[en] The crystal structure and magnetic and piezoelectric properties of the Bi_1−xCa_xFeO_3−x/2 system (x ≤ 0.2) are studied. The crystal-structure transformations occur in the following sequence: rhombohedral (R3c) polar phase (x ≤ 0.06) → modulated polar phase (0.07 ≤ x ≤ 0.1) → modulated antipolar phase (0.11 ≤ x ≤ 0.14). The modulated polar and antipolar phases are weakly ferromagnetic with a spontaneous magnetization of 0.25 G cm³/g (x = 0.09). In the polar weak ferromagnet with x = 0.09, a uniform piezoelec- tric response 2.5 times stronger than in the initial BiFeO₃ compound is detected by the piezoelectric force microscopy.

[en] Mn substitution-driven structural and magnetic phase evolution in the predominantly antiferromagnetic polar rhombohedral (space group R3c) and weak ferromagnetic antipolar orthorhombic (space group Pnam) phases of the Bi_1-xPr_xFeO₃ perovskites was studied by x-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy and magnetometry techniques at room temperature. Mn doping was found to effectively modify both the initial structures to stabilize a new phase demonstrating incommensurable modulation. Magnetic behaviour of the Mn-containing samples changes in correlation with the evolution of their structural state. Within the compositional range of the rhombohedrally distorted compounds, the manganese substitution gives rise to suppression of the dominant antiferromagnetic interaction. In the weak ferromagnetic doping-induced phase, a gradual decrease in room temperature spontaneous magnetization takes place with increasing Mn content. (paper)