基于第一性原理计算的铁电材料BaTiO3相关振动光谱指认

通过固相反应烧结法在1400 ℃下烧结4 h合成了BaTiO₃陶瓷, 并用X衍射确定了其为四方晶系. 进行了拉曼谱和红外谱的测量, 并采用洛仑兹函数以及四参数方法分别对上述光谱进行了拟合. 基于第一性原理的计算, 并考虑了横模纵模劈裂, 对拉曼和红外光谱进行了指认. 为了更好地分析振动模式, 所有振动模用群论的对称坐标进行了分解. 在12个光学模中, 仅具有拉曼活性的B₁模式是O4和O5沿着z轴的相对运动. A₁模式和E₁软模式是从立方BaTiO₃相的F_1u模式劈裂出来的, 对于四方相BaTiO₃的铁电性有着重要作用, 其体现在A₁⁽¹⁾模式造成了铁电z轴的极化, E₁模式导致了大介电常数. 这两个模式都可以看成是Ti 原子相对于O₆八面体笼子沿着z轴或者是xy平面的振动.

Assignment for Vibrational Spectra of BaTiO3 Ferroelectric Ceramic Based on the First-Principles Calculation

ABaTiO₃ ceramic was synthesized using a conventional solid-state reaction, and sintered at 1400 ℃ for 4 h. The pure tetragonal phase was confirmed by Rietveld refinement of the X-ray diffraction data. The Raman spectrum and the far infrared (FIR) reflective spectrum were obtained and analyzed using Lorentz fitting and the four-parameter semi-quantum model fitting, respectively. The Raman and FIR spectra were assigned based on first-principles calculations, and consideration of the splitting of the transverse optical modes and longitudinal optical modes. All the vibrational modes were represented by linear combinations of the symmetry coordinates deduced by group theory analysis. Among the 12 optical modes, the Raman-active-only mode, B₁, can be viewed as the wing-flapping vibration of the O4-O5 plane perpendicular to the z-axis in the O₆ octahedron. The A₁⁽¹⁾ mode and the E⁽¹⁾ soft mode are split by the triply degenerate F_1u mode of cubic BaTiO₃, resulting in the ferroelectric property of tetragonal BaTiO₃. The appearance of the A₁⁽¹⁾ mode leads to crystal polarization along the z-axis and the E₍₁₎ mode causes the large permittivity. These two modes can be described as vibration of the Ti atom against the O₆ octahedral cage along the z-axis A₁⁽¹⁾] and on the xy-plane E₍₁₎].