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Direct and indirect electrocaloric measurements were performed on the new Tellurium (Te) doped Ba0.8Ca0.2TiO3 (BCT) ceramics. The effects of Te addition on structural, electrical, and electrocaloric properties of BCT ceramics were investigated. The incorporation of the Te element in the BCT induced the decrease of the Curie temperature and the enhancement of the electrocaloric effect. The significant electrocaloric temperature change DT = 1.237 K determined by the direct method was obtained at the relatively moderate field of ~25 kV/cm in Ba0.8Ca0.2Ti(1-x)TexO3 with x = 0.02. The corresponding electrocaloric responsivity DT/DE = 0.495 × 10-6Km V-1 is higher than that observed in pure BCT ceramics and is one of the highest reported so far in lead-free ferroelectric materials. The material's coefficient of performance was determined at the phase transition with a maximal value of 14.7. [ABSTRACT FROM AUTHOR]

Artificial superlattices of ferroelectric Bi0.5Na0.5TiO3 and BaTiO3 have been successfully grown on (001) insulator or conductive SrTiO3 substrates by pulsed laser deposition. In these epitaxial layered structures, the BaTiO3 layers were shown to contribute to an improvement of the two dimensional growth of the Bi0.5Na0.5TiO3 layers. The influence of the superlattice period Λ, between 5 and 20 nm, was investigated. We observe an increase in the in-plane tensile strain as Λ is reduced from 20 nm to 10 nm, accompanied by a decrease in the density of dislocations. A concomitant enhancement of the dielectric permittivity was measured, demonstrating the strain tunability of these superlattices. A significant reduction of the dielectric losses is also obtained with decreasing Λ. Furthermore, a minimum value of the coercive field of less than 70 kV/cm, close to that of Bi0.5Na0.5TiO3 bulk ceramics, was reached by decreasing the period. We demonstrate that the dielectric and ferroelectric properties can be explained as the result of the in-plane tensile strain which contributes to improve the structural properties in the superlattices. [ABSTRACT FROM AUTHOR]

We report an x-ray diffraction (XRD) and a Raman-scattering investigation of ferroelectric/paraelectric superlattices [BaTiO3] (1-x)Λ/[BaZrO3]xΛ for which the composition varied, 0.15 ⩽ x ⩽ 0.85, while the superlattice (SL) modulation period Λ was kept constant at about 100Å. The samples were epitaxially grown by pulsed laser deposition on MgO substrates buffered with La0.5Sr0.5CoO3. Based on the XRD analysis and on polarized Raman spectra, we have showed that the large strain in SLs induced ferroelectricity in BaZrO3 (BZ) for all SLs, a material that is paraelectric in the bulk form at any temperature and in the single film. The induced polar axis in BZ layers is perpendicular to the plane of substrate while BaTiO3 (BT) layers exhibit in-plane polar orientation. Raman spectroscopy revealed a lattice ordering in SLs due to the misfit strain generated by the large lattice mismatch between the alternating BZ and BT layers. This strain induced a huge upward frequency of the lowest E(1TO) soft mode from 60 cm-1 in the BT single film to 215 cm-1 in the SL with x=0.85. These results show that in spite of relatively large periodicity of SLs, they are highly constrained and the variation of BZ ratio allowed modifying strains between layers. The temperature dependence of the Raman spectra for BT0.3Λ/BZ0.7Λ and BT0.7Λ/BZ0.3Λ samples revealed giant shift of the ferroelectric phase transition. The phase transition temperature was found to be upshifted by about 300°C with respect to BT single crystal. [ABSTRACT FROM AUTHOR]

Phase transitions in gadolinium potassium niobate GdK2Nb5O15 (GKN) ceramics have been investigated by x-ray diffraction, dielectric susceptibility, differential scanning calorimetry, and Raman scattering. The results of our complementary studies show that GKN exhibits two phase transitions at Tc1 = 238 °C and Tc2 = 375 °C attributed to the ferroelectric-antiferroelectric-paraelectric (FE-AFE-PE) phase transitions. According to the x-ray diffraction analysis, the FE and PE phases were refined in the polar P4bm and centrosymmetric P4/mbm space groups. For the intermediate phase, the structure is refined in the space group P4nc with doubling of the c unit cell parameter, which is compatible with an AFE phase. This result was confirmed by Raman spectroscopy since new low-frequency lines are activated in the AFE phase due to the Brillouin zone-folding effect, typical for the modulated phases. The presence of the AFE phase between the FE and PE phases and the complex nature of tetragonal tungsten bronze crystal structure can explain the large thermal hysteresis observed at the FE-AFE transition between heating and cooling cycle and the strong depression of the Curie-Weiss temperature T0. The semi-phenomenological Ising-like model based on the obtained experimental data is proposed to account for the observed FE-AFE-PE transition sequence. [ABSTRACT FROM AUTHOR]

Multiferroic composites with the composition (1 − x) BaTi0.8Sn0.2O3–(x) La0.5Ca0.5MnO3 (x = 0, 0.3, 0.5, 0.7, 0.9 and 1) were prepared by sol–gel techniques. Rietveld refinements of the composites with 0–3 and 3–0 type connectivities exhibit the formation of pure phases and confirm the fraction of the composite samples. It was found that the composites grain size decreases slightly with increasing the La0.5Ca0.5MnO3 content. Magnetic measurements of the composite ceramics exhibit a gradual increase in saturation and remanent magnetization with increasing La0.5Ca0.5MnO3 content. Dielectric data demonstrate that the composite materials show high values of the dielectric constant in comparison with the parent compound BaTi0.8Sn0.2O3 (BTSO). In addition, an attempt is made to measure the magnetoelectric coupling coefficient and to explain the negative permittivity and losses occurring in these materials at a certain frequency and temperature conditions. [ABSTRACT FROM AUTHOR]

Perovskite manganite La0.5Ca0.5−x□xMnO3 (LCMO) nanomaterials were elaborated using the sucrose modified auto combustion method. Rietveld refinements of the X-ray diffraction patterns of the crystalline structure confirm a single-phase orthorhombic state with Pbnm space group (No. 62). The Ca-vacancies were voluntarily created in the LCMO structure in order to study their influence on the magnetic behaviour in the system. The magnetic susceptibility was found to be highly enhanced in the sample with Ca-vacancies. Paramagnetic-to-ferromagnetic phase transition was evidenced in both samples around 254 K. This transition is, characterized by a drastic jump of the susceptibility in the sample with Ca-vacancies. The maximum of entropy change, observed for both compounds at magnetic field of 6 T was 2.30 J kg−1 K−1 and 2.70 J kg−1 K−1 for the parent compound and the lacunar one respectively. The magnetocaloric adiabatic temperature change value calculated by indirect method was 5.6 K and 5.2 K for the non-lacunar and Ca-vacancy compound, respectively. The Ca-lacunar La0.5Ca0.5−x□xMnO3 (x = 0.05) reported in this work demonstrated overall enhancement of the magnetocaloric effect over the LCMO. The technique used to elaborate LCMO materials was beneficial to enhance the magnetocaloric effect and magnetic behaviour. Therefore, we conclude that this less costly environmentally friendly system can be considered as more advantageous candidate for magnetic refrigeration applications then the commonly Gd-based compounds. [ABSTRACT FROM AUTHOR]

Ceramic with composition Na0.925Bi0.075Nb0.925Mn0.075O3 (NNBM0075) was synthesized by high temperature solid state reaction technique. It was studied using X-ray diffraction (XRD), dielectric measurements and Raman spectroscopy. The sample crystallizes in orthorhombic perovskite structure with space group Pbma at room temperature. Dielectric properties of the ceramic was investigated in a broad range of temperatures (-150 to 450 °C) and frequencies (0.1-103 kHz), and show two different anomalies connected to the symmetry change and electrical conductivity. Dielectric frequency dispersion phenomena in the NNBM0075 ceramic was analyzed by impedance spectroscopy in the temperature range from 55 to 425 °C. The Cole-Cole analysis based on electrical circuit and least square method was used to characterize the conduction phenomenon. A separation of the grain and grain boundary properties was achieved using an equivalent circuit model. The different parameters of this circuit were determined using impedance studies. Four conduction ranges, with different activation energies, were determined using the Arrhenius model. Raman spectra were studied as a function of temperatures and confirmed the X-ray and dielectric results. This composition is of interest for applications due to his physical properties and environmentally friendly character. [ABSTRACT FROM AUTHOR]

Artificial superlattices (SLs) consisting of ferroelectric BaTi[O.sub.3] (BT) and paraelectric BaZr[O.sub.3] (BZ) are grown by a pulsed laser deposition method and the out-of-plane lattice parameters of the SLs constituents are determined by modeling of the x-ray diffractograms. Ferroelectric hysteresis loops measurements have shown a clear ferroelectric behavior for the constrained SLs , while a weak ferroelectricity is observed for the large periods, which is attributed to the strain-induced ferroelectricity in BZ layers for the constrained SLs.

Ferroelectric ceramics (Ba0.85Ca0.15)(Ti0.9Zr0.1−xSnx)O3 (x = 0.00, 0.02, 0.04, 0.06) were elaborated by sol–gel method. Structural investigation revealed co-existence of tetragonal (P4mm) and orthorhombic (Pmm2) symmetries at room temperature for the undoped ceramic, while only a tetragonal structure (P4mm) was observed for the doped ceramics. Dielectric measurements indicate a dielectric relaxation process at high temperatures which is essentially related to the hopping of oxygen vacancies V O ¨ . Furthermore, a down shifting of the Curie temperature (TC) with increasing Sn4+ doping rate has been revealed. The temperature profiles of the Raman spectra unveiled the existence of polar nanoregions above the Curie temperature in all ceramics. The ferroelectric properties were found to be related to the microstructure. Electrocaloric effect was investigated in this system that revealed an electrocaloric responsivity of 0.225 × 10−6 K m/V for the composition with x = 0.04 Sn doping, where other remarkable physical properties were also observed. [ABSTRACT FROM AUTHOR]

Good quality In2S3 films were grown by the Dr Blade method from a powder synthesized in a chemical bath, and oxidized to obtain In2O3 films and films of intermediate composition. The oxidation process and kinetics are studied by means of thermogravimetric analysis, which shows complex phenomena. The study is completed by scanning electron microscopy and energy dispersive x-ray measurements. X-ray diffraction and UV–visible spectroscopy results are also shown. The oxidation process begins significantly above 420°C, and it appears that at least one intermediate crystal phase takes place in the solid solution, suggesting that the replacement of sulfur by oxygen atoms occurs at different temperatures in the different crystallographic sites. The obtained samples show a band gap varying continuously between 1.94 eV and 3.72 eV. [ABSTRACT FROM AUTHOR]

Ferroelectric compound GdK 2 Nb 5 O 15 with tetragonal tungsten bronze type structure thin films were grown by pulsed laser deposition on (001) SrRuO 3 /SrTiO 3 and (001) SrRuO 3 /LSCO/MgO substrates respectively, in the same experimental conditions. Structural properties were investigated using X-ray diffraction in different modes. θ –2 θ diffraction shows a single crystalline SRO and GKN phases, with c -axis perpendicular to the substrate. Phi-scans show for both SRO and GKN an epitaxial cube-on-cube growth. Reciprocal space mapping (RSM) was performed on GKN to determine the in-plane lattice parameters, and showed that GKN grown on MgO is more constrained comparing to STO. Room temperature electrical properties were investigated using platinum (Pt) top electrodes. Results show that GKN thin films deposited on MgO and STO substrates are resistively switchable. It was found that the nature of the substrate affects strongly the resistance ratio: GKN on SRO/LSCO/MgO showed a large hysteresis comparing to GKN on SRO/STO. Resistance switching in GKN is attributed to the oxygen vacancies migration which can be controlled by the substrate or the frequency sweep. [ABSTRACT FROM AUTHOR]

The effect of tin (Sn) incorporation in lead-free Ba(ZrTi)O ceramics was investigated. Single phases with perovskite structure were identified using X-ray diffraction analysis. Dielectric permittivity and pyroelectric current measurements carried out versus temperature revealed ferroelectric-to-paraelectric structural phase transitions showing intermediate ferroelectric-to-ferroelectric sequence anomalies for low Sn-content compounds. Electrocaloric effects were highlighted by recording P-E hysteresis loops as function of temperature and the electrocaloric temperature change ∆ T = 0.2 K was evaluated under 8.7 kV cm applied electric field. Electrocaloric responsivity of 0.23 K mm kV near Curie temperature remained almost constant until x = 0.075 before decreasing for compounds with x ≥ 0.1. Basing on pyroelectric measurement, we highlight the presence of one quadruple point in this system where the simultaneous presence of Zr and Sn in BaTiO matrix seems to stabilize ferroelectric and electrocaloric behaviors instead of Sn element alone. [ABSTRACT FROM AUTHOR]

Pb0.88Ln0.08TiO3 ferroelectric system, where Ln = La, Sm, Eu, and Dy, has been characterized using Scanning Electron Microscopy, Raman spectroscopy, and X-ray diffraction experiments. Softening of the lowest transverse optical phonon mode E (1TO) was evaluated as a function of the rare earths' ionic radius suggesting partial occupation of lanthanide ions at the A and B sites of the perovskite structure. Using Rietveld refinements, it has been established a higher incorporation of Ln3+ ions into the A sites of the perovskite structure than that of the B sites for the studied ceramics. The occupation at B sites increases slightly with the decreases of the ionic radii of the lanthanide. [ABSTRACT FROM AUTHOR]