Phase transformations in multiferroic Bi_{1- x}La_xFe_1-yTi_yO₃ ceramics probed by temperature dependent Raman scattering

Optical phonons and phase transitions of Bi_1-xLa_xFe_1-yTi_yO₃ (BLFTO, 0.02 ≤ x ≤ 0.12, 0.01 ≤ y ≤ 0.08) ceramics have been investigated by Raman scattering in the temperature range from 80 to 680 K. Four phase transitions around 140, 205, 570, and 640 K can be observed. The Raman modes are sensitive to the spin reorientation around 140 and 205 K, owing to the strong magnon-phonon coupling. The transformation around 570 K is a structural transition from rhombohedral to orthorhombic phase due to an external pressure induced by the chemical substitution. The anomalies of the phonon frequencies near Néel temperature T_N have been discussed in the light of the multiferroicity. Moreover, it was found that the structural transition temperature and T_N of BLFTO ceramics decrease towards room temperature with increasing doping composition as a result of size mismatch between substitution and host cations.