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In the paper we propose a theoretical model that takes into account Vegardstrains and perform a detailed quantitative comparison of the theoreticalresults with experimental ones for quasispherical nanoparticles, which revealthe essential (about 100 K) increase of the transition temperature in sphericalnanoparticles in comparison with bulk crystals. The average radius ofnanoparticles was about 25 nm, they consist of K(Ta,Nb)O3 solid solution, whereKTaO3 is a quantum paraelectric, while KNbO3 is a ferroelectric.From thecomparison between the theory and experiment we unambiguously established theleading contribution of Vegard strains into the extrinsic size effect inferroelectric nanoparticles. We determined the dependence of Vegard strains onthe content of Nb and reconstructed the Curie temperature dependence on thecontent of Nb using this dependence. Appeared that the dependence of the Curietemperature on the Nb content becomes nonmonotonic one for the small (< 20 nm)elongated K(Ta,Nb)O3 nanoparticles. We established that the accumulation ofintrinsic and extrinsic defects near the surface can play the key role in thephysical origin of extrinsic size effect in ferroelecric nanoparticles andgovern its main features.

Could the Vegard strains govern extrinsic size effects in nanoparticles?