Morphotropic phase boundary, weak ferromagnetism, and strong piezoelectric effect in Bi1 − xCaxFeO3 − x/2 compounds

The crystal structure and magnetic and piezoelectric properties of the Bi1 − xCaxFeO3 − 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.