  The influence of the flexoelectric and rotostriction coupling on the phase
diagrams of ferroelastic-quantum paraelectric SrTiO3 films was studied using
Landau-Ginzburg-Devonshire (LGD) theory. The phase diagrams in coordinates
temperature - film thickness were calculated for different epitaxial misfit
strains. Tensile misfit strains stimulate appearance of the spontaneous
out-of-plane structural order parameter (displacement vector of an appropriate
oxygen atom from its cubic position) in the structural phase. Compressive
misfit strains stimulate appearance of the spontaneous in-plane structural
order parameter. Gradients of the structural order parameter components, which
inevitably exist in the vicinity of film surfaces due to the termination and
symmetry breaking, induce improper polarization and pyroelectric response via
the flexoelectric and rotostriction coupling mechanism. Flexoelectric and
rotostriction coupling results in the roto-flexoelectric field that is
antisymmetric inside the film, small in the central part of the film, where the
gradients of the structural parameter are small, and maximal near the surfaces,
where the gradients of the structural parameter are highest. The field induces
improper polarization and pyroelectric response. Penetration depths of the
improper phases (both polar and structural) can reach several nm from the film
surfaces. An improper pyroelectric response of thin films is high enough to be
registered with planar-type electrode configurations by conventional
pyroelectric methods.
