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.     

Phase transformation behavior and dielectric characteristics of BaTi1−x(Al1/2Nb1/2)xO3 (0.01≤x≤0.40) ceramics

Microstructure, phase transformation behavior and dielectric properties of BaTi_1−x(Al_1/2Nb_1/2)_xO₃ (0.01≤x≤0.40) ceramics were investigated. A high level of (Al_1/2Nb_1/2)⁴⁺ substitution for Ti⁴⁺ ions was not conducive to the stability of the perovskite structure and resulted in the formation of BaAl₂O₄. As x was increased, lattice constants and unit cell volume decreased, reached a minimum at x=0.10 and then increased. The BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics at room temperature experienced a transformation from ferroelectric to paraelectric phase with increasing (Al_1/2Nb_1/2)⁴⁺ concentration. Meanwhile, permittivity of the BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics was markedly reduced, while Q value was slightly increased. Frequency dispersion of dielectric peak was obviously increased as x was increased from 0.01 to 0.10. It is of great interest that a dielectric abnormity represented by a broad dielectric peak at 200-400 K was observed for the composition with x=0.40.     

Electronic structure, magnetic and electrical properties of multiferroic PbFe1/2Ta1/2O3

Structural analysis of the synthesized lead iron tantalate, PbFe_1/2Ta_1/2O₃ (PFT) is performed by the refinement of the X-ray diffraction data at room temperature using the GSAS code. Energy dispersive X-ray spectrometry analysis is done to find out the chemical composition. The electronic structure of PFT is calculated by the first principles full potential linearized augmented plane wave method. The spin polarized density of states shows the insulating nature. The magnetic moment of 4.3 μB per Fe ion is obtained from the electronic structure calculation using the GGA+U method and compared with the available experimental data. The electronic structure of the PFT is verified by X-ray photoemission spectroscopy. The dielectric spectroscopy is applied to investigate the electrical properties of PFT in the frequency range from 100 Hz to 1 MHz and in the temperature range from 183 to 253 K. The frequency dependent electrical data are analyzed by conductivity formalism. The relaxation mechanism is explained using the Cole-Cole approach.     

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Electrical impedance measurements of Na₃H(SO₄)₂ were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO₄ tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na⁺ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω^s. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.     

Dielectric dispersion in PbO-Sb2O3-As2O3:V2O5 glasses

Dielectric properties, viz. dielectric constant ε′, loss tan δ and a.c conductivity σ_ac (over a wide range of frequency and temperature) and dielectric breakdown strength of PbO-Sb₂O₃-As₂O₃ glasses doped with V₂O₅ (ranging from 0 to 0.5 mol%) are studied. Analysis of these results, based on optical absorption and ESR spectra, indicates that the insulating strength of the glasses is comparatively high when the concentration of V₂O₅ is about 0.3 mol% in the glass matrix.     

Acoustic properties of multiferroic PbFe1/2Ta1/2O3

The longitudinal acoustic wave velocity and attenuation in PbFe_1/2Ta_1/2O₃ ceramics have been measured by pulse-echo technique in the temperature range from 4.2 to 530 K. The anomalies observed in the sound velocity and attenuation behavior versus temperature were correlated with Burns temperature, temperature range of the coexistence of relaxor ferroelectric and antiferromagnetic states, and a suggested second antiferromagnetic phase transition at low temperatures.     

Evolution from relaxor-like dielectric to ferroelectric in Ba[(Fe0.5Nb0.5)1−xTix]O3 solid solutions

Ba[(Fe_0.5Nb_0.5)_1−xTi_x]O₃ (x=0.2,0.4,0.6,0.8,0.85,0.9 and 0.95) solid solutions were synthesized by a standard solid-state reaction technique. X-ray diffraction at room temperature and dielectric characteristics over a broad temperature and frequency range were evaluated systematically. The structure of Ba[(Fe_0.5Nb_0.5)_1−xTi_x]O₃ solid solutions changed from cubic to tetragonal with increasing x. A Debye-like dielectric relaxation following the Arrhenius law similar to that in Ba(Fe_0.5Nb_0.5)O₃ was observed at lower temperature in the composition range 0.2≤x≤0.8, while the relaxor ferroelectric, diffused ferroelectric and normal ferroelectric behavior were observed for x=0.85,0.9 and 0.95, respectively. The process of the evolution of relaxor-like dielectric to ferroelectric suggested the changing from dilute polar micro-domains to polar micro-domains, polar micro/macro-domains and then polar macro-domains in the present ceramics.     

Physical properties of BaMg1/3Nb2/3O3-BaCo1/3Nb2/3O3 solid solutions

Structural, electric and magnetic properties of Ba₃Mg_1−xCo_xNb₂O₉ based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε(T)) and magnetic measurements (χ⁻¹(T)). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, (no. 164), space group (SG) to a simple structure cubic-type, (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0-1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μ_eff) are close to the value that corresponds to Co²⁺ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative-positive-zero (NPO) type dielectric applications.     

Phase evolution and microwave dielectric properties of Bi3SbO7 ceramic

The Bi₃SbO₇ ceramic was prepared by the solid state reaction method and its phase evolution at different temperatures was studied. Low temperature phase α-Bi₃SbO₇ was formed at about 890 °C and it started to transform to high temperature phase β-Bi₃SbO₇ at about 960 °C. Microwave dielectric constants of α-Bi₃SbO₇ ceramic and β-Bi₃SbO₇ ceramic were 43.2 and 37.6, Qf value were 2080 and 5080 GHz, respectively. TCF of α-Bi₃SbO₇ ceramic was near zero and TCF of β-Bi₃SbO₇ ceramic was about −120 ppm/°C. The Bi₃SbO₇ ceramic is a promising candidate for low temperature co-fired ceramic (LTCC) technology due to its large dielectric constant, low dielectric loss at microwave region, low sintering temperature and simple composition.     

Impedance and dielectric studies of ferroelectric SrBi2Nb2O9 ceramics

This paper reports the dielectric and impedance characteristics of ferroelectric SrBi₂Nb₂O₉ (SBN) ceramics in the 100 Hz-1 MHz frequency range at various temperatures (300-823 K). A strong low frequency dielectric dispersion (LFDD) associated with an impedance relaxation has been found to exist in these ceramics in the temperature range 573-823 K. The Z″ of the AC complex impedance showed two distinct slopes in the frequency range 100 Hz-1 MHz suggesting the existence of two dispersion mechanisms. This non-ideal behavior has been explained on the basis of the expression, Z*=R₀/(1+(iω/ω₁)^m+(iω/ω₂)ⁿ) [J. Phys. Chem. Solids 53 (1992) 1] where ω₁ and ω₂ characterize the lattice response and the charge carrier behavior, respectively. The exponents m and n were obtained from the curve fitting. The exponent n was found to exhibit a minimum at the Curie temperature, T_c (723 K) whereas the m was temperature independent.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Anisotropic properties of self-flux grown LiB3O5 single crystals

In this paper, we report the existence of anisotropic behavior along the crystallographic axes in optical, electrical and thermal properties of lithium tri borate, a recently developed vacuum UV-NLO material. The variation of refractive index with the wavelength along the crystallographic axes was investigated by prism coupling method. The results of impedance spectroscopy measurement reveal the presence of a strong anisotropy in ionic conductivity and dielectric constant along the axes and also show the super-ionic conduction behavior along the c-axis with the activation energy of Δ∼0.20 eV. A thermo-mechanical study in the temperature range of 300-900 K indicates the existence of a strong variation in the linear thermal expansion coefficient (positive value along the a-axis, and negative value along the c-axis) of LiB₃O₅ crystals.     

Role of the TiO6 octahedra on the ferroelectric and piezoelectric behaviour of the poled PbMg1/3Nb2/3O3-xPbTiO3 (PMN-PT) single crystal and textured ceramic

Field cooling (FC) poled/unpoled PMN-29%PT single crystal and room temperature (RT) poled/unpoled PMN-34.5%PT textured ceramic were investigated between ∼0 and 300 °C by thermal expansion, dielectric and Raman spectroscopy. New phase transitions are evidenced at 40, 91 and 180 °C in the case of FC PMN-29%PT as well as at 70 and 200 °C for RT PMN-34.5%PT and their order is discussed. The physical properties of the textured ceramics are rather similar to the ones observed for the single crystals that make them low-cost alternative for a wide range of applications. However, the temperatures and character of the phase transitions strongly depend on the kind of the poling conditions. Temperature dependences of the Raman line parameters show that the NbO₆ octahedra remain stable during temperature increase, while TiO₆ ones evolve quasi-continuously. The step transitions of the Pb²⁺ ion sublattice are evidenced. This suggests that the TiO₆ and Pb²⁺ sublattices are especially coupled. The role of the TiO₆ clusters on the structural phase transitions and dielectric properties of the PbMg_1/3Nb_2/3O₃-xPbTiO₃ (PMN-PT) system is discussed. The presence of the Raman modes above the maximum dielectric permittivity reveals that the local symmetry is lower than the cubic one (Pm3m). The decrease of the Raman line intensities vs. temperature indicates precisely the continuous evolution of the local symmetry towards the cubic one. The temperature evolution of the Rayleigh wing parameters appears sensitive to the phase transitions’ presence.     

The structural, optical, and dielectric spectroscopy studies of novel ZnO–As2O3 glass system with Sb2O3 as additive

ZnO–As₂O₃–Sb₂O₃ glasses of varying concentrations of Sb₂O₃ with ZnO (ranging from 5 to 45 mol%) are prepared. A number of studies, including differential thermal analysis, and study of spectroscopic properties (viz., optical absorption and IR spectra) and dielectric properties (constant ε′, loss tan δ and ac conductivity σ_ac) over a wide range of frequency and temperature of these glasses are carried out. Analyses of the results of these investigations have indicated that the glasses containing higher concentrations of Sb₂O₃ are more suitable for non-linear optical (NLO) applications.     

Microstructural, dielectric and spectroscopic properties of Li2O-Nb2O5-ZrO2-SiO2 glass system crystallized with V2O5

Li₂O-Nb₂O₅-ZrO₂-SiO₂ glasses mixed with different concentrations of V₂O₅ were crystallized. The samples were characterized by XRD, SEM and DTA techniques. The SEM pictures indicated that the samples contain well defined and randomly distributed crystal grains. The X-ray diffraction studies have revealed the presence of several crystalline phases in these samples. Optical absorption, ESR and photoluminescence spectral studies on these samples have indicated that a considerable proportion of vanadium ions do exist in V⁴⁺ state in addition to V⁵⁺ state and the redox ratio seems to be increasing with increase in the concentration of crystallizing agent V₂O₅. The infrared spectral studies have pointed out the existence of conventional SiO₄, ZrO₄, NbO₆, VO structural units in the glass ceramic network. The study of dielectric properties suggested a decrease in the insulating character of the glass ceramics with increase in the crystallizing agent. A.C. conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from V⁴⁺↔V⁵⁺ ions.     

Dielectric properties and temperature stability of BaTiO3 co-doped La2O3 and Tm2O3

The influence of La₂O₃ and Tm₂O₃ co-doping on the dielectric properties and the temperature stability of BaTiO₃ was investigated. BaTiO₃ ceramics were prepared with the compositional formula of (Ba_1−xLa_x)(Ti_1-x/4−yTm_y)O₃. La₂O₃ and Tm₂O₃ co-doping in BaTiO₃ mainly had effects on an increase in the dielectric constant and the temperature stability, respectively. The increase of La₂O₃ concentration and the decrease of Tm₂O₃ concentration in BaTiO₃ resulted in a decrease of lattice parameter and tetragonality because La³⁺ ion substituting for Ba site is smaller than Ba²⁺ ion and Tm³⁺ ion substituting for Ti site is larger than Ti⁴⁺ ion. With the increase of La₂O₃ and the decrease of Tm₂O₃, the dielectric constant of BaTiO₃ was enhanced in spite of the reduction of tetragonality. P-E hysteresis measurements revealed that this phenomenon was based on the improvement of remanent polarization with the increase of La₂O₃ concentration. The introduction of excess Tm₂O₃ in BaTiO₃ suppressed the grain growth and BaTiO₃ ceramics showed higher temperature stability due to the stable tetragonal structure and the small grain size with the increase of Tm₂O₃ concentration.     

The impedance spectroscopic study and dielectric relaxation in A(Ni1/3Ta2/3)O3 [A=Ba,Ca and Sr]

We present the results of impedance spectroscopic study with its analytical interpretations in the framework of electric modulus formalism for Barium Nickel Tantalate Ba(Ni_1/3Ta_2/3)O₃ (BNT), Calcium Nickel Tantalate Ca(Ni_1/3Ta_2/3)O₃ (CNT) and Strontium Nickel Tantalate Sr(Ni_1/3Ta_2/3)O₃ (SNT) synthesized by the solid-state reaction technique. The results of powder X-ray diffraction study reveal that BNT and SNT crystallize in cubic structure with lattice parameter a=4.07 Å and 3.98 Å respectively, whereas CNT crystallizes in monoclinic structure having lattice parameters, a=5.71 Å, b=13.45 Å and c=5.47 Å with β=118.3°. The logarithmic angular frequency dependence of the real part of complex dielectric permittivity and loss tangent as a function of temperature indicate significant dielectric relaxation in the samples, which have been explained by the Debye theory. The frequency dependence of the loss peak and the imaginary part of electrical modulus are found to obey the Arrhenius law. The relaxation mechanism of these samples is modeled by the Cole–Cole equation. This confirms that the polarization mechanism in BNT, CNT and SNT is due to the bulk effect arising in semiconductive grains. The scaling behavior of imaginary part of electric modulus M″ suggests that the relaxation describes the same mechanism at various temperatures but relaxation frequency is strongly temperature dependent. The normalized peak positions of tan δ/tan δ_m and M″/M″_m versus log ω for BNT, CNT and SNT do not overlap completely and are very close to each other. These indicate the presence of both long-range and localized relaxation. Due to their high dielectric constant and low loss tangent, these materials may find several technological applications such as in capacitors, resonators, filters and integrated circuits.     

Dielectric relaxation in complex perovskite oxide BaCo1/2W1/2O3

Polycrystalline BaCo_1/2W_1/2O₃ (BCW) is prepared by the solid-state reaction technique. The X-ray diffraction study of the compound at room temperature reveals the monoclinic phase. The field dependence of the dielectric constant and the conductivity are measured in the frequency range from 50 Hz to1 MHz and in the temperature range from 300 to 413 K. An analysis of the real and imaginary parts of the dielectric permittivity with frequency is performed. The frequency-dependent maxima in the imaginary impedance are found to obey an Arrhenius law with an activation energy=0.86 eV. The frequency-dependent electrical data are also analysed in the framework of the conductivity and modulus formalisms.     

An atomistic approach to the dielectric modes of BaTiO3 and SrTiO3

Recently, a clear long-sought Debye mode (D) in barium titanate (BT) was identified [J. Hlinka, T. Ostapchuk, D. Nuzhnyy, J. Petzelt, P. Kuzel, C. Kadlec, P. Vanek, I. Ponomareva, L. Bellaiche, Phys. Rev. Lett. 101 (2008) 167402] and this mode was shown to originate from the polar coordinate that also generates the so-called Slater mode (S). The inter-relations between the D mode and the normal A1-type phonon modes were studied by those authors using a four-mode phenomenological model. The present work is to offer an atomistic support of their work and to better illustrate the nature of and the couplings between these modes. In addition, we extend the as-obtained insights to the investigation of the nature of the low frequency Raman peaks that were found in ¹⁸O-substituted strontium titanate (ST) many years ago [M. Itoh, R. Wang, Y. Inaguma, T. Yamaguchi, Y.-J. Shan, T. Nakamura, Phys. Rev. Lett. 82 (1999) 3540]. Thus, our work provides important information on the connections between the lattice dynamics of BT and ST.