  Solid solution BiFe1-xCoxO3 shows anti-ferromagnetic order and pyroelectric
order, simultaneously. It has been known that BiFe1-xCoxO3 exhibits a
structural phase transition between monoclinic and tetragonal phases as x
increases. This kinds of transition is often called morphotoropic phase
boundary, which is well known to take place in a representative piezoelectric
oxide, PbZr1-xTixO3. In order to theoretically understand the piezoelectric
property in BiFe1-xCoxO3, we performed ab-initio electronic-structure
calculations and studied the structural stability, the magnetic property, and
the electronic polarization by means of super-cell approach. It turns out that
the large electric polarization and the particular pyramidal coordination
suppress the response of the electric polarization under strain. A way to
enhance the piezoelectric effect in BiFe1-xCoxO3 is proposed.
