While commonly used piezoelectric materials contain lead, non-hazardous, high-performance piezoelectrics are yet to be discovered. Charged domain walls in ferroelectrics are considered inactive with regards to the piezoelectric response and, therefore, are largely ignored in this search. Here we demonstrate a mechanism that leads to a strong enhancement of the dielectric and piezoelectric properties in ferroelectrics with increasing density of charged domain walls. We show that an incomplete compensation of bound polarization charge at these walls creates a stable built-in depolarizing field across each domain leading to increased electromechanical response. Our model clarifies a long-standing unexplained effect of domain wall density on macroscopic properties of domain-engineered ferroelectrics. We show that non-toxic ferroelectrics like BaTiO3 with dense patterns of charged domain walls are expected to have strongly enhanced piezoelectric properties, thus suggesting a new route to high-performance, lead-free ferroelectrics.

Head-to-head and tail-to-tail 180 • domain-walls in a finite isolated ferroelectric sample are theoretically studied using Landau theory. The full set of equations, suitable for numerical calculations is developed. The explicit expressions for the polarization profile across the walls are derived for several limiting cases and wall-widths are estimated. It is shown analytically that different regimes of screening and different dependences for width of charged domain walls on the temperature and parameters of the system are possible, depending on spontaneous polarization and concentration of carriers in the material. It is shown that the half-width of charged domain walls in typical perovskites is about the nonlinear Thomas-Fermi screening-length and about one order of magnitude larger than the half-width of neutral domain-walls. The formation energies of head-to-head walls under different regimes of screening are obtained, neglecting the poling ability of the surface. In the nonlinear regimes of screening, this energy is equal to the energy necessary for the creation of electron-hole pairs in the amount sufficient to screen the spontaneous polarization, which is proportional to the band gap of the ferroelectric. It is shown that either head-to-head or tail-to-tail configuration can be energetically favorable in comparison with the monodomain state of the ferroelectric if the poling ability of the surface is large enough. If this is not the case, the existence of charged domain walls in bulk ferroelectrics is merely a result of the domain-growth kinetics. Formation energies of the other possible states: multidomain state with antiparallel domains separated by neutral walls and the state with the zero polarization were compared with the formation energy of the charged domain wall. It was shown that, at large enough sample thicknesses, a charged domain wall can be energetically favorable in comparison with the states mentioned above. This size effect could explain why charged domain walls were observed experimentally in bulk lead titanate but not in barium titanate. The results obtained for the case of an isolated ferroelectric sample were compared with the results for an electroded sample. It was shown that charged domain wall in electroded sample can be either metastable or stable, depends on the work function difference between electrodes and ferroelectric and the poling ability of the electrode/ferroelectric interface.

The interaction of electric field with charged domain walls in ferroelectrics is theoretically addressed. A general expression for the force acting per unit area of a charged domain wall carrying free charge is derived. It is shown that, in proper ferroelectrics, the free charge carried by the wall is dependent on the size of the adjacent domains. As a result, the mobility of such domain wall (with respect to the applied field) is sensitive to the parameters of the domain pattern containing this wall. The problem of the force acting on a charged planar 180• domain wall normal to the polarization direction in a periodic domain pattern in a proper ferroelectric is analytically solved in terms of Landau theory. In small applied fields (in the linear regime), the force acting on the wall in such pattern increases with decreasing the wall spacing. It is shown that the domain pattern considered is unstable in a defect-free ferroelectric. The poling of a crystal containing such pattern, stabilized by the pinning pressure, is also considered. Except for a special situation, the presence of charge domain walls makes poling more difficult. The results obtained are also applicable to zigzag walls under the condition that the zigzag amplitude is much smaller than the sizes of the neighboring domains.

Capacitors are essential building blocks of electrical radiofrequency (rf) circuits. Reconfigurable circuits whose electrical characteristics can be altered dynamically are of interest for space and weight economy, especially in future multifunctional, multiband hand-held electronic devices. Electrically tunable capacitors, that is, varactors, are therefore in demand. Two emerging solutions compete over this market: rf-microelectromechanical systems (rf-MEMS) tunable capacitor technology and ferroelectric thin-film capacitor technology. The former has the advantages of very low loss and relatively low voltage operation, but suffers inherently from low speed and reliability problems due to the moving parts at the microsystem level.[1] The latter involves the use of a ferroelectric material as a tunable element, and is attractive for being a fast-response, low-noise, competitive-loss level, [2] robust, all-solid technology, but is limited by circuit considerations due to the high permittivity of ferroelectrics. [3,4] This communication introduces a new, rather unexpected and yet simple way to overcome the limitation of ferroelectric materials by a self-assembled oriented nanocolumnar composite approach, which lowers the permittivity of the material and simultaneously amplifies its tunable response to electric field. A ferroelectric perovskite BaTiO 3 in the form of oblique nanofibers with lateral dimensions as fine as 2-8 nm embedded in dielectric fluorite CeO 2 matrix results in an appreciable tunability with a very low relative permittivity of 50. As a result, the coefficient of dielectric nonlinearity increases up to 25 times that of conventional BaTiO 3 -based ferroelectrics. The system is attractive also in manifesting low dielectric losses and excellent temperature stability of the dielectric properties. Beyond the interest in this approach for the field of reconfigurable microelectronics, the perovskite-fluorite nanocomposite approach shown here may be of interest to other fields, such as oxide fuel microcells with fluorite electrolytes and perovskite cathodes.The high permittivity of ferroelectrics is of a structural origin. It can be tuned by an external electric field. The tunability of the ferroelectric n, under dc field E, is strongly correlated with its initial permittivity e(0), as expressed by n ¼ "ð0Þ="ðEÞ % 1 þ 3 " 0 "ð0Þ ½ 3 bE 2 , for n % 1, and 3" 0 "ð0Þb 1=3 E 2=3 , for n ) 1, where b is the coefficient of the dielectric nonlinearity and e 0 is the permittivity of vacuum. [3] Therefore, a large permittivity is necessary in order to have a large tunability. However, the circuit consideration (impedance matching) imposes the use of low permittivity materials, especially with the increase of operational frequency, and with the drive toward miniaturization, which requires the use of the material in a thin-film, parallel-plate configuration. Consequently, two conflicting demands, namely low permittivity and high tunability, are required, which hinder the use of ferroelectric materials in reconfigura...

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