  In the paper we propose a theoretical model that takes into account Vegard
strains and perform a detailed quantitative comparison of the theoretical
results with experimental ones for quasispherical nanoparticles, which reveal
the essential (about 100 K) increase of the transition temperature in spherical
nanoparticles in comparison with bulk crystals. The average radius of
nanoparticles was about 25 nm, they consist of K(Ta,Nb)O3 solid solution, where
KTaO3 is a quantum paraelectric, while KNbO3 is a ferroelectric.From the
comparison between the theory and experiment we unambiguously established the
leading contribution of Vegard strains into the extrinsic size effect in
ferroelectric nanoparticles. We determined the dependence of Vegard strains on
the content of Nb and reconstructed the Curie temperature dependence on the
content of Nb using this dependence. Appeared that the dependence of the Curie
temperature on the Nb content becomes nonmonotonic one for the small (< 20 nm)
elongated K(Ta,Nb)O3 nanoparticles. We established that the accumulation of
intrinsic and extrinsic defects near the surface can play the key role in the
physical origin of extrinsic size effect in ferroelecric nanoparticles and
govern its main features.
