First-principles Study on Piezoelectricity and Spontaneous Polarization in Bi(Fe,Co)O3

Solid solution BiFe1−xCoxO3 simultaneously shows antiferromagnetic and pyroelectric orders and exhibits a structural phase transition from rhombohedral to tetragonal phases via a monoclinic one as x increases in bulk. This kind of transition is often called morphotropic phase boundary, which is well known to take place in a representative piezoelectric oxide, PbZr1−xTixO3. In the film form, instead, BiFe1−xCoxO3 shows the different structure transitions from a monoclinic to a tetragonal phase where the piezoelectric effect is rather modest. To theoretically understand the piezoelectric property in BiFe1−xCoxO3, we perform ab-initio electronic-structure calculations and study the structural stability, the magnetic property, and the electric polarization through the supercell approach. It turns out that the modest piezoelectric response in BiFe1−xCoxO3 is caused by Bi and apical O ionic distortions in the peculiar pyramidal coordination. A way to enhance the piezoelectric effect in BiFe1−xCoxO3 is proposed.