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International audience; In this work, structural and dielectric properties of BaTi(1-x)ZrxO3 (BTZ) ceramics prepared by Spark Plasma Sintering (SPS) from powders obtained via Self-propagating High-temperature Synthesis (SHS) are shown to be strongly affected by the sintering temperature. In addition, a post-annealing treatment in air of the as-prepared ceramics leads to a transition from the hexagonal to the tetragonal and cubic phases. The SPS ceramics corresponding to compositions 0.05 ≤ x ≤ 0.20 and obtained at a sintering temperature of 1200 °C exhibit a standard ferroelectric behavior. In contrast, a diffuse phase transition is observed for the case of ceramics sintered at higher temperatures. Finally, the BTZ ceramic containing 5 at.% of Zr displays the best dielectric permittivity and piezoelectric properties as compared to the other compositions taken into account.

International audience; Herein we investigate the technologically relevant blend of the ferroelectric polymer poly(vinylidene fluorideco-trifluoroethylene), P(VDF-co-TrFE), with the semiconducting polymer poly(3-hexylthiophene), P3HT, by means of a combination of Scanning Probe Microscopy techniques, namely Atomic Force Microscopy, Conductive Force Microscopy, Kelvin Probe Force Microscopy and Piezoresponse Force Microscopy. This combination proves to be a powerful tool for the non-destructive morphological reconstruction of multifunctional nano-structured thin films, as those under study. Each modality allows discerning the two blend constituents based on their functionality, and, additionally, probes layers of different thickness with respect to the films surface. The depth-dependent information that is collected allows a qualitative reconstruction of the blend's composition and morphology both in-plane and out-of-plane of the film. We demonstrate that P3HT exhibits the tendency to reside the film surface at an almost constant composition of 15%, independent of blend's composition. Increasing the P3HT content in the blend results in the segregation of P3HT at the upper layers of the films, partially buried below a P(VDF-co-TrFE) superficial layer. The depletion of P3HT from the substrate/film interface is reflected by the poor existence of conducting pathways that connect the top and bottom planes of the film. The three-dimensional morphology of this polymer blend that is revealed thanks to the employed techniques deviates substantially from the ideal morphology proposed for the efficient performance of the targeted memory devices.

Tunable ferroelectric capacitors, which exhibit a decrease in the dielectric permittivity under an electric field, are widely used in electronics for RF tunable applications. Current devices use barium strontium titanate (BST) as the tunable dielectric, but new applications call for tunable materials with specific performance improvements. It is then of crucial importance to dispose of a large panel of electrically characterized materials to identify the most suited compound for a given set of device specifications. Here, we report on the dielectric tuning properties of Ba1−xCaxTi1−yZryO3 (BCTZ) thin films libraries (0 ≤ x ≤ 30% and 0 ≤ y ≤ 28.5%) synthesized by combinatorial pulsed laser deposition (CPLD). An original CPLD approach allowing reliable and statistical ternary phase diagrams exploration is reported. The effects of Ca and Zr content on tunability, breakdown voltage and dielectric losses are explicated and shown to be beneficial up to a certain amount. Compounds close to (Ba0.84Ca0.16)(Ti0.8Zr0.2)O3 exhibit the highest figures of merit, while a zone with compositions around (Ba0.91Ca0.09)(Ti0.81Zr0.19)O3 show the best compromise between tuning ratio and figure of merit. These results highlight the potential of BCTZ thin films for electrically tunable applications.

International audience; BaTiO3-based crystal fibres with mm-sized grains were grown by the micro-pulling down technique from the BaTiO3–CaTiO3–BaZrO3 solid solution with pulling velocities of about 6, 9 and 15 mm h−1. The natural growth direction was identified as (001)pc. For the pulling velocities of about 15 mm h−1 and 9 mm h−1, effective partition coefficients have been calculated from Castaing micro-probe measurements, and gave, respectively, 1.3 and 2 for Zr, and 0.95 and 0.9 for Ca. Laser-induced breakdown spectroscopy measurements reveal a strong inhomogeneity and variations of Zr contents while Ca contents show an opposite variation trend with a more steady distribution. Coexistence of two crystallized perovskite solid solutions is suggested. Most efficient polycrystals with mm-sized grains and 0.5 mol% Zr and 11 mol% Ca as average contents exhibit Curie temperatures higher than 113 °C, electromechanical coupling factors kt up to 41% and piezoelectric charge coefficients d33 up to 242 pC N−1 at room temperature. These values are similar to piezoelectric coefficients reported in the literature for oriented flux-grown single crystals with close compositions. Both chemical and physical results obtained in the BCTZ system make the μ-PD technique a promising way to improve the piezoelectric response of lead-free solid solution-based single crystals.

Piezoelectric thick films are of real interest for devices such as ceramic Micro-ElectroMechanical Systems (MEMS) because they bridge the gap between thin films and bulk ceramics. The basic design of MEMS includes electrodes, a functional material, and a substrate, and efforts are currently focused on simplified processes. In this respect, screen-printing combined with a sacrificial layer approach is attractive due to its low cost and the wide range of targeted materials. Both the role and the nature of the sacrificial layer, usually a carbon or mineral type, depend on the process and the final device. First, a sacrificial layer method dedicated to screen-printed thick-film ceramic and LTCC MEMS is presented. Second, the recent processing of piezoelectric thick-film ceramic MEMS using spark plasma sintering combined with a protective layer approach is introduced. Whatever the approach, the focus is on the interdependent effects of the microstructure, chemistry, and strain/stress, which need to be controlled to ensure reliable and performant properties of the multilayer electroceramics. Here the goal is to highlight the benefits and the large perspectives of using sacrificial/protective layers, with an emphasis on the pros and cons of such a strategy when targeting a complex piezoelectric MEMS design.

International audience; The design and processing of vibrational energy harvester based on screen-printed piezoelectric lead zirconate titanate (Pb(ZrxTi1-x)O3 (PZT)) are here described. Two different structures, a simple cantilever and a complex zigzag geometry made of PZT layer sandwiched between gold electrodes and supported on a metallic stainless steel substrate have been successfully fabricated by screen printing thick film technique. Compared to bulk PZT ceramics, the main limiting features at different scales are porosity, interfaces and bending issues. The microstructural analysis of the interfaces in the cantilever has highlighted the formation of an interface between the substrate and the bottom electrode which ensures cohesion of the structure but can limit its dynamic. Bending has shown to be dependent on the thickness of the active piezoelectric layer. Dielectric and electromechanical characterizations performed on multilayers, bulk ceramics, and free-standing screen-printed disks are compared and discussed on the basis of interface issues.

Multilayered doped BaTiO3 thin films have been fabricated by physical vapor deposition (PVD) on low-cost polycrystalline substrates with the aim to improve dielectric properties by controlling point charge defects at the interfaces. We show that carefully designed interfaces lead to increasing the relative permittivity of the BaTiO3 thin films, in contradiction with the common belief that interfaces behave as dead layers. High relative permittivity up to 1030 and tan&delta, = 4% at 100 kHz and room temperature were obtained on BaTiO3 multilayered films deposited on Si/Pt substrates by PVD. The large permittivity is suspected to be an extrinsic contribution due to band bending at the interfaces, as inferred by in-situ X-ray photoelectron spectroscopy. A 20-nm depletion layer was found to be associated with an interdiffusion of dopants, as measured by depth profiling with time-of-flight secondary ion mass spectrometry. The films exhibit high permittivity and low dielectric losses stable between 200 and 400 K, which meet the requirement of electronic applications.

International audience; Interfaces are a major issue when designing ferroelectric nanostructured materials with tailored properties. In a context of integration and multifunctionality in the field of electronics, several strategies have been developed to control the microstructure and defect chemistry of interfaces that strongly impact the macroscopic properties. The suitability of the core-shell approaches that allow a subtle tuning of interface phenomena at different scales has been widely demonstrated. We focus here on the flexibility of the core-shell approach devoted to the processing of nanostructured ferroelectric composites. Our strategy relies on the use of advanced synthesis processes to design ferroelectric grains coated with shells of different nature, morphology and crystallinity. Typical examples will be reviewed with a specific attention on their impact on both microstructure and dielectric properties. Our approach, based also on the use of fast sintering technique, provides a guidance to design 3D bulk nanostructured ferroelectrics while controlling and/or exploiting size, interface and defects chemistry. The contribution of specific spectroscopies to probe interfacial chemistry and defects is underlined. The high density of interfaces in core-shell materials is obviously an advantage to target additional functionality such as magneto-electric coupling. This is illustrated in 3D composites and one dimensional nanostructures that coaxially combine electric and magnetic materials. The core-shell approach described here could be transferred to a much broader range of materials covering many functionalities provided a deeper understanding of the interfaces at the atomic scale is achieved and a further development of low temperature processing is reached.

${\mathrm{BaZrO}}_{3}$ is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with sharp and well-defined features. Here, we report the evolution of the Raman spectrum of ${\mathrm{BaZrO}}_{3}$ single crystals in a broad temperature range (4--1200 K) and discuss its origin with the support of detailed first-principle calculations of the lattice dynamics. Phonon calculations are performed not only for the cubic phase of ${\mathrm{BaZrO}}_{3}$, but also for the low-symmetry phases with octahedra tilts that have been suspected to exist at the nanoscale. We show that the Raman spectrum shows no direct evidence for these nanodomains, but can instead be explained by classical second-order Raman scattering. We provide an assignment of the dominant features to phonon mode combinations. In particular, we show that the high frequency range of the spectrum is dominated by overtones and shows an image of the phonon density of states corresponding to the stretching modes of the oxygen octahedra.

International audience; We report the growth of BaZrO3 single crystals by the optical floating zone technique and the investigation on its flux growth using BaB2O4 as a solvent. 6 mm long colorless and transparent single crystals were obtained with a mirror furnace without the need for post-treatment annealing. Its properties are determined and compared with those of two commercial crystals grown by the tri-arc Czochralski method. The chemical composition was investigated using glow discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS), which indicate minor impurities of Sr, Hf, Ca and Ti, with maximal concentrations for Sr and Hf in the range of 0.3–0.5% at. The optical band gap determined by UV-visible spectroscopy is found to be ∼4.8 eV and indicates the high quality of the BaZrO3 crystals grown by the optical floating zone technique. Raman spectroscopy at ambient conditions and at low temperatures down to 4.2 K reveals a relatively sharp second-order spectrum and does not reveal any structural phase transition. Prospective high-temperature solution growth using BaB2O4 self-flux was investigated and led to 150–200 μm BaZrO3 crystals. This solvent opens the way to grow BaZrO3 at half its melting point by the flux method.

From DFT based calculations establishing energy-volume equations of state and electron localization mapping, the electronic structure and crystal chemistry changes from Sn2TiO4 to Sn2TiO6 by oxidation are rationalized; the key effect being the destabilization of divalent tin SnII towards tetravalent state SnIV leading to rutile Sn2TiO6 as experimentally observed. The subsequent electronic structure change is highlighted in the relative change of the electronic band gap which increases from ~1eV up to 2.2 eV and the 1.5 times increase of the bulk modulus assigned to the change from covalently SnII based compound to the more ionic SnIV one. Such trends are also confronted with the relevant properties of black SnIIO characterized by very small band gap., Comment: 21 pages; 5 Figs, 1 table

Monocrystalline titanium dioxide (TiO2) micro-spheres support two orthogonal magnetic dipole modes at terahertz (THz) frequencies due to strong dielectric anisotropy. For the first time, we experimentally detected the splitting of the first Mie mode in spheres of radii inline imagem through near-field time-domain THz spectroscopy. By fitting the Fano lineshape model to the experimentally obtained spectra of the electric field detected by the sub-wavelength aperture probe, we found that the magnetic dipole resonances in TiO2 spheres have narrow linewidths of only tens of gigahertz. Anisotropic TiO2 micro-resonators can be used to enhance the interplay of magnetic and electric dipole resonances in the emerging THz all-dielectric metamaterial technology.

International audience; Polycrystals and centimeter-sized BaTiO3-based single crystals were grown by top seeded solution growth from the BaTiO3–CaTiO3–BaZrO3 system. High effective partition coefficients of Zr ranging from 15 to 6 with small Zr content have been calculated from Castaing microprobe measurements, whereas those of Ca increase slightly from 0.45 to 0.7. A spinodal decomposition mechanism is emphasized during the growth leading to the emergence of two phases with close compositions. Chemical analysis displayed periodical Zr and Ca content fluctuations within the whole boules, and Rietveld measurements highlighted two phases belonging to perovskite structures with tetragonal P4mm and orthorhombic Amm2 space groups. Samples with various calcium and zirconium contents were characterized by means of dielectric and piezoelectric measurements. Most efficient samples are indistinctly polycrystals or oriented single crystals where electromechanical performances are compositional-dependent. Polycrystalline samples and single crystals oriented along (001)pc and (110)pc displayed Curie temperatures ranging from 50 to 111 °C. Electromechanical coupling factor up to 58% and piezoelectric charge coefficient d33 = 496 pC·N–1 were obtained at room temperature. The miscibility gap between the two perovskite solid solutions as well as Ca and Zr element content variation in single crystals lower crystal piezoelectric response that remains nonetheless of the same efficiency compared to that of ceramics of the same composition.

International audience; (K,Na)NbO 3 -based ferroelectric single crystals have recently undergone a substantial development, resulting in improved crystal quality and large piezoelectric coefficients, exceeding 700 pC/N, over a broad temperature range. However, further development necessitates a detailed understanding of the mechanisms defining the domain structure and its temperature evolution. This paper presents the investigation into the crystallographic structure and domain configurations of a (K,Na,Li)(Ta,Nb)O 3 single crystal over a broad temperature range. The crystal was grown by the submerged-seed solution growth technique and investigated using in situ transmission electron microscopy, X-ray diffraction, dielectric measurements, and polarized light microscopy. The lattice distortion, structural phase transitions, and domain configurations are reported. A transition from the lamellar orthorhombic to the rectangular tetragonal domain structure is observed upon heating. Moreover, the milky optical appearance of the crystal was investigated and found to result from the presence of regions with different domain configurations and domain sizes. The formation of these regions is related to the growth defects, which govern the domain formation when cooling below the Curie temperature.

The functional properties of P(VDF-co-TrFE) are strongly dependent on its structure, which, in turn, depends on processing conditions applied. In this work we investigate the P(VDF-co-TrFE) processing structure-function relationships, in order to find the thermal conditions that result in optimum ferroelectric performance in thin film configuration. Our results show that annealing temperature affects mostly the remnant polarization value, P, while annealing time has a severe effect on the coercive field, E-c, An optimized ferroelectric functionality, in terms of high P-r of about 90 mC/m(2) and low E-c of 50 MV/m, is achieved and rationalized through structural analysis by means of GIWAXS. The best performing structure exhibits a high degree of crystallinity, a preferential orientation of the crystallites with the polymer chains parallel to the substrate and the occurrence of three ferroelectric phases. A deconvolution study demonstrates the presence of a moderately unstable ferroelectric phase that is designated to facilitate ferroelectric switching. Our findings show that a single step of 15 min annealing at 135 degrees C leads to high performance P(VDF-co-TrFE) structure, proving that the 2 h-long annealing step that is traditionally applied is not necessary. (C) 2018 Elsevier Ltd. All rights reserved.

International audience; The dielectric properties of Ba1-xCaxTi1-xZrxO3 (BCTZ) thin films are explored by varying x from 0 to 0.18 through an RF magnetron co-sputtering process. Thin films are analyzed by Rutherford Backscattering Spectroscopy, X-Ray Diffraction and dielectric measurements on parallel plate capacitors as a function of temperature (from 100 to 420 K), frequency (from 100 Hz to 1 MHz) and static electric field (from 0 to 250 kV cm−1). The evolutions of their dielectric properties with composition are systematically compared to bulk ceramic counterparts. In both cases, a continuous crossover from classical-to-relaxor ferroelectric behavior along with a decrease of the temperature of the permittivity maximum are observed as x increases. Besides, a phase convergence region (PCR) between cubic, tetragonal, orthorhombic and rhombohedral symmetries is evidenced for compositions close to x = 0.12. This is the first time that such a PCR is observed in thin films. At this point in the phase diagram, all of the dielectric parameters undergo specific variations. In particular, the enhanced permittivity dependence to external electric field for these Ca and Zr contents is attributed to an emerging relaxor behavior together with its vicinity of the PCR. These results make BCTZ thin films potential candidates to achieve electrically tunable devices.

International audience; Ferroelectric single crystals exhibit the largest known piezoelectric constants and therefore show high potential for application in electronic devices, including sensors, actuators, and transducers. Their large properties, however, can only be fully exploited if the behavior is not influenced by lattice defects, formed during high-temperature processing. Such defects can inhibit domain-wall movement, increase leakage currents, and are predominantly responsible for the poor performance of the emerging ferroelectric crystals. Here, an approach to considerably enhance the piezoelectric and ferroelectric properties of (K,Na,Li)(Nb,Ta,Sb)O3 crystals by oxygen annealing is investigated. As compared to the non-annealed crystals, polarization, strain, and piezoelectric constant of the annealed sample were enhanced by a factor of two, resulting in an outstanding room-temperature value of piezoelectric constant d33 = 732 pC/N and a peak value of d33 = 1431 pm/V at the orthorhombic-tetragonal transition. The behavior was analyzed using electromechanical measurements, Mössbauer spectroscopy, and impedance spectroscopy, revealing decreased concentrations of both oxygen vacancies and Sb3+ ions after annealing. The findings indicate that the control of the defect chemistry enables these ferroelectric (K,Na)NbO3-based single crystals to outperform their polycrystalline counterparts and reach the values of lead-containing compositions. Moreover, the present strategy enables further detailed studies of the inherent crystal anisotropy and the possibilities for domain engineering.

International audience; The re-oxidation kinetics of BaTiO3 ceramics sintered by Spark Plasma Sintering (SPS) was investigated using in-situ impedance spectroscopy. Thanks to the flexibility of the SPS process, the grain size of the dense ceramics was tuned from 0.5 μm to 10 μm. The re-oxidation kinetics are found to be very fast regardless of the grain size and a full re-oxidation of the ceramics are achieved after 20 h of exposure to an ambient environment at only 600 °C. The residual density of charge carriers is reduced when using finer starting powders. SPS ceramics made with micrometer size grains demonstrate a residual charge-carrier density that is one tenth that of ceramics made from 10 μm particles. Grain-boundary conduction is dominant through fine-grain SPS ceramics. This latter feature is similar to BaTiO3 sintered using the conventional route with 10 μm size grain. Finally, the critical grain size for optimal dielectric permittivity is found to shift from 0.7 μm in standard ceramics to 1.5 μm in SPS ceramics.

We show that the type-II multiferroic properties of bulk MnWO4 are kept in nanostructured ceramics of 50 nm grain size prepared via spark plasma sintering (SPS). This means that ferroelectric polarization is robust against downsizing, which is at variance with standard ferroelectrics like BaTiO3. We ascribe this stability to the spin-driven nature of the ferroe-lectric correlations in type-II multiferroics while it is resulting from lattice distortion in other cases. This may open the way for persistent type-II multiferroicity with no need for external stabilization like substrate-generated strain.

International audience; In the framework of the crystal chemical investigation of Tetragonal Tungsten Bronze (TTB) niobates, the Ba2NdFeNb4−xTaxO15 solid solution has been investigated in order to gain insights on the relaxor to ferroelectric crossover and its features. Single-phase ceramics were synthesised by conventional two-step firing and sintering and they underwent extended chemical, structural, microstructural and physical characterizations. For Nb-rich compositions (x ≤ 0.5), the temperature dependence of the dielectric permittivity, the pyroelectric coefficient, and also the ferroelectric hysteresis loops revealed a nonconventional relaxor to ferroelectric crossover, which was confirmed by piezoelectric measurements for x ≤ 0.1. A thermal hysteresis of the ferroelectric transition revealed the existence of a metastable ferroelectric phase in these TTB niobates. An accurate and reliable dielectric phase diagram is established from these observations, which allows discussing the original sequence of dielectric phase transitions encountered in this solid solution.

International audience; Lattice structure transformations in nanopowders of ferroelectric BaTiO3 and BaTiO3@SiO2 core-shell nanostructured ceramics were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) at the temperatures 120–450 K and particle size of 300 and 500 nm. NMR spectra of all studied samples in the paraelectric phase are identical to the spectra in bulk material indicating their perfect perovskite structure without visible influence of particle surface. However, we have found that surface of particles essentially influence the ferroelectric phase transitions detected by both NMR and EPR techniques. The strongest changes as compared to bulk material were observed in BaTiO3@SiO2 core-shell ceramics. Thorough analysis of NMR spectra suggests that the orthorhombic-like symmetry phase coexists here with other polar phases up to the Curie temperature. Depending on temperature, its relative volume varies from 25% to 100%. We assume that the orthorhombic-like symmetry phase is stabilized by anisotropic components of surface stresses which increase also global stability of polar state in nanoceramics to the temperature in bulk material. We summarize our results in a phase diagram.

International audience; A reliable and flexible synthesis route was used for processing high density Ni–BaTiO3 nanocable arrays based on wet chemical impregnation and subsequent electrodeposition within a highly ordered unidirectional porous alumina membrane. The core–shell structure was carefully investigated by bright field scanning transmission electronic microscopy coupled with energy dispersive X-ray spectroscopy. The strength of the dipolar interaction arising from the packing density of the magnetic nanowires was correlated with the BaTiO3 wall thickness through magnetometry and ferromagnetic resonance measurements. Our approach opens a pathway to obtain optimized nanostructured multiferroic composites exhibiting tunable magnetic properties.

International audience; A new strategy to elaborate (1-3) type multiferroic nanocomposites with controlled dimensions and vertical alignment is presented. The process involves a supported nanoporous alumina layer as a template for growth of free-standing and vertically aligned CoFe2 nanopillars using a room temperature pulsed electrodeposition process. Ba0.70Sr0.30TiO3–CoFe2O4 multiferroic nanocomposites were grown through direct deposition of Ba0.7Sr0.3TiO3 films by radio-frequency sputtering on the top surface of the pillar structure, with in situ simultaneous oxidation of CoFe2 nanopillars. The vertically aligned multiferroic nanocomposites were characterized using various techniques for their structural and physical properties. The large interfacial area between the ferrimagnetic and ferroelectric phases leads to a magnetoelectric voltage coefficient as large as ~320 mV cm−1 Oe−1 at room temperature, reaching the highest values reported so far for vertically architectured nanocomposite systems. This simple method has great potential for large-scale synthesis of many other hybrid vertically aligned multiferroic heterostructures.

International audience; The present work aims to obtain Hydroxyapatite (HA) – Barium Titanate (BT) piezocomposites with improved electrical properties by retaining OH-ion and tuning BT grain size. Dense piezocomposites of HA-BT were obtained using single step Spark Plasma Sintering (SPS). The presence of OH-ions in the sintered piezocomposites of HA-BT was confirmed by high-temperature X-ray diffraction and Fourier transform infrared transmittance. No decomposition/ interaction between HA and BT phases were observed in the complete range of temperature studied. The dielectric and pyroelectric properties were studied as a function of temperature (100 K to 450 K) and frequency (100 Hz to 2 MHz). The influence of the ferroelectric grain size (300 and 500 nm) on the electrical properties of HA-BT has been analyzed. The dielectric permittivity of HA-BT ceramics can be enhanced with appropriate ferroelectric grain size and volume fraction of BT (

International audience; Ferroelectrics materials have been tremendously attractive since the 40 s with the discovery of ferroelectricity in metal oxide perovskite materials and more precisely in barium titanate. Due to their high potential for industrial applications, intensive research has been carried out to better understand their behavior and develop processes to produce them. Trying to face the down scaling demand of high quality particles towards the nanometer range, some conventional methods such as the solid state one reach their limits. The development of other processes are thus required and the synthesis in supercritical fluids can be considered as a promising alternative. This technology exhibits very interesting characteristics such as fast continuous synthesis (few seconds) of high quality nanoparticles (well crystallized nanoparticles with narrow size distribution) with controlled composition (Ba1−xSrxTiO3 with 0 ⩽ x ⩽ 1) at intermediate synthesis temperatures (

International audience; Centimeter-sized single crystals of Ba2LaFeNb4O15 were grown from a high-temperature solution by using LiBO2 flux and a sealed platinum assembly. The obtained single crystals display the same physical properties as their ceramic counterparts. A frequency-dependent dielectric permittivity maximum was found (Tm = 100 K at 5 kHz), which indicates relaxor behavior. Magnetic susceptibility measurements revealed purely paramagnetic behavior between 10 and 350 K. X-ray diffraction measurements of Ba2LaFeNb4O15 single crystals revealed an incommensurate structure at room temperature with a bidimensional modulation characterized by vectors q1 = (α, α, 1/2) and q2 = (α, -α, 1/2) with α = 0.295(1). This crystal growth method offers a promising elaboration route to centimeter-sized crystals of niobate-based compounds, which may not be grown from the pure liquid phase, especially those with a tetragonal tungsten bronze (TTB) structure.

We have grown by flux method centimeter-sized single crystals from pseudo-hexanary Li2O–Na2O–K2O–Nb2O5–Ta2O5–Sb2O3 system. Based on chemical analysis, crystals of compositions (Li0.023Na0.583K0.394)(Nb0.925Ta0.037Sb0.038)O3 and (Li0.034Na0.609K0.357)(Nb0.896Ta0.047Sb0.057)O3 were characterized by X-rays diffraction which revealed a tetragonal structure. The dielectric analysis confirmed that the ferroelectric behavior of these crystals is very sensitive to little changes in composition as previously observed on ceramics. Such high flexibility of the ferroelectric properties in crystals opens the way towards improved understanding of the relations between structure and polarization in solid solutions which may be an alternative to the lead-based materials.

This review will deal with several types of free charge localization in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localization at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localization of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons is so high as to drive a net lattice of interacting polarons. On the other hand, in ferroelectric materials like BaTiO3 and LiNbO3, the density of polarons is usually very small but they can influence strongly the macroscopic conductivity. The contribution of such polarons to the dielectric spectra of ferroelectric materials is described. Even residual impurities as for example Iron can induce well-defined anomalies at very low temperatures. This is mostly resulting from the interaction between localized polarons and the highly polarizable ferroelectric network in which they are embedded. The case of such residual polarons in SrTiO3 will be described in more detail, emphasizing the quantum polaron state at liquid helium temperatures. Recently, several nonferroelectric oxides have been shown to display giant effective dielectric permittivity. It is first shown that the frequency/temperature behavior of such parameters is very similar in very different compounds (donor-doped BaTiO3, CaCu3Ti4O12, LuFe2O4, Li-doped NiO, etc.). This similarity calls for a common origin of the giant dielectric permittivity in these compounds. A space charge localization at macroscopic interfaces can be the key for such extremely high dielectric permittivity.

International audience; Orientation and temperature dependence of dielectric and electromechanical properties of Mn-doped (Li,Na,K)(Nb,Ta)O3 single crystals were investigated. Samples exhibited very low dielectric losses, with tanδ between 0.03 and 0.05 over a broad temperature range between room temperature and 480 °C. Influences of the crystallographic structure and external electric field on polarization and strain parameters are discussed. The orientation-dependent electrical properties were ascribed to the anisotropic polarization rotation in the crystals. Higher maximum polarization, coercive field, and negative strain were achieved when the electric field was oriented along one of the spontaneous polarization directions. The highest maximum unipolar strain of 0.42% (at 3 kV/mm) and the normalized strain d 33 * of 1391 pm/V were obtained in the [001]PC-oriented sample at 100 °C, which was much higher than the values obtained for the [110]PC-oriented sample. Further insight of the phase transition behavior is given by comparing the temperature-dependence of the small- and large-signal dielectric and piezoelectric properties. The observed changes are rationalized by the different increase rates of the dielectric permittivity and piezoelectric coefficients with temperature.

A series of centimeter-sized lead-free piezoelectric Li+- and Ta5+-modified (Na,K)NbO3 single crystals with an ABO3 perovskite structure was successfully grown by the top-seeded solution growth method. Effective segregation of the elements was considered in order to further develop the growth of Li+- and Ta5+-modified (Na,K)NbO3 single crystals. The crystallographic structure and electrical behaviour along the [001]PC orientation were studied. X-ray diffraction, Raman spectroscopy and dielectric studies reveal that the increase in Ta5+ content reduces the orthorhombic–tetragonal phase transition. Bipolar and unipolar strain curves were investigated at 2 kV mm−1. The highest bipolar and unipolar strains associated with a large-signal piezoelectric constant of 368 pm V−1 were achieved for Li0.02Na0.626K0.354Nb0.808Ta0.192O3. The asymmetric bipolar strain curves were observed and related to the existence of internal bias fields induced by defect dipoles, which were created during the crystal growth process.

International audience; All-oxide composites are increasingly investigated in the field of microelectronics and telecommunications because of their multifunctionalities. Advanced control of interfaces is required to fully tune the macroscopic properties of the composites. Processing ferroelectric Ba0.65Sr0.35TiO3 (BST65/35)-TiO2 composites by Spark Plasma Sintering (SPS), we show that the SPS electric current promotes an interphase growth whose thickness depends on the BST/TiO2 stacking and on the current direction. In all cases, the interface is composed of Ba1.14(2) Ti8O16-δ a highly defective, but stable phase which can be used to tune the overall dielectric properties of the composites.

International audience; The production of BaTi1–yZryO3 (0 ≤ y ≤ 1, BTZ) nanocrystals is known to be challenging due the low reactivity of zirconium precursors. Here we have successfully studied the impact of zirconium on the BTZ particle formation in sub- and supercritical fluid conditions along the entire solid solution. In situ synchrotron wide angle X-ray scattering (WAXS) analyses were conducted in batch at 150 and 400 °C to follow, in real time, the BTZ crystallite synthesis. This approach revealed the complexity behind the nucleation and growth mechanisms of ABO3 nanocrystals, especially toward high zirconium content (more than 50 atomic %). This type of substitution induces, among other things, microstrain within the structure. Moreover, for the cases of BaTi0.4Zr0.6O3 and BaTi0.2Zr0.8O3, the experiments showed the apparition of two crystallite size populations. In the BaTi0.4Zr0.6O3 case, at 400 °C, these two size populations merged into a single one after at least 8 min; in contrast to what was observed for the case of BaTi0.2Zr0.8O3. This is a manifestation of how with increasing the zirconium content the particles become more refractory and in these cases the temperature is not high enough to enable their ripening. It is important to note that this behavior was not observed for particles produced at 400 °C using a flow synthesis method, with a residence time of only 50 s. There, the particles presented a single size population close to the one obtained after 8 min in batch. This thus suggests that for batch syntheses a longer time is required to achieve a similar product quality to the one obtained with a flow process.

International audience; A major challenge in the development of efficient magnetoelectric nanocomposites is the adequate control of the interfaces, in order to avoid the formation of undesirable interphases and to ensure an optimal strain mediated coupling. In this work we used a combination of low-cost impregnation and electrodeposition processes within the ordered nanochannels of a porous anodic alumina template, to elaborate CoFe2O4–PbZr0.52Ti0.48O3 core/shell nanocable arrays with high interfacial areas between the ferroelectric and the magnetic phases. We have shown in this way that the thermal annealing steps required for phase's crystallization and oxidation of metallic CoFe2 into spinel CoFe2O4 are critical with respect to interdiffusion phenomena through the interfaces. The impact of the processing temperature on the morphological and structural features of the nanocables was then discussed. We demonstrated that an optimization of process variables in the synthesis of CoFe2O4–PbZr0.52Ti0.48O3 nanocables allows a significant improvement in their microstructure and ensures the chemical integrity of the two-phase materials, an important concern when seeking for enhanced multiferroic properties.

Ba 2 NdFeNb 4 O 15 tetragonal tungsten bronze (TTB)/BaFe 12 O 19 (BaM) hexaferrite bilayers have been grown by RF magnetron sputtering on Pt/TiO 2 /SiO 2 /Si (PtS) substrates. The BaM layer is textured along (0 0 1) while the TTB layer is multioriented regardless of the PtS or BaM/PtS substrate. Dielectric properties of TTB films are similar to those of bulk, i.e. , e ∼ 150 and a magnetic hysteresis loop is obtained from TTB/BaM bilayers, thanks to the BaM component. This demonstrates the possibility of transferring to 2 dimensional structures the composite multiferroic system TTB/BaM previously identified in 3 dimensional bulk ceramics.

International audience; New ferroelectric ceramics of ABO3 perovskite type were synthesized in the Ba(Ni1/3Nb2/3)xTi1−xO3 (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 TC or Tm (for BaTiO3 TC = 340 K, for x = 0.06, TC = 270 K and for x = 0.16, Tm = 150 K). The evolution from classic ferroelectric behavior for (0 ≤ x x ≤ 0.2) is emphasized.

The hysteresis, piezoelectric and pyroelectric properties were measured in the temperature range near the ferroelectric–paraelectric phase transition. The BaTi0.9(Fe1/2Nb1/2)0.1O3 ceramic exhibits typical ferroelectric P –E hysteresis behavior with a remanant polarization, Pr, of about 7.52 μC/cm2 detected at 155 K. The electromechanical properties of this composition were measured using the resonance method. The ceramic provides high piezoelectric performance at the temperature of transition (Tmax = 216 K): the piezoelectric constant is d31 = 140 pC/N and the electromechanical coupling factor was kP = 22%. The pyroelectric study confirms the dielectric and ferroelectric measurements. The pyroelectric coefficient is about 125 nC/cm2 K at Tmax. Raman spectra of BaTi0.9(Fe1/2Nb1/2)0.1O3 ceramic were taken at various temperatures and measured over the wave number range from 150 to 1300 cm–1. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; New ferroelectric ceramics of ABO3 perovskite type were synthesized in the BaTi1-x(Zn1/3Nb2/3)xO3 system by solid state reaction technique. The effect of the replacement of titanium by zinc (Zn) and niobium (Nb) in the B cationic site on structural and dielectric properties were investigated. As a function of composition, these compounds crystallize with tetragonal or cubic symmetry. The material is classical ferroelectric for 0 ≤ x ≤ 0.05 and presents a relaxor behavior for 0.075 ≤ x ≤ 0.2 and for 0.75 ≤ x ≤ 0.975. Dielectric permittivity in the temperature range from 85 to 500 K with frequencies range from 0.1 to 200 kHz, was studied. In the region when 0.75 ≤ x ≤ 0.975, ΔTm presents the important values which go more then 100 K for BaTi0.05(Zn1/3Nb2/3)0.95O3 composition with values of Tm near room temperature. These values make these ceramic compositions in the families of relaxors with interest properties for applications. © 2007 Elsevier B.V. All rights reserved.

International audience; Spark Plasma Sintering (SPS) is an efficient technique to produce highly densified ferroelectric–dielectric composites with good control of the interfaces and microstructures. We present an effective approach based on precursor preparation to produce BaTiO3/MgO with adjustable hardness for the inclusions. The influence of their rigidity on the sintering composites was investigated and the possibility of being able to tailor the permittivity simply by changing the morphology of the MgO inclusions was demonstrated.

International audience; Accurate control of residual defect density is required for reliable investigation and use of ferroelectric materials. After reviewing the long term endeavor to decrease defect contributions in bulk materials, which reached mass production decades ago, recent challenges are underlined. These mostly result from the continuous trend towards integration which has reached the nanometre range. The contribution of solid state chemistry is of key relevance for improving the present processing routes and suggesting alternative ones, for example by controlling a large density of charged defects to reach unprecedented functionalities. Some of these breakthroughs are reviewed.

International audience; n a non-magnetic dielectric sphere of high-permittivity (

International audience; Macroscopic anisotropy in polycrystalline materials is of key interest since it may help filling the gap between randomly oriented polycrystals like ceramics and single crystals. Non-destructive X-ray Computed Micro Tomography (XCMT) is a necessary step towards the full control and modelling of such anisotropy, beyond the standard scheme of interfaces. To ascertain this progress, XCMT is applied to 3D mixtures of ferroelectric and dielectric oxides processed by Spark Plasma Sintering (SPS). In such conditions, not only is this anisotropy seen in the overall dielectric parameters but it also shows up in ferroelectric properties. Experimental macroscopic parameters are linked to the 3D morphological anisotropy of individual MgO inclusions induced during SPS.

International audience; Polycrystalline samples of Mn1-xCuxWO4 (x ≤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 °C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO4 caused by replacing Jahn-Teller-inactive Mn(2+) (d(5), S = 5/2) ions with Jahn-Teller-active Cu(2+) (d(9), S = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ≤ x ≤ 0.25 are solid solution that forms in the monoclinic P2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO4 structure and suggest random distributions of Jahn-Teller-distorted CuO6 octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO4. Type-II multiferroicity survives a weak Cu substitution rate (x < 0.15). Multiferroic transition temperature and Néel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for x = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO4 strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration.

We have investigated the sintering behaviour of cubic BaTiO3 nanoparticles obtained under supercritical conditions. The ferroelectric properties of samples sintered between 950 and 1300 °C were measured from room temperature up to 200 °C and in the frequency range 100 Hz–10 kHz. The influence of the initial powder characteristics on the microstructure and dielectric properties of ceramics have been investigated. The ferroelectric transition temperature was used as an intrinsic probe for the ceramic characterization and was then found to be dependent on the Ba:Ti ratio of the initial powder. Understanding the role of the key parameters at different scales (bulk and nanoscale composition, tetragonal distortion, microstructure) on the permittivity, losses and transition temperature would lead to a possible modulation of ferroelectric properties.

The efficient use of ferroelectric thin films in radio-frequency agile devices faces several limitations. One of them is imposed by the dielectric losses which are usually above 1%, i.e. above the threshold as set by the electronic industry. Following the same route as for bulk ceramics, we have processed composite stacks made of BST/SiO2 multilayers using radio-frequency magnetron sputtering. Doing so, we were able to repeatedly achieve dielectric losses well below 0.5% while keeping a high dielectric susceptibility and a suitable tunability. All of these improvements have been observed at low frequencies (f 1 GHz) is currently in progress.

BaTi1−xZrxO3 thin films have been prepared on Si and Pt/TiO2/SiO2/Si substrates by radio-frequency magnetron sputtering varying the deposition parameters, such as the chamber pressure, the substrate temperature and the gas composition. The films have been probed by X-ray Diffraction (XRD), Wavelength Dispersive Spectrometry (WDS), Scanning Electron Microscopy (SEM) and Rutherford Backscattering Spectrometry (RBS). The dependence of their composition, crystallinity and dielectric properties on these parameters was investigated. The films show dielectric susceptibilities up to 130, dielectric losses of 2% and tunabilities up to 17% under 650 kV/cm at 100 kHz.

The effect of grain size on the dielectric properties of Ba0.6Sr0.4TiO3 (BST) ceramics is investigated. Attrition milling is chosen to obtain nanometre particle size from micrometre particle size powders. Fine grained ceramics are obtained by hot uniaxial pressing (HUP). Additionally, the present study is focused on the effect of the nature of milling balls on loss tangent and permittivity. For that, three kinds of balls are tested: calcia, yttria or ceria stabilised zirconia balls. The properties of these samples are evaluated in the range of MHz. The balls induce an involuntary doping of powders which modifies the dielectric properties, especially the Curie temperature and loss tangent.

At the present time, the most widely-used piezoelectric materials belongs mainly to the PZT solid solution system (1-x) PbTixO3 − × ZrTiO3. Because of the possible evaporation of lead oxides during...

Ferroelectric tantalates with the tetragonal tungsten bronze (TTB) type structure were studied over a wide frequency range from 10 2 to 10 9 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 10 8 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 Ta 5+ ions are considered.

International audience; Nano-sized ferroelectric powders have been embedded for the first time in an amorphous silica shell. Massive composites made of these nano-objects have the same ferroelectric phase-transition temperature as the related pure ferroelectric phase, showing that chemical interdiffusion between the core and the shell is moderate. This is of key importance in building physical models for computing the electrical properties of the composites.

The relaxation map of highly concentrated sucrose water mixtures was built using mechanical and impedance spectroscopies. Data of α- and β-relaxation processes obtained with both techniques complete calorimetric and rheological measurements. The temperature evolutions of the relaxations were extrapolated using the measured data and the equations commonly used to describe the relaxations: Arrhenius and WLF behaviours for respectively the β- and α -relaxations. The temperature/frequency domain when α and β processes merge for 99% sucrose solution is discussed with respect to scenery proposed in the literature.