Theoretical analysis based on the Landau-Ginzburg-Devonshire (LGD) theory is
used to show that the joint action of flexoelectric effect and rotostriction
leads to a large spontaneous in-plane polarization (~ 1-5 muC/cm2) and
pyroelectric coefficient (~10^-3 C/m2K) in the vicinity of surfaces of
otherwise non-ferroelectric ferroelastics, such as SrTiO3, with static
octahedral rotations. The origin of the improper polarization and
pyroelectricity is an electric field we name flexo-roto field whose strength is
proportional to the convolution of the flexoelectric and rotostriction tensors
with octahedral tilts and their gradients. Flexo-roto field should exist at
surfaces and interfaces in all structures with static octahedral rotations, and
thus it can induce surface polar states and pyroelectricity in a large class of
otherwise nonpolar materials.Comment: 16 pages, 3 figures, 1 table, supplementary material