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Tunable ferroelectric capacitors, which exhibit a decrease of the dielectric permittivity (∈) under electric field, are widely used in electronics for RF tunable applications (e.g., antenna impedance matching). Current devices use barium strontium titanate as the tunable dielectric, and the need for performance enhancement of the tunable element is the key for device improvement. We report here on libraries of Ba0.97Ca0.03Ti1–xZrxO3 thin films (0≤x≤27%) with a thickness of about 130 nm deposited on IrO2/SiO2/Si substrates using combinatorial pulsed laser deposition allowing for gradients of composition on one sample. A total of 600 capacitors on a single sample were characterized in order to statistically investigate the dielectric properties. We show that the tunabilty is maximum at intermediate compositions, reaching values up to 60% for an electric field of about 400 kV cm–1. We attribute the high tunability in the intermediate compositions to the paraelectric-ferroelectric phase transition, which is brought down to room temperature by the addition of Zr. In addition, the piezoelectric coefficient is found to be decreasing with increasing Zr content. [ABSTRACT FROM AUTHOR]

Optimal properties like piezoelectricity can be found in polarizable materials for which the structure changes sharply under small composition variations in the vicinity of their morphotropic phase boundary or the triple point in their isobaric temperature-composition phase diagram. In the latter, lead-free (Ba0.850Ca0.150)(Ti0.900Zr0.100)O3 ceramics exhibit outstanding piezoelectric coefficients. For the first time, we report the growth of piezoelectric lead-free single crystals in the BaTiO3-BaZrO3-CaTiO3 pseudo-ternary system. The stoichiometry control in the CaO-BaO-TiO2-ZrO2 solid solution led to single crystals with various compositions ranging from (Ba0.857Ca0.143)(Ti0.928Zr0.072)O3 to (Ba0.953Ca0.047)(Ti0.427Zr0.573)O3. We evidenced a continuous cross-over from a ferroelectric state at high titanium content to a relaxor one on increasing the zirconium content. Such a property tuning is rather seldom observed in lead-free ferroelectrics and confirms what was already reported for ceramics. Single crystal with (Ba0.838Ca0.162)(Ti0.854Zr0.146)O3 composition, which has been grown and oriented along [001] crystallographic direction, displayed electromechanical coefficients d31 and k31 of 93 pC.N-1 and 0.18, respectively, near the room temperature (T=305K). [ABSTRACT FROM AUTHOR]

Lead-free (1 − x)NaNbO3/ xBa(Ti0.5Sn0.5)O3 ( x = 0.1, 0.125, 0.15, 0.175, 0.2, and 0.3) ceramics were elaborated by the conventional ceramic technique. Sintering has been made at 1523 K for 2 h. The crystal structure was investigated by X-ray diffraction with CuKα radiation at room temperature. As a function of composition, these compounds crystallize with tetragonal or cubic symmetry. Dielectric measurements show that the materials have a classical ferroelectric behavior for compositions in the range 0.10 ≤ x ≤ 0.15 and relaxor one for compositions in the range 0.15 < x ≤ 0.30. Temperatures TC or Tm decrease as x content increases. The ferroelectric behavior has been confirmed by hysteresis characterization. For x = 0.1, a piezoelectric coefficient d31 of 42.146 pC N-1 was obtained at room temperature. The evolution of the Raman spectra was studied as a function of temperature for x = 0.1. [ABSTRACT FROM AUTHOR]

The theory of dc-electric field nonlinear susceptibility in the doped incipient ferro- electrics is developed in a random field approach. The calculations of the susceptibil- ity χ(E, T) had shown that the coefficients χnl(k))(T, n) in power serie of susceptibility χ(E, T) = ∑ χnl(k)E2k (k = 0 and k = 1, 2, 3 … correspond respectively to linear and nonlinear susceptibility of the first, second etc. order) diverge at the same temperature T = Tm that depends on the impurity dipoles concentrations. The critical indexes of the divergency for the concentration of the dipoles which correspond to mixed ferroglass phase with coexistence of short- and long-range polar order were shown to be the same as those calculated in mean field approximation. The comparison of the theory with experimental data obtained for KTa1-xNbxO3 (0.0075 ≤ x ≤ 0.05) is performed. The calculations describe the main peculiarities of temperature, electric field and concentrational dependence of observed nonlinear dielectric susceptibility. Since the temperature Tm corresponds to the transition from dipole glass to ferroglass phase the transition is proven to be equilibrium one in the disordered ferroelectrics. [ABSTRACT FROM AUTHOR]

Non-linear dielectrics which display electric-field dependent dielectric properties can be used in microwave devices such as tunable resonators or phase shifters. The required performances are essentially moderate permitivity (100-1000), low dielectric losses (≤1%) in the GHz frequency range, good thermal stability and high tunability of the dielectric susceptibility (>10%). The mixed perovskite Ba 0.6 Sr 0.4 TiO 3 [BST] is an attractive candidate as its ferroelectric transition occurs close to room temperature and it displays high tunability but also strong temperature sensitivity and high frequency dielectric losses. To overcome these two deficiencies, ceramic/ceramic composites based on BST and a non-ferroelectric oxide such as MgO, MgTiO 3 , SnO 2 and SiO 2 were synthesized and compared to BST. For a better control of the losses a new concept based on a core-shell principle is presented. Through soft chemistry routes, a core made of BST grains is coated by a simple binary oxide shell. Modulation of the dielectric properties is expected by changing the core-shell structure. High frequency experiments are underway in order to check the GHz losses of these composites. [ABSTRACT FROM AUTHOR]

Abstract: Tetragonal Tungsten Bronze Ferroelectrics (TTB) are currently revisited for the elaboration of new multifunctional materials. Recent studies on Ba2LnFeNb4O15 ceramics (Ln=Nd, Sm and Eu) demonstrated that these materials display both ferroelectric and ferromagnetic behaviors at room temperature due to the presence of barium hexaferrite as a secondary phase. In this paper, we report for the first time the growth of Ba2LnFeNb4O15 single crystals (Ln=La, Pr, Nd, Sm and Eu) from high temperature solution using LiBO2 flux. Inclusion free millimeter-sized single crystals were successfully grown. Structural and dielectric characterizations were performed and chemical analysis confirmed that the TTB matrix is reluctant to accommodate small rare earth as it was observed for ceramics. A paramagnetic behavior was observed for all compositions, which corroborates the composite nature of the TTB ceramic multiferroics at room temperature, while dielectric measurements on Ln=La and Pr crystals confirmed their relaxor behavior. [Copyright &y& Elsevier]

Abstract: New ferroelectric ceramics of ABO3 perovskite type were synthesized in the Ba(Ni1/3Nb2/3) x Ti1−x O3 (0≤ x ≤0.2) system by solid state reaction technique. The effect of the replacement of titanium by nickel and niobium in the B site on structural and dielectric properties was investigated. For this, we have use X-ray diffraction and dielectric characterizations. Samples in cubic phase were generally obtained with a sintering temperature of 1350°C. Dielectric measurements revealed that the very small change in composition by the substitution of titanium by nickel and niobium in the BaTiO3 has a strong influence on the dielectric proprieties: fast decreasing of T C or T m (for BaTiO3 T C =340K, for x =0.06, T C =270K and for x =0.16, T m =150K). The evolution from classic ferroelectric behavior for (0≤ x <0.1) to relaxor behavior for (0.1≤ x ≤0.2) is emphasized. [Copyright &y& Elsevier]

Abstract: Investigation of crystal structure, dielectric, magnetic and local ferroelectric properties of the diamagnetically substituted Bi1− x A x FeO3− x /2 (A=Ca, Sr, Pb, Ba; x=0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Piezoresponse force microscopy has revealed signs of existence of the ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 giving rise to the appearance of room-temperature weak ferromagnetism. [Copyright &y& Elsevier]

For microwave agile devices such as tunable resonators or phase shifters, low loss components exhibiting electric-field dependent dielectric properties are seeked. Ferroelectric/low loss dielectric composites are good candidates to improve dielectric properties of BaxSr1-xTiO3 (BST) for such applications. Nanosized ferroelectric powders have been successfully encapsulated in a thin surrounding shell made of a binary oxide. The conservation of the ferroelectric transition in the composite has been shown while preserving the size of the ferroelectric cores after sintering.The dielectric properties of the composite can be tuned by changing the relative core/shell sizes, the intercore distance, the nature of the dielectric phase. Various synthesis routes have been used according to the required design of the final nanocomposite. Dielectric performances are presented and compared among the different nano-ferroelectric core–shell based particles obtained. [Copyright &y& Elsevier]

Ferroelectric tantalates with the tetragonal tungsten bronze (TTB) type structure were studied over a wide frequency range from 102 to 109 Hz. Complementary microwave measurements at 36 GHz and micro-Raman experiments were performed in order to extend the dielectric study. A high frequency relaxation was observed above 108 Hz and fitted to the Cole–Cole formula. Different qualitative models are proposed to explain the understanding of the origin of the high frequency relaxation in ferroelectrics. Among these, polar clusters of ferroelectric active off-centred Ta5+ ions are considered. [Copyright &y& Elsevier]