A mechanism for the 150 μC cm-2 polarization of BiFeO3 films based on first-principles calculations and new structural data

Following our experimental report of a giant ferroelectric polarization in the region of 150 μC cm-2 in BiFeO3 (BFO) films, we have performed first-principles calculations based on the local density approximation to density functional theory, aiming to clarify its mechanism. Upon optimization of lattice constants we have shown that the natural tetragonal structure of BFO has a giant tetragonality ratio of 1.26 and large ionic off-centring. Experimentally this structure has been detected in BFO films deposited on La-doped SrTiO3 substrates. The spontaneous polarization calculated ab initio for this structure is 143.5 μC cm-2, in agreement with the remanent polarization of hysteresis loops measured at 90 K. These results suggest that the giant polarization of our BFO films may occur upon stabilization of the optimal tetragonal phase with giant tetragonality. Future experimental effort aiming to routinely obtain such values of spontaneous polarization should concentrate on how to isolate this phase without compromising the insulating and switching properties of BFO. (letter to the editor)