| The natural materials exist mostly in the form of microcrystal or polycrystal. There are very few single crystals exist as natural jewels. Therefore, comparaing with the polycrystals, the development and application history of crystals is much shorter. Because of scarcity in nature and demands for people of the single crystals, in the nineteenth century, the study of the crystal growth started. Nowadays, 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 can also be grown by artificial methods.Nowadays, the quality of artificial crystals such as diamond is greater than the natural one, 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 growing. 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.The floating zone method is a vertical zone melting method. The liqure zone is achieved by focusing the infrared light from the halogen or xenon lamps by elliptical mirror. The melting zone is maintained by the surface tension, and crystal grow along the vertical direction. The feed rods and the seed rods rotated in opposite direction, which can make the melting zone heated uniformly in all direction and the dopant distributed uniformly. Due to the free of impurity, fast growing-rate and free of 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. Many useful single crystal, such as piezoelectric crystal BaTiO3-CaTiO3, laser crystal Nd:GdVO4 and 12CaO·7Al2O3 crystals et al., have been grown successfully by the optical floating zone methods.Spinels, including normal (AB2O4) and inverse (A2BO4) spinels, have awakened great interests due to their important technological applications, such as magnetic materials, superhard materials, high-temperature ceramics and high-pressure sensors with Cr3+ doping. One of them, zinc titanate (Zn2TiO4) is a kind of the high-effect regenerable catalytic sorbents for H2S, As, Se and other contaminants removal from hot coal gases. Zn2TiO4 is also a good candidate for microwave dielectric applications. In recent years, numerous studies have been focused on its physical, electrical and optical properties under different conditions to investigate its other potential applicationsBinary niobate ceramics, with the formula M2+Nb2O6 where M2+= Ca. Mg, or a transition metal (TM), have the orthorhombic columbite structure. There is a growing interest in the columbites as microwave dielectric ceramics, due to their lower processing temperatures, less complicated processing, the lower cost of niobium compared with tantalum and low-temperature cofired ceramics (LTCC) with Cu2+Zinc niobate (ZnNb2O6), as one of the best-known members of this group, has been investigated and found to be a promising candidate for application in microwave devices due to its excellent dielectric properties. It is also a suitable reference material for investigating defects introduced in LiNbO3 substrates for waveguide fabrication and a key precursor for the preparation of lead zinc niobate (PZN) ferroelectric ceramics, which is difficult to synthesize via the conventional solid-state reaction process using oxides as starting materials. MgNb2O6 also exhibits excellent dielectric and optical properties. Moreover, it has been widely used as a precursor in the synthesis of single-phase PMN (Pb(Mgl/3Nb2/3)O3).Crack-free and transparent Zn2TiO4 single crystals of 4-6 mm in diameter and 30 mm in length have been grown by the optical floating zone method. The powder X-ray diffraction (XRD) results show that the as-grown crystals have the spinel-type Zn2TiO4 structure. The Zn2TiO4 single crystals grow along the (4 0 0) direction (a-axis).The formation of bubble inclusions was effectively suppressed by lowering rotation rate. The melting incongruently ZnNb2O6 single crystal of 4-6 mm in diameter and 30 mm in length have been grown by the optical floating zone method. The outside of as-grown crystals are colorless. And the inside of as-grown crystals are blue and transparent. After being annealed, the crystals fade to colorless. The crystal grows along the c-axis and the plane along the growth direction formed during crystal growing is the (010) plane.MgNb2O6 single crystals are grown by the optical floating zone method. The as-grown crystals are dark brown and have dimensions ofΦ4-6 mm×L 87 mm, with the largest crystal domain beingΦ5 mm×L 32 mm. After being annealed, the crystals fade to light brown. The powder X-ray diffraction analysis shows that the crystals have a columbite-type MgNb2O6 structure. The crystal grows along the c-axis and the cleavage plane is the (010) plane.Transmission polarized light microscopy measurements show that all the three crystals are free of low-angle grain boundaries and inclusions.All the crystals have been characterized by polarized Raman scattering, which reflects the Raman selection rules. Temperature dependence Raman spectra of all the crystals show that there is no phase transition in the whole temperature range (-190～560℃). The temperature -dependent shift of Raman phonons is also discussed in the context of thermal expansion.The transmittance and refractive spectra of ZnNb2O6 and Zn2TiO4 single crystals have been measured in the range of 200-1000 nm. The wavelength dependent refractive index, extinction coefficient, real and imaginary parts of the complex dielectric constant and other related parameters of the ZnNb2O6, and Zn2TiO4 crystal have been derived from the measured T and R spectra. Based on the theory of band to band transitions, the indirect energy gap and the phonon energy of Zn2TiO4 crystal has been determined as Egi=3.29 eV and Ep=0.09 eV, respectively. The existence of this phonon was also confirmed by the Raman spectra, which indicates that the indirect transitions for the Zn2TiO4 crystal are mainly due to the participation of the phonon mode at 721 cm-1. The direct energy gap of ZnNb2O6 has also been determined as 3.84 eV.The excitation spectra of the ZnNb2O6 exhibits intensity absorption at 285 nm by monitoring fluorescence at a wavelength of 450 nm. The peaks of excitation, at about 285 nm.can be associated with charge transfer bands of [NbO6]/- in the ZnNb2O6 systemUnder 285 and 282 nm excitation, respectively, the emission spectra of ZnNb2O6 and MgNb2O6 exhibit a broad and strong blue emission band centered at 450 nm and 435 nm, respectively. The edge-shared NbO6 groups are efficient luminescent centers.
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