Effects of uniaxial stress on dielectric properties of lithium modified potassium sodium niobate ceramics

In this work, the solid solution of ((K_0.5Na_0.5)_1−xLi_x)NbO₃ ceramics with x=0.03, 0.04, 0.05, 0.06 and 0.07 was prepared by a conventional mixed-oxide and solid-state sintering method. The structural phase formation and microstructure were characterized by X-ray diffraction technique and scanning electron microscopy. The ceramics were identified by XRD as a single-phase perovskite structure with symmetry gradually changing from orthorhombic to tetragonal. The grain size and the optimum density of the sintered ceramics were noticeably compositional-dependent. The dielectric properties of the ((K_0.5Na_0.5)_1−xLi_x)NbO₃ ceramics under the uniaxial compressive stress were observed at stress up to 180 MPa. The results showed that the dielectric constant and the dielectric loss tangent increased with applied stress. The change in the dielectric properties with stress was seen to depend on the composition and grain size. The observations were interpreted in terms of the intrinsic and extrinsic contributions to the changes in dielectric properties upon the applied compressive stress.     

Anisotropic dielectric properties of PbYb1/2Ta1/2O3 single crystals

PbYb_1/2Ta_1/2O₃ single crystals were obtained for the first time. They were grown by the flux method. The PbOPbF₂B₂O₃ system was used as a solvent. Dielectric investigations were carried out in 1 0 0_c, 1 1 0_c and 1 1 1_c pseudocubic directions. These studies pointed to anisotropy of dielectric properties. Frequency-independent ε′(T) and ε″(T) maxima related to the antiferroelectric–paraelectric (AFE—PE) phase transition are observed for all directions at 562 K. The frequency-dependent ε′(T) and ε″(T) maxima near 400 K related to the ferroelectric (FE)–AFE phase transition are observed only in 1 1 1_c direction. The hysteresis loops were observed in this direction only. These results point that ferroelectric relaxor properties appear only in 1 1 1_c direction. We propose to consider the ferroelectric phase as ferrielectric one.     

Phase transitions and metastable states in TlGaSe2

The results of measurements of the dielectric constant of TlGaSe₂ in temperature range of successive phase transitions are presented. An anomaly in the temperature dependence of the real part of dielectric constant in TlGaSe₂ has been observed at about 242?K in addition to anomalies at 115, 108, and also near 65?K as reported in previous publications. The presence of temperature hysteresis effects in temperature interval between 115 and 242?K allowed making a conclusion about possible existence of an incommensurate phase in the mentioned temperature range. A model of succession of the structural phase transitions in TlGaSe₂ has been suggested.     

Phase transitions and electrical characteristics of tungsten substituted barium titanate

In the present work, phase transitions and electrical characteristics of tungsten substituted barium titanate are reported. Structural analysis of the prepared samples was carried out to confirm the formation of the compounds in proper phase. Scanning electron micrographs of the compounds have been used to investigate the grain morphology of the material. The dielectric properties of the samples have been studied as a function of temperature and frequency. The variation of dielectric constant (ε_r) with temperature shows a diffuse kind of ferro–paraelectric phase transition. The diffusitivity of the phase transitions has been calculated. Complex impedance spectroscopy analysis was carried out to investigate the electrical characteristics of the specimen.     

Effect of Nb-doping on phase transitions and selected properties of (Ba0.90Pb0.10)TiO3 ceramics

Dielectric, pyroelectric characteristics as well as the changes of electric conductivity were investigated for Nb₂O₅-doped (Ba_0.90Pb_0.10)TiO₃ ceramics. The influence of this dopant on the ceramic microstructure and the lattice parameters were also studied. The analysis of the results of electric conductivity and Seebeck coefficient has led to the conclusion that Nb⁵⁺ ions reducing the conductivity by about two orders play the role of donors. The correlation between investigated electric characteristics and the dopant concentration was confirmed. Due to these findings some progress in understanding the influence of Nb-dopant was achieved.     

Anomalies of dielectric properties of cs2hgci4 crystals in the vicinity of phase transitions

A short review of the published experimental data on various physical properties and some new results of the measurement of dielectric and electromechanical properties of Cs₂HgCI₄ crystals in a wide temperature range are presented. The crystal is shown to reveal anomalies at seven temperatures associated with structural phase transitions and hence to have eight phases. In two temperature regions there exists a small spontaneous polarization and the anomalous dielectric permittivity is a nonlinear function of the electric biasing field. Within the phase followed by the first polar phase on cooling the crystal, the effects of thermal, electrical and mechanical memory, specific for incommensurate phases, are observable.     

Effect of glycine substitution on the ferroelectric phase of betaine arsenate [(CH3)3NCH2COO·H3AsO4]

The present work reports an experimental investigation on the influence of glycine (NH₂CH₂COOH) substitution in the polar properties and the critical dynamics of the molecular ferroelectric betaine arsenate, (CH₃)₃NCH₂COO·H₃AsO₄. The dielectric dispersion (20 Hz<ν<3 MHz) and the thermally induced displacement currents are investigated in detail over the extended Curie region of the system (130 K<T<100 K). The results obtained for a single crystal with nominal glycine content of 20% are analyzed, compared with those obtained for pure betaine arsenate and discussed within the scope of a phenomenological Landau model previously used to describe a system with competing ferroelectric and structural instabilities.     

Influence of the A and B vacancies on the dielectric and structural properties of the PLZT 8/60/40 ferroelectric ceramic system

The lanthanum modified lead zirconate titanate (PLZT) ferroelectric ceramics, considering La/Zr/Ti ratio of 8/60/40 and vacancies in A or B sites of the perovskite structure, have been analyzed by using X-ray diffraction, dielectric response, polarized light microscopy and ferroelectric hysteresis. No remarkable differences are observed between samples with A or B vacancies from the X-ray diffraction patterns and ferroelectric hysteresis loops. However, the dielectric measurements have revealed a significant decrease in the maximum values for the real part of the dielectric permittivity and an enhancement of the relaxor behavior in the systems where A vacancies have been considered. On the other hand, the anomalies observed in the temperature dependence of the remanent polarization suggest a frozen out in the dynamic of Nanometric Polar Regions (PNRs) around the same region of temperatures for both samples.     

Effect of uniaxial stress on the dielectric properties of BaTiO3+0.1wt.%Eu2O3 ceramics

ABSTRACT

BaTiO₃+0.1wt.%Eu₂O₃ ceramics were prepared by a solid-state reaction method. The dielectric behavior of these ceramics as a function of uniaxial pressure has been systematically studied. The external stress showed obvious effects on these properties. An increase of the Curie point (T_c) and decrease of the Curie–Weiss temperature (T₀) was observed with increasing pressure, resulting in an increase in the first-order nature of the phase transformation (T_c–T₀ increases). Broadening and flattening of the permittivity versus temperature curves near their maximum was found. The pressure behavior of thermal hysteresis and the ??/?T vs. T plot suggests that the phase transition changes to second-order type with increasing pressure. Furthermore, the Curie–Weiss constant obtained from a modified Curie–Weiss law strongly decreases with increasing pressure, suggesting that the mechanism of phase transition is going to order–disorder type.     

The study of the influence of uniaxial stress on impurity complexes in silicon

The influence of external uniaxial stress on the different indium-donor complexes in silicon has been studied using the perturbed γ –γ angular correlation (PAC) method. Such effect of an applied stress is detected by means of the probe atoms situated at different complexes in the sample. The current results showed that the responses of the probes in an extrinsic silicon samples are found to be dissimilar for the same value of stress. Such change in the local environments of the probe atoms could be associated with the various strain field created by the implantations of varied size of impurities. The phosphorous implantation in silicon has even lead to the complete absence of observable effect of the applied stress suggesting significant lose of the elasticity of the sample.     

Thermal hysteresis and memory effects in TlGaSe2 crystal with incommensurate phase

Temperature dependencies of dielectric permittivity of TlGaSe₂ have been measured under various thermal cycles. Peculiarities of anomalies in temperature dependencies of dielectric permittivity corresponding to structural phase transitions at 108 and 115?K are discussed. The coexistence of two different incommensurate structures in TlGaSe₂ was proposed. The phase transitions at 108 and 115?K are considered as commensurate lock-in transitions. As a result a new model of the structural phase transitions in TlGaSe₂ has been suggested.     

Effect of Dichroic dye on dielectric properties of ferroelectric liquid crystals: A comparative study

In order to study the effect of mixing dye molecules in ferroelectric liquid crystals, we have investigated two ferroelectric liquid crystal samples CS1016 and Felix 17/000 along with their mixture with Anthraquinone dye. The measurements have been made in the frequency range 100 Hz-10 MHz, with the variation of temperature from 30 to 90 °C. The dielectric behaviour of dye mixed CS1016 is quite different from that of Felix 17/000. This different behaviour has been explained by determining other parameters like distribution parameter, dielectric strength and relaxation frequency, etc. The different nature shown by two different samples has also been explained by electro-optical measurements.     

Relaxations and phase transitions phenomena in structurally related tyrosine-derived polyarylates

Abstract

A series of four tyrosine-derived polyarylates with different number of methylene groups in the backbone (y = 2, 4, 6 and 8) were studied by measuring thermally stimulated depolarization currents (TSDC). The low temperature TSDC spectra (80—240K) were analyzed at two different degrees of hydration to assess the effects of small amounts of water in the observed broad and complex β band. Higher temperature TSDC spectra (250 to 350 K) provided the α relaxation peaks associated with the glass transition temperature. The direct signal analysis (DSA) method allowed the decomposition of the complex peaks into elementary processes. The β_i components result from localized motions of molecular segments of variable length molecules whose identities were established in a prior study of polycarbonates. The changes observed in the low and high temperature polarization with the number of backbone methylene groups were interpreted as a combination of flexibility and entanglement phenomena.     

Second harmonic generation in layered TunS2 in the vicinity of the low-temperature phase transitions

Second-harmonic generation in the ternary layered semiconductor TlInS₂ excited with the wavelength λ = 1.06 μm of YAG: Nd^{3 +} laser is investigated in the temperature range corresponding to the low-temperature phase transitions. It is shown that the intensity of the second-harmonic signal corresponding to the non-linear coefficients d_eff, d₂₁, d₂₂ reveals peculiarities close to the commensurate-incommensurate phase transitions. The temperature hysteresis of the second-harmonic signal in the low-temperature region (below and close to T_i2 = 206K) is explained assuming that an incomplete lock-in transition in TlInS₂ takes place at T_c1 =204K within the temperature range between a ferroelectric (T_c2 = 201K) and an incommensurate (T_i2 = 206 K) phases. It is shown that the monoclinic point group symmetry C₂ is preserved also at temperatures lower than the phase transition temperature Tc4 = 79K to a weak ferroelectric state.     

Mechanical and dielectric dissipation related to phase transitions

Abstract

(1) First Order Phase Transition There always appear two dielectric loss and/or two mechanical dissipation peaks (P₂ and P₃ ) related to domain walls in many martensitic and ferroic materials. The P₂ near T_c (or M_s ) originates from first-order phase transition or phase-like transition and shows static hysteresis character, but in a certain frequency range (kHz) manifests relaxation behavior, with t—t ₀exp[? B/(T_c—T)]. The transient componant of P₂ is also reviewed.

(2) Glass Transition The peak P ₃ at kHz often appears near a freezing temperature Tf as in KDP, TGS and occasionally in Mn-Cu at Hz, with T = T ₀exp[U/(T- Tf)], a Vogel-Fulcher relation, showing a “Transition” type of glass-transition of domain wall. Whereas the glass-transition in C₆₀ crystal obeys the Arrhenius relation, exibiting “Kinetic” type of glass-transition.

(3) Flux Melting The peak just below T_c was measured for melt-textured YBa₂Cu₃O_{7 +}δ superconductors in an applied magnetic field and various frequencies. The peak hight satisfies the power law Q ^?1 ∝ ω ^?n (1 < n < 2), being explained by the melting and viscous motion of flux.     

Li and Nb codoping in dielectric properties and phase transitions of KTaO3: Reentrant dipolar glass and long-range dipole ordering

Abstract

The highly polarizable perovskite-type oxide, KTaO₃ doped simultaneously with Li⁺ and Nb⁵⁺ (K_1?xLi_xTa_1?yNb_yO₃, KLTN), reveals unexpected properties and ordering effects. Studies of the dielectric permittivity ?'(T, f) (10—300K, 100Hz-1 MHz) for x = 0.0014 and y = 0.024 show collective dipolar ordering effects with a transition from paraelectric into a mixed phase (coexisting dipole-glass-like and long-range ordered ferroelectric phases) taking place near 39 K. At 15 K another phase transition into a reentrant dipolar glass-like state is observed. Such a sequence of transitions and the existence of a reentrant glass state are unknown for electrical dipolar systems.     

Solid solutions on the base of bismuth vanadate: Preparation, structure, phase transitions, dielectric and transport properties

Ceramic solid solutions (Bi_1-yLa_y)₄(V_1-xMe_x)₂O_11-y with x, y < 0.2, Me-Zr, Ga, Fe, Cu, have been prepared by the solid state reaction method. Crystal structure parameters, phase transitions, dielectric and transport properties of ceramic samples have been studied. Concentration and temperature stability regions of monoclinic α-, orthorhombic β- and tetragonal γ- or γ’- polymorph modifications have been determined.Annealing of samples, containing large amount of Cu and/or La dopants, at 973 K in the reducing atmosphere resulted in their decomposition, though compositions containing low content of La, Ga or Zr dopants, remained pretty stable.     

The p–T phase diagram of KNbO3 by a dielectric constant measurement

A dielectric constant measurement was carried out on perovskite-type ferroelectrics KNbO₃ over a wide range of temperature under high pressure. The temperature- and pressure-dependence of the dielectric constant clarified that all temperatures of the transitions from the ferroelectric rhombohedral to orthorhombic, to tetragonal and then to the paraelectric cubic phase, decrease with increasing pressure. These results indicate that the orthorhombic–tetragonal transition takes place at 8.5 GPa and the tetragonal–cubic transition at 11 GPa, at room temperature.     

Symmetry breaking at the phase transitions in ferroelectric pyridinium salts

The phase transitions in ferroelectric pyridinium tetrafluoroborate (PyBF₄) and pyridinium perchlorate (PyClO₄) have been characterized by structural studies. The continuous ferroelectric phase transition at 238.7 K in PyBF₄ appears to be a unique case for multiaxial ferroelectrics, while the first order phase transition in PyClO₄ at 248 K is consistent with the Landau theorem for deducing the character of phase transitions from symmetry considerations. The phase transition in both materials is caused by ordering of the pyridinium cation as well as tetrahedral anions.