The perovskite solid solution BiFe_1-xCo_xO₃ (BFCO) is composed of BiFeO₃ and BiCoO₃, which have rhombohedral and tetragonal structure, respectively. BFCO is a promissing lead-free piezoelectric material, since the morphotropic phase boundary (MPB) was discovered around 0.2 < x < 0.4 region at room temperature, where monoclinic phase is present [1]. A typical multiferroic material, BiFeO₃ shows ferroelectricity and G-type antiferromagnetic order at room temperature. On the other hand, BiCoO₃ shows high tetragonality and C-type antiferromagnetic order. The enhancement of piezoelectric coefficients and polarization rotation can be expected in a vicinity of the MPB as in PZT. The monoclinic crystal structure is √2×√2×1 perovskite cell which accommodates the antiferromagnetic configuration. The spontaneous polarization and the piezoelectric coefficients have not been measured in the bulk system so far, owing to the large leakage current. In this context, we performed first-principles calculations of the spontaneous polarization and the piezoelectric response of a Fe/Co chemically ordered model of BFCO. Based on the results, we discuss the ferroelectric distortion and the microscopic mechanism of the piezoelectric effect in BFCO.

[1] K. Oka et al., Angew. Chem. Int. Ed. 51, 7977 (2012).