New trends in fundamental research due to the spontaneous flexoelectric effect in nanosized and bulk ferroelectrics

Abstract

Physical processes taking place at the surface and its vicinity are so rich and versatile that sometimes seems to be inexhaustible subject of fundamental researches, which open new ways of technical applications. In particular theoretical and experimental researches of the versatile flexoelectric phenomena at nanoscale remain hot topic during last 10 years. Among them the special attention attracts the spontaneous flexoelectric effect inherent to nanosized and nanostructured ferroelectrics, as originated from the intrinsic polarization inhomogeneity near the surface of the system. The effect revealed in 2009 is very important, since it leads to the strong increase of effective flexoelectric coefficients values due to size effect. So that at small sizes the spontaneous flexoeffect strongly influences polar, piezoelectric and dielectric properties as well as the correlation radius. Motivated by above argumentation the author review describes the way of nanoscale ferroelectric properties and phase diagram calculations in the framework of Landau-Ginzburg-Devonshire thermodynamic theory with special attention to flexoelectric effect contribution. The comparison of obtained results with available experimental data is given whenever possible.The review highlights recent calculations, which have shown that the spontaneous flexoelectric effect and chemical pressure can be the physical reason of ferroelectric phase conservation and reentrance observed in spherical ferroelectric nanoparticles up to 5 nm sizes. Fundamental results devoted to the finding of “forbidden” shear surface wave propagating near the surface of all ferroelectrics due to omnipresent flexoelectric coupling are discussed. These missed surface waves are waiting for experimental observation, which could be the source of unique information about surface impact on the dynamics, structural peculiarities and possible phase transitions induced by the surface. Also we analyze how significantly the flexoelectric coupling can change static and dynamic polar properties in ferroelectrics, govern the formation and growth of unusual domains structures and influence optic and acoustic soft modes. The special attention in the review in paid to recent results, which have shown that flexocoupling induces soft acoustic mode in response of spatially modulated phase appearance in flexoelectrics.We'd like to underline that theoretical and experimental studies of flexoelectricity and related phenomena in nanosized and bulk ferroelectrics are very important for applications in nanoelectronics and memory devices, opening the way of true minimization of multilayer ceramic capacitors, piezoelectric transducers, pyroelectric elements and ferroelectric memories.