Exploration And Characterizations Of Several Niobate Crystals

Niobate crystals are one of the mainstream in the research of crystal materials,they possess many interesting properties such as excellent piezoelectric,ferroelectric,electro-optic and nonlinear coefficients,and can be used in piezoelectric devices,laser frequency doubling,capacitors,electro-optic modulator and filter,etc.With the rapid development of science and technology,in order to meet the development of equipment intelligence and integration,miniaturization and multi-functinalization of optoelectronic devices,the exploration and research of noval niobate crystals have important theoretical and realistic significance.It is necessary to explore more practical niobate crystal materials,study their properties and application prospects.Several niobate crystals TbNbO₄,Ba₆Ti₂Nb₈O₃₀ and Sr₆Ti₂Nb₈O₃₀ were explored and studied in this paper.Their properties and possible application prospects were studied.The main research work and conclusions are as follows:（1）Crystal growthAccording to the growth characteristics of these crystals,the synthesis of polycrystalline,the structure of the temperature field,the process of crystal growth and the annealing process were designed,the bulk TbNbO₄,Ba₆Ti₂Nb₈O₃₀ and Sr₆Ti₂Nb₈O₃₀ crystals were grown successfully by the Czochralski method.（2）The properties characterization of TbNbO₄ single crystalThe single crystal structure of TbNbO₄ was solved by the single crystal X-ray diffraction.The result confirmed that the as-grown crystal belongs to monoclinic phase with fergusonite structure.The thermal properties of the as-grown TbNbO₄ crystal,including specific heat,thermal expansion coefficient,thermal diffusion coefficient,and thermal conductivity,were investigated systematically.The specific heat increased continuously when the temperature increased,indicating that TbNbO₄ can tolerate more thermal energy at higher temperatures.Thermal expansion coefficients of TbNbO₄ are obtained as:αa = 21.63 × 10-6 K-1,αb=11.71× 10-6 K-1,αc’=-7.238 × 10-6 K-1.TbNbO₄ has relatively large anisotropy in the thermal expansion,and the as-grown crystal TbNbO₄ belongs to c axis negative expansion material,zero thermal expansion could be achieved.Zero thermal expansion materials have significant advantages in the thermal shock resistance and they have been the research hot spot.They have great significant to improve the geometric thermal stability of military electronic devices and structural devices.Moreover,they also have potential application value in the precision control of instruments and equipment.The emission spectrum of the as-grown crystal excited at 392 nm was measured and exhibited four characteristic peaks which could be assigned to Tb3+.The peak centered at 550nm with a half-width of 13nm is the strongest,and it is expected to achieve a continuously tunable green laser output of 541-554 nm.The absorption of TbNbO₄ was also measured.In the range of 350-1500 nm,TbNbO₄ crystal has high transmittance.X-ray photoelectron spectroscopy（XPS）was used to study the binding energy and the valence state of the elements in the TbNbO₄ crystal.It lays the foundation for studying the influence of the microscopic electronic structure on the macroscopic properties.（3）The properties characterization of Ba₆Ti₂Nb₈O₃₀ single crystalThe thermal expansion of Ba₆Ti₂Nb₈O₃₀ crystal was studied.The thermal expansion along c axis is negative and the thermal expansion coefficient is small.The electro-elastic constants of Ba₆Ti₂Nb₈O₃₀ crystal were measured and calculated by the resonance method.At room temperature,the relative dielectric constant ε₃₃/ε₀,electromechanical coupling coefficient k31,elastic compliance constant s11,piezoelectric coefficients d31 and d33 were obtained as 106.99、18.04%、4.2×106pm²/N、11.38 and 65 pC/N.The results indicate that BTN has excellent piezoelectric property and can be used in piezoelectric devices,such as filters,resonators and sensors.The temperature dependence of the relative permittivity and dielectric loss with different frequency were measured.The relative permittivity showed marked anomaly at the transition point of 250 ℃,and the value up to 2×10⁴.At room temperature,the dielectric loss is low,but with the temperature increased,the dielectric loss of Ba₆Ti₂Nb₈O₃₀ has obvious fluctuation.The coercive electric field,residual polarization and spontaneous polarization were obtained as 1.0 kV/mm,0.75 μC/cm² and 0.6 μC/cm².Under this test condition,Ba₆Ti₂Nb₈O₃₀ crystal has weaker ferroelectricity.The pyroelectric property of Ba₆Ti₂Nb₈O₃₀ crystal was investigated.At 40℃,the pyroelectric coefficient is 138.71 μC/（m²·K）.The pyroelectric coefficient reached the maximum when the temperature increased to 130℃,and pmax = 363.95 μC/（m²·K）.The result indicates that BTN has large pyroelectric coefficient and can be used in the pyroelectric infrared detector.According to the temperature dependence of pyroelectric coefficient and thermal expansion of c axis,we found that the phase transition of BTN crystal occurred in a wider temperature range and BTN may belongs to the displacive transition.（4）Research on Sr₆Ti₂Nb₈O₃₀ crystal defectsAfter annealing,the as-grown Sr₆Ti₂Nb₈O₃₀ crystal has obvious defects.We analyzed the crystal phase of the transparent and opaque parts,found that the components of opaque crystal has a deviation.The causes of defects were analyzed and the preliminary scheme for improving the crystal quality was proposed.In the subsequent work,we will improve the quality of STN crystal and study their properties.