Crystal growth and structure-property relationships for ferroelectric strontium bismuth tantalate.

摘要

This thesis reports on the crystal growth of SrBi₂Ta ₂O₉ ISBN, the measurements of intrinsic properties and the determination of structure-property relationships. Data are reported for the first time for the spontaneous polarization, dielectric anisotropy, electric resistivity, elastic coefficients, piezoelectric activity, heat capacity, and thermal expansion.; Details are reported for the high-temperature solution growth of SBT from a Bi₂O₃ self-flux. Sizable single crystals (up to 6 x 2 x 1.7 mm3) were grown. High resolution TEM imaging verified the basic layer structure, and occasionally identified defects including stacking faults in the SBT structure. The crystal structure was determined by x-ray diffraction. The refinement was consistent with ion exchange between Bi and Sr sites. A theoretical value for the spontaneous polarization was calculated from ionic displacements in the refined structure, and found to be in excellent agreement with experimental data (17 μC/cm2).; Well-saturated ferroelectric hysteresis loops were observed for the first time in this thesis for SBT single crystals, from which the spontaneous polarization was obtained. Dielectric constant values were determined along the three principal directions, and were at room temperature K_a = 300, K_b = 186, and K_c = 93. The temperature dependence of their anisotropy was also investigated, with a strong anomaly at a lower transition temperature of ∼300°C. In addition, the electrical resistivity was also determined for the three principal directions. The value along c (ρ_c = 3.6 × 1014 ·CM) was more than an order of magnitude greater than along the other two principal directions (ρ_a = 1.4 × 1013 ·cm and ρ_b = 2.2 × 1013 ·cm). The full tensors of elastic and piezoelectric coefficients were obtained by Brillouin scattering method. The piezoelectric coefficients for SBT are weak (e.g., d₁₁∼24 pC/N), and should not give rise to any significant strain on switching P_S.; Two phase transformations were identified. The higher-temperature phase transformation (∼580°C) was attributed to a tetragonal-orthorhombic transformation on cooling (4/mmm→2mmm). The lower-temperature phase transformation (∼305°C) was identified to be within the same point group (2mm) and the same Aizu species. Raman spectroscopy as a function of temperature identified a soft mode with A₁ symmetry which disappeared at the lower-transition temperature on heating. Raman data suggest a structural change with a lower transition from A2₁am to F2mm on heating. Thermal analysis indicated the latent heat of transformation was small. Thermal properties including thermal expansion coefficient and heat capacity were also determined.; Discoveries in this thesis suggest that Sr and Bi exchange between the layers and compensated doping, and the weak piezoelectric properties, are the major mechanisms by which SBT shows excellent resistance to fatigue.