  We present a first-principles investigation of ferroelectricity in layered
perovskite oxide La2Ti2O7 (LTO), one of the compounds with highest Curie
temperature known (1770 K). Our calculations reveal that LTO's ferroelectric
transition results from the condensation of two soft modes that have the same
symmetry and are strongly coupled anharmonically. Further, the leading
instability mode essentially consists of rotations of the oxygen octahedra that
are the basic building block of the perovskite structure; remarkably, because
of the particular topology of the lattice, such O6 rotations give raise to a
spontaneous polarization in LTO. The effects discussed thus constitute an
example of how nano-structuring -- provided here by the natural layering of LTO
-- makes it possible to obtain a significant polar character in structural
distortions that are typically non-polar. We discuss the implications of our
findings as regards the design of novel multifunctional materials. Indeed, the
observed proper ferroelectricity driven by O6 rotations provides the ideal
conditions to obtain strong magnetoelectric effects.
