Flexo-chemo effect in nanoferroics as a source of critical size disappearance at size-induced phase transitions

Based on Landau-Ginzburg-Devonshire approach, we explore the critical size disappearance at size induced phase transitions and reentrant phase occurrence in nanoferroics. Our calculations have shown that the physical mechanism of the exciting phenomenon can be the flexo-chemo effect, being the synergy of the spontaneous flexoelectric stresses and the chemical pressure induced by ion vacancies via Vegard effect. The flexo-chemo effect can lead to the remarkable changes of the nanoferroics phase diagrams, such as non-monotonic increase of the transition temperature and long-range order enhancement appearing under the size decrease and fulfilment of definite conditions. Since the flexo-chemo effect should exist in any nanostructured ferroics, obtained analytical results can be predictive for many of them. As a concrete example of the primary ferroics, we consider ferroelectric nanoparticles and have shown that a commonly expected transition from the ferroelectric to paraelectric phase at some small critical size is absent, so that the critical size loses its sense. Contrarily, the stabilization of the ferroelectric phase manifests itself by the enhancement of the transition temperature and polarization with the particle size decrease (ferroelectric phase reentrance), which was observed earlier in the tetragonal BaTiO3 nanospheres of radii 5-50 nm and stayed unexplained up to now. (C) 2016 AIP Publishing LLC.