Ferroelectric phase transitions in KNO3 embedded into porous glasses

This paper presents the results of dielectric and dilatometric measurements of porous glasses filled with KNO₃ from water solution and from the melt. It was shown that ferroelectric phase transition is observed during the cooling process only in KNO₃ nanocrystals introduced into the porous matrix. The phase diagram of transition temperatures versus average values of pore dimensions was obtained. The phenomenological model describing the phase transition and size effects in KNO₃ crystals is proposed.     

Time‐dependent dielectric behaviour in incommensurate TlInS2 crystal

The results of measurements of temperature and time dependencies of the dielectric constant of TlInS₂ in the temperature interval of successive incommensurate and commensurate phase transitions have been presented. Considerable decrease of the dielectric constant in a temperature interval including successive phase transitions was observed after annealing the sample at a fixed temperature in the ferroelectric state. Additionally, it has been revealed that the time dependencies of the real part of dielectric susceptibility and dielectric losses angle tangent of TlInS₂ crystal in the incommensurate phase exhibits an unusual behaviour, which is accompanied by exponential decreasing of the dielectric constant and increasing of dielectric losses angle tangent by time. The observed effects are qualitatively explained using defect‐density‐wave model and phenomenological theory of incommensurate systems containing discommensurations undergoing nucleation and evolution processes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric properties of thallium gallium diselenide layered crystal in the incommensurate phase

The dielectric measurements of the layered crystal were studied in temperature range of successive phase transitions. The measurements revealed that the phase transition occurred in 242 K is an incommensurate phase transition. When the sample is annealed at a stabilized temperature in the incommensurate phase, a remarkable memory effect has been observed on cooling run. The mechanism of the memory effect in the incommensurate phase of the semiconducting ferroelectric TlGaSe₂ can be interpreted in the frame of the theory of defect density waves. This theory claims that the memory effect is the result of pinning of the incommensurate structure by the lattice inhomogeneities. With decreasing the annealing temperature the phase transition temperature shifts to lower temperatures gradually. Moreover, the peak intensities also increase gradually. In addition to these effects, the phase transition temperature shifts to lower temperatures with increasing annealing time. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric and optical properties of surface crystallized TeO2– LiNbO3 glasses

Glasses with composition (100 − x) TeO₂–x LiNbO₃ (10⩽x⩽50) were prepared by conventional melt-quenching. The glass-formation ability, dielectric and optical properties of these glasses were studied. LiNbO₃ microcrystallites were directly precipitated on the surface of the glass, with the composition 50 TeO₂ – 50 LiNbO₃, by a single step heat treatment. The polar nature and second-order optical non-linearity of the transparent surface crystallized glasses are manifested in their strong pyroelectric response, ferroelectric hysteresis and intense second harmonic generation of 1064 nm wavelength radiation.     

Structural instabilities,incommensurate modulations and <Emphasis Type="Italic">P</Emphasis> and <Emphasis Type="Italic">Q</Emphasis> phases in sodium niobate in the temperature range 300–500 K

The relation between the phase transition in the vicinity of 420 K discovered earlier and the formation of an incommensurate phase in the temperature range 410–460 K is established based on the X-ray, dielectric, and dilatometric studies of sodium niobate (NaNbO₃) single crystals and ceramics. It is also established that this phase is characterized by temperature and temporal instabilities. Anomalies in some physical characteristics in the vicinity of 350 K are revealed. It is also shown that the thermodynamic history of the samples is important for the coexistence of the regions of the ferroelectric Q and antiferroelectric P phases in NaNbO₃.     

Phase transitions and macroscopic properties of NaNO3 embedded into porous glasses

The crystal structure and dielectric response of nanocomposite materials on base of porous glasses with average pore diameters of 320, 46 and 7 nm with embedding sodium nitrate have been studied by neutron diffraction and dielectric spectroscopy in low and high temperature phases up to melting. In porous glasses with 46 and 7 nm pores NaNO₃ forms dendrite nanoclusters with "diffraction" sizes of 50(2.5) and 20(2) nm. Decreasing of particle sizes results in decreasing of T_c (temperature of order-disorder orientational transition) and T_melt and in smearing of structure phase transition. The values of critical exponent β for orientational transition are estimated from temperature dependences of intensities of superstructure elastic peaks for these three types of nanocomposite materials.     

Dielectric and pyroelectric response of PVDF loaded with BaTiO3 obtained by mechanosynthesis

Electroactive composites based on ferroelectric ceramic and polymer components are still of interest since their properties can be easily tailored to the requirements of smart structures, sensors and actuators. We studied the dielectric and pyroelectric response of poly(vinylidene fluoride) loaded with BaTiO₃ nanograins obtained by mechanically activated synthesis from BaO and TiO₂. The BaTiO₃ nanopowder was characterized by X-ray diffraction, transmission electron microscopy and NIR Raman scattering. The relaxation processes in the polymer matrix were found to determine the dielectric response of the composites but with higher permittivity values due to the presence of BaTiO₃. The activation energy of the segmental motion of the polymer matrix was found to increase with increasing contents of the filler. The dielectric response of the composites with volumetric fraction of BaTiO₃ equal to and greater than 0.24 was found to be dominated by a broad maximum at ∼320 K, which we relate to the Curie point of the ferroelectric nanograins.     

Dielectric constant,loss factor and ac conductivity of Ni2+‐doped K2ZnCl4 crystals in the ferroelectric‐commensurate,incommensurate and normal phases

The dielectric constant (ϵ), dielectric loss (tanδ) and ac conductivity (σ_ac) of virgin and thermally cycled undoped and Ni²⁺‐doped K₂ZnCl₄ (KZC) crystals in the ferroelectric‐commensurate (FC), incommensurate (IC) and normal phases (N) have been studied. Anomalous behaviour at the two‐phase transition points was observed while measuring ϵ along the polar a‐axis for the undoped sample. With increasing Ni²⁺concentration a systematic shift of the phase transition temperature towards lower values and a continuous inhibition of the peak height were detected. ϵ changed linearly with lnf up to f =10⁵ Hz. tanδ along the a‐axis declared the phase transitions by peak changes. After Ni²⁺‐doping this behaviour was preserved at the FC‐IC phase transition point while the IC‐N phase transition was manifested by a change in the slope of the straight line representing the tanδ‐T dependence. The ac conductivity changed lineally with frequency according to a relation of the form σ_ac = σ_o f ^β where 0>β>1.9. σ_ac increased monotonically with increasing temperature and doping concentration in the low‐temperature phases tending to merge in one straight‐line with high activation energy that might be due to superionic dc conduction in the high temperature N‐phase. Doping with Ni²⁺ pinned stripples, decreased the soliton‐soliton interaction, weakened discommensuration effects, shortened the IC‐phase and strongly affected the ‘chaotic' behaviour at the T_C‐IC. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure and phase transitions in the new crystals of [(CH3)2NH2]2CuCl4[(CH3)2NH2]Cl

Crystals grown from a solution of dimethylammonium and copper chlorides are studied using electron paramagnetic resonance (EPR) and X-ray diffraction. The dielectric properties of the crystals grown are measured. It is established that the crystals have the composition [(CH₃)₂NH₂]₂CuCl₄[(CH₃)₂NH₂]Cl and, in phase I at room temperature, are described by the orthorhombic space group Pna2₁ with the unit cell parameters a = 11.338 Å, b = 9.981 Å, and c = 15.675 Å. At temperatures of 279 K and 253 K, the crystals undergo jumpwise phase transitions into the incommensurate modulated ferroelectric phase II and commensurate modulated phase III, respectively. __________ Translated from Kristallografiya, Vol. 49, No. 1, 2004, pp. 92–100. Original Russian Text Copyright ? 2004 by Kirpichnikova, Pietraszko, Bednarski, Waplak, Sheleg. Dedicated to the 80th Birthday of L.A. Shuvalov     

Dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with Cole‐Cole plots technique

In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce³⁺ doped Sr_0.61Ba_0.39Nb₂O₆ with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce⁺³ doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ferroelectric and ferroelastic properties of Rb2MnCl4·2H2O

Rb₂MnCl₄·2H₂O crystal belonging to the family of tetrahalogeno‐metallates dihydrates has been subjected to thermal, optical and dielectric studies. At room temperature the optical study under a polarization microscope has revealed a ferroelastic domain structure. At 240 K a dielectric anomaly, characteristic of the week ferroelectric phase transition, has been also observed. Moreover, below the temperatures of the phase transitions dielectric dispersion has appeared. Ferroic properties of Rb₂MnCl₄·2H₂O crystal have been compared to those of other tetrachlorides dehydrates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ferroelectricity and antiferroelectricity in CuCrO2-type multiferroic semiconductors

Multiferroic semiconductors of the CuCrO₂ type are considered. These materials, in the presence of free charge carriers, allow for the existence of opposite domains (in the ferroelectric FE _d phase), along with ordinary 180° domains (in the FE phase). The magnetization phase transition in a chiral multiferroic, allowing for piezoelectric effects in an antiferromagnet with a layered triangular structure which result in an incommensurate (helicoidal) spin structure, is phenomenologically described. The behavior of the ferroelectric polarization in the considered phases is characterized. The antiferroelectric is considered a system of parallel layers alternating in chirality sign with oppositely directed polarization vectors. The possibility of antiferroelectric phase transition due to the specific features of the dipole-dipole interaction is discussed.     

The properties of xPbTiO- (0.9 — x) PbZrO- 0.1 PbW0.5Cd0.5O3 solutions in the region of coexistence of tetragonal and rhombohedral phases

The changes of the dielectric permittivity η, dielectric loss tg δ, piezoelectric coefficient d₃₃, pyroelectric coefficient p, remanent polarization P_r and reorientation processes of polarization in dependence on percent content of PbTiO₃ in the xPbTiO₃−(0.9 — x) PbZrO₃− 0.1 PbW_0.5Cd_0.5O₃ solid solutions in the phase coexistence region (PCR) wree investigated. It was pointed out that the characteristics of mentioned parameters possess singularities in this region. The values of average Curie temperature obtained from structural, dielectric and pyroelectric measurements were compared. Presented results let us connect dielectric properties of studied solid solutions with their structure.     

Chromium phase behavior in a multi-component borosilicate glass melt

This paper reports the phase behavior of a multi-component borosilicate glass melt with 0–3 mass% Cr₂O₃ at 800–1500 °C in equilibrium with air. Both upper and lower liquidus temperatures were observed. When the temperature was between the upper and lower liquidus temperatures, eskolaite (Cr₂O₃) formed in melts with >2 mass% Cr₂O₃. Below the lower liquidus temperature, a dispersed chromate phase appeared in the melt that eventually became macroscopically segregated. The chemical durability of the glasses was virtually unaffected by chromium concentration. The particular glass studied was prototypic for vitrifying high-Cr high-level radioactive wastes stored in underground tanks at the Hanford site. The results suggest a significant potential cost benefit for Hanford tank waste cleanup through increased waste solubility in glass.     

Growth,structure and some properties of oriented Sb (S,O) I films

X-ray and electron microprobe analyses of Sb(S, O)I films structure and composition have been performed. The films under investigation were prepared by crystallization from the melt in close spacing. It was revealed that the SbS_0.8O_0.2I structure was isomorphic t the SbSI one. The ferroelectric phase transition was observed at 75°C. The electromechanical coupling factor was 0.3 at 50°C.     

Crystal growth and physical properties of monoclinic L-arginine hydrochloride monohydrate,C6H14O2N4HCl · H2O,and L-arginine hydrobromide monohydrate,C6H14O2N4HBr · H2O

Single crystals of optical quality of L-arginine HCl · H₂O and L-arginine HBr · H₂O with dimensions up to 80 × 60 × 30 mm have been grown from aqueous solutions by controlled evaporation at about 310 K. The isotypic crystals exhibit a quite similar behaviour with respect to morphological, pyroelectric, dielectric, optical, thermal expansion, electrostrictive, electrooptic, elastic, thermoelastic and non-linear optical properties. A distinct anisotropy of the longitudinal elastic stiffness is observed with a maximum along the direction of the alignment of the arginine ions. Nearly in the same direction a minimum of thermal expansion, dielectric constant and index of refraction is found. The pyroelectric, electrostrictive, electrooptic and non-linear-optical effects are of only mediocre magnitude except the electrostrictive constant d₂₁₁ which surmounts the longitudinal electrostrictive constant d₁₁₁ of alpha-quartz by a factor three.     

Low‐temperature elastic and dielectric properties of K3Na(SO4)2 crystals

Elastic and dielectric properties of glaserite K₃Na(SO₄)₂ single crystal were examined using the method of composite oscillators, Brillouin light scattering methods and dielectric spectroscopy. Measurements were performed in the temperature range from 18 K to 300 K. Anomalies in the temperature dependencies of Brillouin shift and dielectric permittivity at about 70 K confirmed the earlier predicted phase transition at 75 ± 25 K. Temperature dependences of the resonance frequency of the vibrating composite oscillator, Brillouin shift measured in the [110] direction, components of dielectric permittivity tensor reveal an anomaly at about 50 K. Moreover, thermal hysteresis of the dielectric permittivity suggested the presence of an incommensurate state between T ₁ = 50 K and T ₂ = 70 K. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preform fabrication and drawing of KNbO3 modified tellurite glass fibers

For fiberization of tellurite glasses the 70TeO₂–25ZnO–5Na₂O composition is selected based on its good thermal stability and refractive index compatibility with most of the ferroelectric oxides. A modified built-in casting method is used to fabricate preforms. The fiber drawn by the rod-in-tube technique consists of two layers of cladding glass and a core with 5 mol% ferroelectric KNbO₃. In this study we also address the low mechanical strength problem with tellurite glass fibers, by subjecting the preforms to polishing and wet chemical etching. Apart from the fabrication methods, we also report here a selective core heat-treatment and ferroelectric phase crystallization in a low dimensional system.     

Crystal growth and characterization of tetragonal tungsten bronze FerroNiobates Ba2LnFeNb4O15

Tetragonal Tungsten Bronze Ferroelectrics (TTB) are currently revisited for the elaboration of new multifunctional materials. Recent studies on Ba₂LnFeNb₄O₁₅ ceramics (Ln=Nd, Sm and Eu) demonstrated that these materials display both ferroelectric and ferromagnetic behaviors at room temperature due to the presence of barium hexaferrite as a secondary phase. In this paper, we report for the first time the growth of Ba₂LnFeNb₄O₁₅ single crystals (Ln=La, Pr, Nd, Sm and Eu) from high temperature solution using LiBO₂ flux. Inclusion free millimeter-sized single crystals were successfully grown. Structural and dielectric characterizations were performed and chemical analysis confirmed that the TTB matrix is reluctant to accommodate small rare earth as it was observed for ceramics. A paramagnetic behavior was observed for all compositions, which corroborates the composite nature of the TTB ceramic multiferroics at room temperature, while dielectric measurements on Ln=La and Pr crystals confirmed their relaxor behavior.