Search

X-ray diffraction (XRD) and dielectric studies of dense hot-pressed K2Sr4Nb10O30ceramics with a tetragonal tungsten bronze–type structure revealed texturing of the samples and substantial anisotropy of the dielectric permittivity. XRD studies of the K2Sr4Nb10O30powder show a diffuse phase transition in the 140 – 160°C range while a diffused and frequency-independent maximum of the dielectric permittivity is observed at about 120–125°C for ceramic samples. Substantial anisotropy of the dielectric permittivity (more than 50% in the phase transition region) as well as sample's texturing and a difference between phase transition temperatures of the powder and ceramic samples are supposed to be caused by anisotropic residual stresses. [ABSTRACT FROM PUBLISHER]

The multicomponent Ising-like network model was developed to describe the ferroelectric phase transition in Tetragonal Tungsten Bronze (TTB) compounds. Using Monte Carlo simulation with Metropolis algorithm we reproduced the phase diagram of this model as a function of the temperature and exchange interaction and demonstrated it coherence with experimental data. [ABSTRACT FROM AUTHOR]

New compositions of Tetragonal Tungsten Bronze (TTB) ferroelectrics Pb1-xK2xNb5O15(PKN) with x = 0.1, 0.2, 0.25 and 0.3 were synthesized using the solid state reaction method. Structural and electrical properties were examined as a function of the ratio Pb/K using X-ray diffraction, Raman spectroscopy and dielectric measurements. The dielectric permittivity and the loss tangent of each composition have been measured in a frequency range 10 Hz–1 MH and in a temperatures range 25–600°C. It was shown that the Pb substitution induces the diffuse phase transition. [ABSTRACT FROM PUBLISHER]

The multi-component Ising-like network model on x-y rectangular lattice was developed to describe the ferroelectric phase transition in Tetragonal Tungsten Bronze (TTB) compounds. Using Monte Carlo simulation with Metropolis algorithm we reproduced the phase diagram of this model as a function of the temperature and exchange interaction and demonstrated it coherence with experimental data. [ABSTRACT FROM AUTHOR]

Ionic conduction is evidenced in PbK2LiNb5O15 single crystal using electric impedance measurements. The phase transition is of first order type and it occurs around 640 K as evidenced by a discontinuity accompanied by a hysteresis of about 10 K, and a global hysteresis up to 720 K. The ionic conduction is attributed to Li+ ion mobility through the structural tunnels by a hoping mechanism. [ABSTRACT FROM AUTHOR]

Substitution of Pb with Gd in the Pb 2 KNb 5 O 15 phases leads to a new composition of solid solution with chemical composition Pb 2(1 - x ) Gd x K 1 + x Nb 5 O 15 (0 ≤ x ≤ 1) which crystallizes with TTB type structure. Powders are pressed and heated into ceramics and their compaction is about 90%. Thermal evolution of the real part of the dielectric permittivity presents different behavior according to the composition. The dielectric permittivity of compounds with Gd composition above x = 0.30 shows two anomalies. The first one appears at T C and is associated to the ferroelectric-paraelectric phase transition. The origin of the last one at which occurs at T 1 is up to now unknown. Domain structure should exhibit strong changes both with temperature and composition. [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]

The Bi 0.7 (Ba 0.8 Sr 0.2 ) 0.3 Fe 0.7 Ti 0.3 O 3 ceramic is synthesized by a solid state reaction method and characterized by X-ray diffraction (XRD), SEM, Raman spectra and impedance spectroscopy. Besides, the XRD pattern shows perovskite phase with rhombohedral structure (R3c space group) at room temperature. Furthermore, the ferroelectric-paraelectric phase transition ( T C ) and G-type antiferromagnetic Neel temperature ( T N ) is detected by Raman spectroscopy. Meanwhile, the electrical conductivity of the sample is investigated in the temperature and the frequency range 713–773 K and 100–1000 kHz respectively by means of impedance spectroscopy. The Nyquist plots (−Z″ versus Z′) frequency plots are well fitted to an equivalent circuit model. The ac conductivity data obey the universal power law. The nature of variation of dc conductivity suggests Arrhenius type of electrical conductivity. The temperature dependence of the alternative current conductivity ( σ g ) and direct current conductivity ( σ dc ) confirm the presence of the ferroelectric–paraelectric phase transition. [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]

Abstract: Ferroelectric ceramics Pb2−x K1+x Li x Nb5O15 (PKLN), with , having the tetragonal tungsten bronze (TTB) structure, were synthesized using a solid-state reaction and characterized by X-ray diffraction. The temperature dependence of dielectric permittivity was measured from 35 to 550 °C in the frequency range from 20 Hz to 1 MHz. These measurements permit us to present the phase diagram of this family and the evolution of the transition temperature as function of the chemical composition. [Copyright &y& Elsevier]