LaCoO₃ And TiO₂ Single Crystal Grown By Floating Zone Method And Their Electromagnetic Properties

The natural materials are mostly in the form of microcrystal. The single crystals as large as natural gems is minimal. Therefore, the development and application history of crystals is much shorter than that of polycrystals. Because of the single crystals scarcity in nature and demands for people, so the study of the crystal growth started in the nineteenth century. Today, almost all of the mineral crystals can be grown or synthetic by artificial methods, and a mass of single crystals which does not exist in nature have been grown by artificial methods.Today, the quality of artificial crystals such as diamond has been greatly exceeds the natural, and a mass of large crystals which does not exist in nature can be synthesized such as laser crystals and silicon, etc. With the development of modern science and technology, the requirements of crystal materials are improved. This promotes the development of some high-performance crystals, so that crystal materials as an industry play a very important position in the field of materials science, the national economy and high-tech, and this trend continues incessantly. The significance of crystal growth, the first is in the development of high technology, the crystals are important basic substances in the information age, is the supported matrix of mutual transformation among sound, light, heat, electricity, magnetism and force, it has extensive and important applications in the sustainable economic development and national defense construction. The second is in the basic research, many of the basic properties of materials are only carried out by crystals to research and to apply. Although the development of new crystal materials have made significant progress, but there is still a great disparity with the need of technology development. And the species, amount and cubage of crystals lie a place unsatisfactory, so the study of the crystal growth is very important and meaningful in present and future.Floating zone method is a melt growth method, starting from 1950s, first used in the growth of single crystal silicon. Because of non-polluting and fast growing without the crucible, it is widely used in various crystal growth, in particular used in the crystal growth with strong response and high melting oxides and intermetallic compounds. In the past half of century, the floating zone method has been improved and innovated constantly. Now the optical floating zone crystal growth furnace is widely used with elliptical mirrors and halogen lamps or xenon lamps. The melt zone remains stable mainly by the surface tension, and the crystal grows along the vertical direction.LaCoO₃ is a typical ABO₃ perovskite-type compounds. It has excellent chemical stability and good electrical conductivity under high temperature, due to their potential applications in energetics both as cathodes and solid electrolytes in solid oxide fuel cells, and both in environmental catalysis for pollutant decomposition and colossal magnetoresistance materials. LaCoO₃ has attracted considerable interest in the past due to its unique low-temperature magnetic behavior. TiO₂ is the most white thing in the world. Titanium dioxide has three crystal forms, brookite, anatase and rutile, respectively, and rutile is the high-temperature stable phase. TiO₂ is an n-type semiconductor material and widely used in heterogeneous catalysis, as a photocatalyst, in solar cells for the production of hydrogen and electric energy, as gas sensor, as white pigment, as a corrosion-protective coating, as an optical coating, as varistors, in bone implants, as dilute magnetic semiconductors. Both LaCoO₃ and TiO₂ have very high research value. The existing research results are almost based on different forms, such as polycrystals, nanoparticles and films, but not good in indication the nature of materials. Single crystal has long-range order in the macro level and has no defects, it can give more fundamental characteristics of the material. In this thesis, the single crystal growth of LaCoO₃ and TiO₂ by optical floating zone method and their properties have been investigated.We investigated the effects of the atmosphere and the pressure on the crystal growth of LaCoO₃ and the optimum growth conditions. The best LaCoO₃ single crystal growth atmosphere is 0.3 Mpa high pure oxygen atmosphere. The as-grown LaCoO₃ single crystal is a crack-free and bright black cylindrical rod with 4-6 mm in diameter and 30 mm in length. XRD and room temperature Raman show that the as-grown crystal grows parallel to the ( 121) direction and the as-grown crystal is highly crystalline with the rhombohedral perovskite structure. The electrical resistivity of the single crystal is much lower than that of the previous reports. It indicates that there is no the scattering introduced by crystal boundaries in the as-grown crystal and the single crystal is high-qulity single crystal. In low magnetic fields, the magnetic susceptibility for the as-grown single crystal shows paramagnetic below 35 K and above 90 K. Theχ^T curve appears a wave crest over the interval 55 K≤T≤90 K, the maximum value ofχlocated at 70 K, and theχunder FC is lower than that under ZFC in the temperature range of 55⁹0 K. The ZFC and FC magnetic susceptibility curves overlap and the 1/χ(T) curve shows a linear variation over the interval 5 K3 at low temperature may not be the intrinsic property, and comes from the incompleteness of prepared sample. The ferromagnetic order in LaCoO₃ may be caused by the well-known ferromagnetic double-exchange interaction between Co2+ (or Co4+) ions and the Co3+ ions. The Co2+ or Co4+ ions comes from the oxygen deficiencies or La vacancies in the crystal, as well as the surface and grain boundary of the polycrystal sample.Because of the high melting point, it is generally difficult to obtain a practical value of high-quality large rutile single crystal, oxygen vacancies and low angle grain boundary are also hindered on rutile single crystal in the optical application. Although the rutile single crystal grown by floating zone growth was starting from twenty years ago, but obtained TiO₂ single crystals are in deep yellow or blue and have poor light transmission. This is due to oxygen vacancies which not well controlled. Doped TiO₂ and oxygen vacancies have attracted considerable interest in the past due to TiO₂ can be used as the basis material of diluted magnetic semiconductors. So, it is important to discover how to control the level of oxygen vacancies in crystal growth progress and how to grow high-quality rutile TiO₂ single crystal.The TiO₂ single crystals with different oxygen vacancies had been grown by floating zone method. The apparatus used for the crystal growth was an image furnace with four ellipsoids (Crystal Systems Inc.: FZ-T-10000-H-VI-VP). Four halogen lamps of 1.5 kW as an infrared source were set at the each focal point of the ellipsoidal mirror. The growth chamber was isolated with a quartz tube, which ensured the strict control of the growth atmosphere. The as-grown crystals were in three colors: light yellow, yellow and pale blue, whose dimensions were about 6 mm in diameter and 40⁵0 mm in length. The light yellow and yellow crystals were grown in 0.5 and 0.3 MPa high purity oxygen atmosphere, respectively. And the pale blue crystal was grown in 0.1 MPa argon atmosphere. The as-grown crystals with different colors have different oxygen vacancies.The XRD results show that the crystals with different color grow parallel to the [110] direction, and the oxygen vacancies have no effect on preferential orientation. The room temperature Raman spectra show that the light yellow crystal was the Raman vibration anisotropy, and the other crystals (yellow and pale blue) were the Raman vibration isotropy. It indicates that the oxygen vacancies result in the lattice distortion partly on crystals, which influences the anisotropy. UV emission spectroscopy analysis shows that the oxygen vacancies have no effect on the band gap, but affect on the transition probability of the band gap. In addition, a luminescence band (424 nm) was observed in the light yellow crystal, and another luminescence band (434 nm) was found in the pale blue sample. It indicates that there is a new energy band under the conduction band caused by the oxygen vacancies. The magnetic susceptibility measurements show that the pale blue crystal with high oxygen vacancies is diamagnetic. The oxygen vacancies have no effect on the magnetism of TiO₂.In this thesis, the single crystal of LaCoO₃ and TiO₂ with controlled oxygen vacancies grown by optical floating zone method has been investigated. It proves that the ferromagnetic order in LaCoO₃ at low temperature is not the intrinsic property. It is found that oxygen vacancies in rutile TiO₂ single crystal have effect on the anisotropy and the transition probability of the band gap, and there is a new energy band under the conduction band caused by the oxygen vacancies, but oxygen vacancies have no effect on the magnetism of TiO₂.