可见和近红外波段，并且发现在486nm处有比较明显的吸收。使用棱镜耦合仪测试了Tb₂（MoO₄）₃晶体在632．8nm时的折射率，测试结果为n₁=n_a=1．8479，n₂=n_b=1．8483，n₃=n_c=1．8990，表明Tb₂（MoO₄）₃晶体为正光性的双轴晶。（001）面的锥光干涉也验证了这一点。利用阻抗分析仪测试了Tb₂（MoO₄）₃晶体电容随温度的变化曲线，确定了Tb₂（MoO₄）₃晶体的居里温度为162．3℃。由此计算了该晶体C方向的介电常数ε₃₃，发现在低频条件下介电常数的反常才比较明显。通过化学腐蚀，观察到了Tb₂（MoO₄）₃晶体的畴结构为90°畴。在频率f=2 Hz，温度t=110℃的条件下测得了Tb₂（MoO₄）₃晶体的电滞回线。结果显示，其回线不饱满，存在一定的内偏场强。五、掺杂Co³⁺、Cr³⁺离子钼酸铽晶体的生长探索进行了掺杂过渡金属磁性离子Co³⁺、Cr³⁺：Tb₂（MoO₄）₃晶体的生长的探索，但总的来说并未获得良好的实验结果。更多还原

【Abstract】 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 0.604mm²·s^-1, 0.631 mm²·s^-1and 0.644 mm²·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.更多还原