The Structure And Properties Research On Tetragonal Tungsten Bronze Structure Crystal Ca_0.28Ba_0.72Nb₂O₆

As tetragonal tungsten bronze niobates have excellent piezoelectric, ferroelectric and non-linear optical characters, they have been widely used in laser frequency doubling, electro-optic modulation, optical information and other related areas. SBN crystal has very excellent electrical and optical characters, so it has been widely used. But the Curie temperature of SBN is 60℃, the application in high temperature is very limited. In 2001, CBN-28 crystal was grown by a group of Kolon University Germany for the first time. They found that the Curie temperature of CBN-28 is 265℃, which is nearly 200℃higher than SBN. It predicated that CBN-28 crystal has more potential applications.Not only good piezoelectrical and ferroelectrical properties but also good spectral, thermomechanical, thermodynamic characters are very important to a new ferroelectrical crystal. They affect the application of a new crystal directly. So it is important to do some properties research of the crystal. In this thesis, the spectral and thermal experiments results of CBN-28 were introduced, and were compared with some other tetragonal tungsten bronze crystals. Some dielectric experiments were also introduced in this thesis. Some conoscopic interferential pictures of tetragonal tungsten bronze niobates crystals were shown. The main works of thesis are as follows:1. Based on the space group theory, the lattice vibrational modes of CBN-28 were predicted. The modes were observed by Raman scattering and infrared absorption spectroscopy, and the results were identical to the theory. There are 3 characteristic Raman peaks and 2 characteristic infrared absorption spectroscopy peaks belonging to the internal vibrational modes of octahedral structure in tetragonal tungsten bronzea crystals. We identified the characteristic peaks in different geometry Raman scattering and infrared absorption spectroscopy. The Raman peaks relaxation of the line width is due to the distortion of octahedral ions.2. The specific heat were measured by the differential scanning calorimetry method, which the value is 0.455-0.770J/g·℃from the temperature 20℃to 500℃. A specific heat peak at Curie temperature of CBN-28 crystal was found. Thermal expansion of CBN-28 and Cu:KNSBN were measured by thermomechanical analysis. The thermal expansion coefficients of the two crystals changed greatly at Curie temperature. Thermal diffusion coefficient was measured by flash method, and the thermal conductivity of CBN-28 was also calculated.3. The transmission of CBN-28 crystals was measured by a UV/VIS/NIR spectrophotometer, and the transmission was compared with other tetragonal tungsten bronze crystals. The ultraviolet absorption edge of non-doped crystal was usually at 380 nm. Polarized transmission of CBN-28 was also measured.4. Dielectric constant and dielectric loss were measured. We found the Curie temperature of CBN-28 is 260℃. There is a dielectric loss peak at the frequency of 1KHz in 240℃.5. Some photos of conoscopic interference of tetragonal tungsten bronze structure crystals were shown in this thesis. Both SBN and Cu:KNSBN are uniaxis crystals. The phenomenons of convergent light for a uniaxis crystal with optical axis vertical and parallel to surface are also discussed in this thesis.