Strain-induced polarization enhancement in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>BaTi</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> core-shell nanoparticles

Abstract

Despite fascinating experimental results, the influence of defects and elastic strains on the physical state of nanosized ferroelectrics is still poorly explored theoretically. One of the unresolved theoretical problems is the analytical description of the strongly enhanced spontaneous polarization, piezoelectric response, and dielectric properties of ferroelectric oxide thin films and core-shell nanoparticles induced by elastic strains and stresses. In particular, the 10-nm quasi-spherical $\mathrm{BaTi}{\mathrm{O}}_{3}$ core-shell nanoparticles reveal a giant spontaneous polarization up to $130\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$, where the physical origin is a large Ti off-centering. The available theoretical description cannot explain the giant spontaneous polarization observed in these spherical nanoparticles. This work analyzes polar properties of $\mathrm{BaTi}{\mathrm{O}}_{3}$ core-shell spherical nanoparticles using the Landau-Ginzburg-Devonshire approach, which considers the nonlinear electrostriction coupling and large Vegard strains in the shell. We reveal that a spontaneous polarization greater than $50\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ can be stable in a (10--100)-nm $\mathrm{BaTi}{\mathrm{O}}_{3}$ core at room temperature, where a 5-nm paraelectric shell is stretched by (3--6)% due to Vegard strains, which contribute to the elastic mismatch at the core-shell interface. The polarization value $50\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ corresponds to high tetragonality ratios (1.02--1.04), which is further increased up to $100\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ by higher Vegard strains and/or intrinsic surface stresses leading to unphysically high tetragonality ratios (1.08--1.16). The nonlinear electrostriction coupling and the elastic mismatch at the core-shell interface are key physical factors of the spontaneous polarization enhancement in the core. Doping with the highly polarized core-shell nanoparticles can be useful in optoelectronics and nonlinear optics to increase beam coupling efficiency, electric field enhancement, reduced switching voltages, ionic contamination elimination, catalysis, and electrocaloric nanocoolers.

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