Interband electronic transitions and phase transformation of multiferroic Bi_1-xLa_xFe_1-yTi _yO₃ ceramics revealed by temperature-dependent spectroscopic ellipsometry

Optical properties and phase transition of Bi_{1- x}La_xFe_1-yTi_yO ₃ (BLFTO) ceramics with different composition (0.02 ≤ x ≤ 0.10, 0.01 ≤ y ≤ 0.06) have been investigated by spectroscopic ellipsometry (SE) in the temperature range of -70-450 °C. The real part of the complex dielectric function ε₁ increases with the temperature. Meanwhile, the imaginary part ε₂ in the low-energy region decreases with the temperature and has an opposite trend in the high-energy side. Four typical interband transitions (E_a ∼ 2.50 eV, E _b ∼ 2.70 eV, E_c ∼ 3.60 eV, and E_d ∼ 4.25 eV) can be observed from the second derivative of the complex dielectric functions with aid of the standard critical point model. The critical point (CP) transition becomes broadening and shifts to a lower energy side as La and Ti compositions increase. Moreover, the CP transition energies show a red-shift trend with increasing the temperature until 320 °C, due to the lattice thermal expansion and electron-phonon interaction. The typical interband transitions and partial spectral weight present anomalies in the proximity of antiferromagnetic transition owing to the coupling between magnetic and ferroelectric order parameters and spin-lattice coupling for BLFTO multiferroic materials. It was found that the Néel temperature of BLFTO ceramics decreases from 364 to 349 °C with increasing doping composition of La and Ti elements. These phenomena can be attributed to the modification of electronic structure and magnetic order because the differences of electronegativity and ionic radii between Bi and La, Fe and Ti induce the variations on the bond angle and bond length between cations and anions. Moreover, the substitution for magnetic Fe³⁺ ions with nonmagnetic Ti⁴⁺ ions can reduce the exchange interaction between adjacent magnetic moments. Therefore, SE technique can be sensitive for detecting the phase/structural transitions of multiferroic oxides.