  Using Landau-Ginzburg-Devonshire (LGD) approach we proposed the analytical
description of the chemical strains influence on the spontaneous polarization
and electrocaloric response in ferroelectric core-shell nanorods. We postulate
that the nanorod core presents a defect-free single-crystalline ferroelectric
material, and the elastic defects are accumulated in the ultra-thin shell,
where they can induce tensile or compressive chemical strains. The finite
element modeling (FEM) based on the LGD approach reveals transitions of domain
structure morphology induced by the chemical strains in the BaTiO3 nanorods.
Namely, tensile chemical strains induce and support the single-domain state in
the central part of the nanorod, while the curled domain structures appear near
the unscreened or partially screened ends of the rod. The vortex-like domains
propagate toward the central part of the rod and fill it entirely, when the rod
is covered by a shell with compressive chemical strains above some critical
value. The critical value depends on the nanorod sizes, aspect ratio, and
screening conditions at its ends. Both analytical theory and FEM predict that
the tensile chemical strains in the shell increase the nanorod polarization,
lattice tetragonality, and electrocaloric response well-above the values
corresponding to the bulk material. The physical reason of the increase is the
strong electrostriction coupling between the mismatch-type elastic strains
induced in the core by the chemical strains in the shell. Comparison with the
earlier XRD data confirmed an increase of tetragonality ratio in tensiled
BaTiO3 nanorods compared to the bulk material.
