The effect of the cooling rate on the electrical properties was investigated in the 0.75BiFeO3-0.25BaTiO3 ceramics. The air-quenched samples had superior ferroelectric and piezoelectric properties to the slowly cooled samples. The quenching effect weakened when the quenching temperature was less than 700 °C and eventually disappeared at 500 °C and below. The X-ray diffraction and transmission electron microscopy showed that the cooling rate had a significant effect on the crystal structure and domain structure. The slowly cooled sample showed a very small rhombohedral distortion and a poorly developed domain structure, which leads to weak ferroelectric and piezoelectric properties at room temperature. The quenched and slowly cooled samples had a ferroelectric rhombohedral structure (R3c) at room temperature and a paraelectric cubic structure (Pm-3m) at temperatures above 650 °C. On the other hand, the slowly cooled sample had a centro-symmetric orthorhombic (Pbnm) structure at intermediate temperatures, while the quenched sample had a noncentrosymmetric orthorhombic structure (Amm2). The diffusion of oxygen vacancies in the slowly cooled sample is believed to lead to a more symmetric orthorhombic structure at intermediate temperatures between 500 °C and 650 °C during the slow-cooling process and consequently very small rhombohedral distortion at room temperature.

Neutron diffraction has been carried out to study temperature evolution of crystal and magnetic structure parameters of the multiferroic (0.9)BiFeO3 + (0.1)BaTiO3 over region (300 – 1000) K. Crystal structure is rhombohedral over whole temperature region and it is described by the R3c space group. The lattice parameters increase with temperature. The Ba ions are placed in the Bi sublattice and the Ti ions partly occupy the Fe sublattice. Assuming that the sample has a modulated magnetic structure with the propagation vector k = [0.0045, 0.0045, 0], we obtained a temperature dependence of the Fe-ion magnetic moment. The value of the moment is equaled to be μ = (3.46 ± 0.05) μB at 300 K and becomes zero at 600 K.