We present a first-principles investigation of ferroelectricity in layered perovskite oxide La2Ti2O7 (LTO). Our calculations indicate that LTO's high-temperature (1770 K) ferroelectric transition results from the condensation of two soft modes that have the same symmetry and are strongly coupled anharmonically. The leading instability mode essentially consists of rotations of the oxygen octahedra that are the basic building blocks of the perovskite structure; remarkably, because of its particular lattice topology or connectivity, such O6 rotations give rise to a spontaneous polarization in LTO. The effects discussed thus constitute an example of how nanostructuring - provided here by the natural layering of LTO - makes it possible to obtain a significant polar character in structural distortions that are typically nonpolar. We discuss the implications of our findings as regards the design of multifunctional materials, noting that the observed proper ferroelectricity driven by O6 rotations provides the ideal conditions to obtain strong magnetoelectric effects. © 2011 American Physical Society.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 8 Aug 2011|