Enhanced piezoelectric and magnetic properties of Bi_1-xCa _xFe_1-x/2Nb_x/2O ₃ solid solutions

An investigation of the crystal structure, the magnetic and the piezoelectric properties of polycrystalline Bi_1-xCa _xFe_1-x/2Nb_x/2O ₃ (I) and Bi_1-xPb_xFe_{1-x /2}Nb_x/2O₃ (II) systems was performed by x-ray diffraction, Mssbauer spectroscopy, vibrating sample magnetometry, and piezoresponse force microscopy. It is shown that an increasing niobium content induces a polar-to-nonpolar morphotropic boundary near x=0.19 (I) and x=0.3 (II). Within the polar region of the system (I), (0.15<x<0.18), the solid solutions are homogeneous weak ferromagnets whereas compounds of the system (II) do not exhibit an appreciable spontaneous magnetization. It is assumed that chemical substitutions leading to a decrease of the initial volume of the unit cell favor the stabilization of the weak ferromagnetic state within the rhombohedral ferroelectric phase. The piezoresponse is significantly enhanced near the morphotropic boundary. The piezoelectric properties of the parent antiferromagnet BiFeO₃, harboring a cycloidal spatially modulated spin structure, are compared with those of the polar weak ferromagnet Bi _0.82Ca_0.18Fe_0.91Nb_0.09O ₃.