Theoretical prediction of new high-performance lead-free ferroelectrics

We predict the occurence of large ferroelectric polarization and piezoelectricity in the hypothetical perovskite-structure oxides, bismuth aluminate (BiAlO$_{3}$) and bismuth gallate (BiGaO$_{3}$), using density functional theory within the local density approximation. We show that BiGaO$_{3}$ will have a similar structure to PbTiO$_{3}$, although with much stronger tetragonal distortion and therefore improved ferroelectric properties. Likewise, BiAlO$_{3}$ shares structural characteristics with antiferrodistortive PbZrO$_{3}$, but it is also a ferroelectric with large polarization. Therefore we propose the Bi(Al,Ga)O$_{3}$ system as a replacement for the widely used piezoelectric sensor, Pb(Zr,Ti)O$_{3}$ (PZT) that will avoid the environmental toxicity problems of lead-based compounds. Finally we show that, in both BiAlO$_{3}$ and BiGaO$_{3}$, the large distortions from the prototypical cubic structure are driven by the stereochemical activity of the Bi lone pair.