Polar-Toroidal Phase Transformation in Inhomogeneous Nanoscale Ferroelectric Systems: A Novel Strategy for the Design of
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Polar-Toroidal Phase Transformation in Inhomogeneous Nanoscale Ferroelectric Systems: A Novel Strategy for the Design of Energy Conversion Nanodevices Weijin Chen, Shuai Yuan, Ye Ji, Gelei Jiang, Jian Shao and Yue Zheng* State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou, China. * Corresponding author: [email protected] ABSTRACT The ordering of polarization field of inhomogeneous ferroelectric systems were investigated. We found that these systems exhibit rather complex polarization ordering behaviors with the coexistence of polar and toroidal ordering, and particularly, a novel and tunable polartoroidal phase transformation under external mechanical, electrical or thermal fields. Accompanying with this polar-toroidal phase transformation, there is a large change of polarization and strain. As a result, large eletromechanical and thermomechanical performance can be achieved in these systems. The polar/toroidal phase boundaries can be regarded a new kind of morphotropic phase boundary (MPB). The polar-toroidal phase transformation in nanoscale ferroelectric systems should provide us a novel strategy to develop energy conversion nanodevices. INTRODUCTION Ferroelectrics have been exploited in many energy conversion and storage techniques for their outstanding coupling behaviors between electric polarization and external fields and for their high dielectric constants. In particular, ferroelectrics are well-known to exhibit large electromechanical responses near morphotropic phase boundaries (MPBs) where the energy penalty of polarization rotation is small. In previous works, much attention has been paid to composition-induced MPBs found in ferroelectric solid solutions such as Pb(Zr,Ti)O3 (PZT) [1] and Pb(Mg,Nb)O3–PbTiO3 (PMN-PT) [2]. It is also known that MPBs can be induced by strain in ferroelectrics such as BiFeO3 [3]. MPBs are likely to occur when the system approaches a critical point around which the energy surface is rather flat. As a consequence, polarization and strain become sensitive to composition, temperature and external fields. However, for ferroelectric nanostructures, MPBs are believed to be depressed by surface effects. The strong depolarization field and surface tension tend to impose a strong polarization anisotropy to the nanostructures, leading to the difficulty of polarization rotation and generally small piezoelectric responses. To develop nanoscale energy conversion devices, it is of particular importance to search new kinds of MPBs in ferroelectric nanostructures. Recently, abnormal electromechanical behaviors in ferroelectric nanowires that are caused by novel phase transitions have been reported [4, 5]. An unusual MPB due to polar-toroidal phase transition was predicted by Fu et al. [4] in PZT nanowires by applied modulated electric field. Interestingly, Chen et al. [5] demonstrated that BaTiO3/SrTiO3 composite nanowires can naturally form polar/toroidal phase boundaries with superior electromechanical responses. In this work, we show
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