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Using pulsed laser deposition, we grew a ferroelectric [BaTiO3]Λ/2/[BaZrO3]Λ/2 (BT/BZ) superlattice with a stacking periodicity of Λ = 256 A and a BT single film on a La½Sr½CoOa-buffered (00l)MgO substrate, and then studied the phase transitions of these materials. At room temperature, the polarized Raman spectra of the BT film corresponded to a ferroelectric orthorhombic C2v14 phase with the polar axis oriented in the plane of the substrate. A ferroelectric-paraelectric phase transition in the BT film occurred at ~ 450 K. Upon cooling to ~ 300 K, a phase transition to the monoclinic Cs3 phase occurred. These experimental results agree well with a theoretical "temperature-misfit strain" phase diagram of the BT film. We found no evidence of phase transitions in the BT/BZ superlattice below room temperature. The phase transition to the paraelec-tric phase in the BT/BZ superlattice increased in temperature because of lattice mismatch between the BT and BZ layers. A desirable Curie temperature can be tailored rather precisely by varying the layer thicknesses or the BT/BZ ratio in the superlattice. The BT/BZ superlattices are very good for device applications because their ferroelectric phase with in-plane polarization can remain stable at least from 0 K up to 723 K, and even at higher temperatures in superlattices with smaller periodicities. [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]

BaTiO3/SrTiO3superlattice has been grown on (001)MgO substrate by pulsed laser deposition. The unit cell parameters of BaTiO3and SrTiO3layers and modulation period have been determined using X-ray diffraction. It was found that BaTiO3and SrTiO3layers revealed the pseudo-tetragonal distortion with out-of-plane and in-plane polarization, respectively. Electrostatic interaction of the alternating layers leads to the polarization rotations and to the symmetry lowering of the superlattice to orthorhombic or monoclinic with partial depolarization of the Raman spectra. It was found that the phase transition to the paraelectric phase occurs at ∼480 K. [ABSTRACT FROM PUBLISHER]

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]

A prominent central peak in the sub-THz frequency range was observed in the Raman spectra of BaTiO3/(Ba,Sr)TiO3 (BT/BST) superlattice grown on (001)MgO substrate. Both soft and central mode show an anomaly around 200 K and 280 K, which can be correlated with orthorhombic to monoclinic phase transition of BST and BT, respectively. The observed temperature dependence of the central mode enabled us to explain rather broad temperature dependence of the dielectric permittivity previously observed in BT/BST superlattices. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract We investigate the lead free ferroelectric perovskite ceramic, (Ba 0.85 Ca 0.15)(Zr 0.1 Ti 0.9)O 3 (BCZT), prepared by sol-gel method. The as-synthesized precursor powder is calcined at 1000 °C for 4 h in order to obtain single-phase perovskite structure. X-ray diffraction (XRD) analysis at room temperature shows the formation of single-phase compound with a tetragonal structure refined in P 4mm space group. The dielectric measurements in a frequency range of [100 Hz-1 MHz] and in a temperature range [35–400 °C] in the ceramic sintered at 1350 °C for 2 h, reveal a high maximum dielectric constant ∼5200 (100 Hz) at 107 °C, which is associated with ferroelectric paraelectric phase transition. The sol-gel method has displayed a very important advantages comparing to the conventional solid-state method. Special emphasis was done to characterize diffuse phase transition (DPT) that occurs close to ∼107 °C. We have shown that the real part of the permittivity close to DPT is well described by Santos–Eiras phenomenological model. Impedance spectroscopy analysis over the frequency range of 100 Hz to 1 MHz illustrated mainly bulk contribution up to 300 °C while grain and grain-boundary effects were present above 300 °C using the impedance spectroscopy technique. Highlights • The XRD study showed a pure perovskite structure without impurities with tetragonal symmetry refined to P4mm space group. • The sol-gel method has displayed a very important advantages comparing to the conventional solid-state method. • The real part of the permittivity close to DPT is well described by Santos–Eiras phenomenological model. [ABSTRACT FROM AUTHOR]

Near- and sub-Terahertz dynamics of soft and Debye-type central modes was studied by the polarized Raman spectroscopy in ferroelectric BaTiO 3 /Ba 0.3 Sr 0.7 TiO 3 (BT/BST) superlattice in the temperature range of 80–400 K where system undergoes the series of phase transitions: paraelectric-tetragonal–monoclinic in BT layers and paraelectric-orthorhombic-monoclinic in BST layers. It was shown that temperature evolution of these modes can be described within the model of coexisted damped harmonic oscillator and Debye relaxator. The occurrence of the pronounced central mode can explain the recently observed relaxor-like dielectric anomaly in such superlattice. [ABSTRACT FROM AUTHOR]

The exact mechanism responsible for the ferroelectricity in hexagonal manganite has been the subject of intense debate. Whether ferroelectricity and ferroelastic order appear at the same temperature and the role of covalency in the ferroelectric order are still discussed. High-temperature phase transitions are here investigated through the prism of lattice-dynamics evolution with temperature. Comparison is made between the YMnO3 and YbMnO3 behavior using polarized Raman spectroscopy. While YMnO3 shows two phase transitions (isosymetric at about 900 K and ferroelastic at 1200 K) YbMnO3 presents no lattice instability up to 1350 K. Phonons involved in the lattice instability and the zone tripling transition are identified. Moreover, peculiar hardening on heating of some YbMnO3 phonons is revealed and discussed, which further highlights the dynamical difference between these two hexagonal manganites. [ABSTRACT FROM AUTHOR]

Electrocaloric effect of lead-free Ba(Zr 0.1 Ti 0.9 ) 1− x Sn x O 3 ( x =0 and x =0.075) ceramics is investigated. In the composition x =0, the electrocaloric strength ( ξ ) was about 0.23 K mm/kV at Curie temperature. The incorporation of a small amount of tin (Sn) ions in the matrix of Ba(Zr 0.1 Ti 0.9 )O 3 was found to decrease the Curie temperature T C toward room temperature while the value of the electrocaloric strength remained practically unchanged ( ξ =0.22 K mm/kV). [ABSTRACT FROM AUTHOR]

Highlights: [•] Lead free Ba0.8Ca0.2Ti0.8Ge0.2O3 ceramics. [•] Dielectric and ferroelectric properties. [•] Low coercive field for polarization switching. [•] Change of the nature of BaTiO3 phase transition by co-doping. [Copyright &y& Elsevier]

Lead-free compounds with formula Ba 0.975 Ln 0.017 (Ti 0.90 Zr 0.05 Sn 0.05)O 3 (BLnZ5TS) and Ba 0.975 Ln 0.017 (Ti 0.75 Zr 0.20 Sn 0.05)O 3 (BLnZ20TS) [Ln = Eu, Ho] were synthesized using the solid-state reaction technique. Scanning electron micrographs of ceramics showed a decrease in the average grain size as the Ln3+ radii increases. The structural study allowed the identification of tetragonal and cubic symmetries at room temperature for BLnZ5TS and BLnZ20TS, respectively. Dielectric measurements were carried out as a function of temperature and frequency. The results revealed that BLnZ5TS exhibited normal ferroelectric behavior with diffuse phase transition, whereas BLnZ20TS displayed relaxor ferroelectric properties. The diffuse phase transition parameters were determined from the modified Curie–Weiss law, while the relaxor behavior in BLnZ20TS was highlighted by a good fit to the Vogel–Fülcher relationship. The photoluminescence of the prepared specimens was investigated and the results showed that these compositions bear interest for green and red emission. [Display omitted] [ABSTRACT FROM AUTHOR]