  Using Landau-Ginzburg-Devonshire theory and phase field modeling, we explore
the complex interplay between the long-range structural order parameter (oxygen
octahedron rotations) and polarization in EuxSr1-xTiO3 thin epitaxial films. In
biaxially tensile strained films, we discover the presence of a low symmetry
monoclinic phase with in-plane ferroelectric polarization that is stabilized by
antiferrodistortive oxygen octahedra tilts. The monoclinic phase is stable in a
wide temperature range, and is characterized by the large number of
energetically equivalent polar and structural twin domains, which stimulates
easy twinning of the film and thus enhances its effective piezoelectric
response. The flexoelectric coupling and rotostriction give rise to additional
spontaneous polarization, piezo- and pyro-electricity in the ferroelastic twin
boundaries.
