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In the framework of the thermodynamic approach Landau-Ginzburg-Devonshire(LGD) combined with the equations of electrostatics, we investigated the effectof polarization surface screening on finite size effects of the phase diagrams,polar and dielectric properties of ferroelectric nanoparticles of differentshapes. We obtained and analyzed the analytical results for the dependences ofthe ferroelectric phase transition temperature, critical size, spontaneouspolarization and thermodynamic coercive field on the shape and size ofnanoparticles. The pronounced size effect of these characteristics on thescaling parameter, the ratio of the particle characteristic size to the lengthof the surface screening, was revealed. Also our modeling predicts asignificant impact of the flexo-chemical effect (that is a joint action offlexoelectric effect and chemical pressure) on the temperature of phasetransition, polar and dielectric properties of nanoparticles when theirchemical composition deviates from the stoichiometric one. We showed on theexample of the stoichiometric nanosized and fine SrBi2Ta2O9 particles thatexcept the vicinity of the critical size, where the system splitting intodomains has an important role, results of analytical calculation of thespontaneous polarization have little difference from the numerical ones. Werevealed a strong impact of the flexo-chemical effect on the phase transitiontemperature, polar and dielectric properties of SryBi2+xTa2O9 nanoparticleswhen the ratio Sr/Bi deviates from the stoichiometric value of 0.5 from 0.35 to0.65. From the analysis of experimental data we derived the parameters of thetheory, namely coefficients of expansion of the LGD functional, contribution offlexo-chemical effect and the length of the surface screening.

Surface and finite size effects impact of the phase diagrams, polar and dielectric properties of (Sr,Bi)Ta2O9 ferroelectric nanoparticles