Dielectric properties and microstructural characterization of cubic pyrochlored bismuth magnesium niobates

Cubic bismuth pyrochlores in the $\mathrm{Bi}_{2}\mathrm{O}_{3}$ –MgO– $\mathrm{Nb}_{2}\mathrm{O}_{5}$ system have been investigated as promising dielectric materials due to their high dielectric constant and low dielectric loss. Here, we report on the dielectric properties and microstructures of cubic pyrochlored $\mathrm{Bi}_{1.5}\mathrm{MgNb}_{1.5}\mathrm{O}_{7}$ (BMN) ceramic samples synthesized via solid-state reactions. The dielectric constant (measured at 1 MHz) was measured to be ${\sim}120$ at room temperature, and the dielectric loss was as low as 0.001. X-ray diffraction patterns demonstrated that the BMN samples had a cubic pyrochlored structure, which was also confirmed by selected area electron diffraction (SAED) patterns. Raman spectrum revealed more than six vibrational models predicted for the ideal pyrochlore structure, indicating additional atomic displacements of the A and $\mathrm{O}'$ sites from the ideal atomic positions in the BMN samples. Structural modulations of the pyrochlore structure along the [110] and [121] directions were observed in SAED patterns and high-resolution transmission electron microscopy (HR-TEM) images. In addition, HR-TEM images also revealed that the grain boundaries (GBs) in the BMN samples were much clean, and no segregation or impure phase was observed forming at GBs. The high dielectric constants in the BMN samples were ascribed to the long-range ordered pyrochlore structures since the electric dipoles formed at the superstructural direction could be enhanced. The low dielectric loss was attributed to the existence of noncontaminated GBs in the BMN ceramics.     

Magnetic and transport properties of some light rare earth tungstates

Measurements relating to molar magnetic susceptibility, dc electrical conductivity and thermoelectric power of Nd, Sm, Eu and Gd are reported. The ac electrical conductivity at a few temperature ranges is also given. It is found that it follows the Curie-Weiss law behaviour and this has been attributed to the crystal field effect. The experimental value of Bohr magneton for the magnetic ions has been found to be in good agreement with theory. Thermoelectric power is negative in the measured temperature range suggesting these materials to bep-type semi-conductors and holes as the dominant charge carriers. The results are explained using band theory.     

X-ray K-absorption edge of niobium in some of its niobates

The position and extended fine structure of X-ray K-absorption edge of niobium in its various niobates has been investigated using 400-mm bent crystal (mica, 100 planes) spectrograph. The shift of the K-absorption edge of niobium in its various niobates is related to the size of the next nearest cation; large the size, smaller the shift of the absorption edge towards higher energy side.     

Optical properties of Dy ions in sodium gadolinium tungstates crystal

Dy³⁺-doped NaGd(WO₄)₂ crystal with sizes of about Φ20×40 mm² was grown by the Czochralski technique along the (0 0 1) orientation. Polarized absorption spectra, fluorescence spectra, and fluorescence decay curve of Dy³⁺-doped NaGd(WO₄)₂ have been recorded at room temperature. Based on the Judd-Ofelt (J-O) theory, the intensity parameters from the measured line strengths were evaluated. The J-O parameters were used to predict radiative transition probabilities, radiative lifetimes and branching ratios for various excited levels of Dy³⁺-doped NaGd(WO₄)₂ crystal. The luminescent quantum efficiency of the ⁴F_9/2 level was determined to be approximately 63% for this material. The emission cross-section of the ⁴F_9/2→⁶H_13/2 transition was estimated by using the Füchtbauer-Ladengurg method.     

Phase transitions and electrical properties of solid solutions of sodium, lithium, and strontium niobates

An additional study of solid solutions in the (Na, Li, Sr_0.5)NbO₃ ternary system was performed. More detailed information concerning its phase diagram and physical parameters in a wide range of component concentrations is obtained. The compositions obtained are of interest for application in highly sensitive and high-frequency transducers.     

Dielectric properties of microcomposite ferroelectrics

The dielectric response in two-phase composites of ferroelectric and dielectric materials was studied within the quasistatic approximation. Using different effective medium theories (Maxwell-Garnet theory, the effective medium approximation and the Bergman representation) the dielectric function was represented as a distribution of polar modes. One of the main results concerns the existence of modes due to geometric resonances which among other effects can lead to new low-frequency absorption peaks near the percolation threshold of the ferroelectric material. These peaks appear always higher than the transverse polar mode frequencies of the bulk components but below their corresponding longitudinal mode frequencies. The soft mode with renormalized mode strength but unchanged frequency exists only in the composite with percolated ferroelectric clusters. When the ferroelectric clusters are only finite the soft mode does not soften completely.     

Dielectric properties of evaporated films

The evidence for vacancy-pair formation in alkali halide crystals is reviewed. Existing information on the dielectric properties and structure of thin films is discussed with a view to using the high defect concentrations in vacuum-deposited films to confirm vacancy-pair orientation effects. Experimental measurements of the dielectric constant and dissipation factor of evaporated films have been made at room temperature over a range of frequencies from 100 c.p.s. to 100 kc/s. Even under vacuum these films show pronounced losses at low frequencies which are attributed to excess vacancy concentrations. Such losses are contrary to the theory of ideal lattices and are not shown by single crystals.

In spite of the excess defect concentrations no evidence of vacancy-pair orientation has been found. Ageing effects have been observed in all cases, the losses decreasing with time as the defect concentration decreases. In the alkali halides the magnitude of the losses depends on the cation mobility but they decrease at a rate determined by the anion mobility. This leads to a proposed dielectric relaxation mechanism in which individual crystallites form the polarizable units, becoming polarized by migration of the cation vacancies towards one end. The losses decrease as the defects are gradually eliminated by simultaneous condensation of positive and negative vacancies at grain boundaries. This is essential to maintain electrical neutrality and the rate is determined by the diffusion of the slower anion vacancies. The measured rates are in agreement with anion activation energies obtained by tracer methods. These results cannot be explained by vacancy-pair formation even if it is assumed that vacancy pairs can form but are incapable of orientation and hence we must conclude that there is little or no vacancy-pair formation.

Measurements at atmospheric pressure show that moisture has a pronounced effect in all cases, producing dielectric losses which completely obscure the vacancy effects. The changes in dielectric properties during and after adsorption cannot be explained as conductivity effects and are in complete opposition to any modification of the Maxwell-Wagner theory. The ageing effects show that after the initial adsorption, the water molecules migrate over crystallite surfaces to positions where they are more strongly bound and contribute to the dielectric polarization by a form of hindered rotation closely analogous to mechanisms proposed for ice. At these equilibrium sites, H-bonds are frequently formed between the adsorbed molecules and the halide ions of the crystal lattice.     

Dielectric properties of polycrystalline ferrites

The well-known dielectric relaxation of polycrystalline ferrites is usually seen as being related to an inhomogeneous polarization. According to that theory, the inhomogeneities are formed by poorly conducting grain boundaries in the well-conducting bulk material. Experimental results reported here suggest, that in some cases the dispersions are not due to an inhomogeneity polarization. Previously reported results of measurements taken on single crystals are also inconsistent with this model. Therefore an electric dipole relaxation model is introduced.     

Dielectric properties of electrolytic solutions

The variation in dielectric properties of water with the addition of ionic salts have been measured using automated frequency domain experimental microwave C-band. The dielectric properties, that is dielectric constant (ε′) and dielectric loss (ε″) of two electrolyte solutions for various concentrations have been measured at 5-GHz frequency at room temperature. It has been observed that for concentration between 0.2 to 1.0 mole the dielectric constant of water is smaller and some larger than that of pure water and dielectric loss increases with increasing concentration of these salts. It has been also observed that the variation in dielectric loss is different, though the ionic concentration of the two salts are equal.     

Dielectric and ferroelectric properties of NBT-BT systems

ABSTRACT

The (1-x)Na_0.5Bi_0.5TiO₃- xBaTiO₃ ceramics (x = 0.1, 0.135 and 0.17) were fabricated by a conventional solid phase sintering process. The bulk density of the obtained samples exceeded 95% of the theoretical relative density as determined by Archimedes method. Dielectric and ferroelectric measurements of these ceramics were performed. Measurements of the ferroelectric properties show that, above the depolarization temperature T_d, the shape of the hysteresis loops approaches that of linear dielectrics. The dielectric study results correlate with the hysteresis loops measurements. The relaxor-like behavior of the investigated materials was revealed.     

Magnetic studies of rare-earth tungstates

Measurement of magnetic susceptibility (χ_m) of rare-earth tungstates RE₂(WO₄)₃ with RE = Gd, Tb, Dy, Ho, Er, Tm and Yb have been reported in the temperature range 3–300 K at a magnetic field of 3.2 × 10⁵ Am^-1. All the tungstates are found to be ferrimagnetic with a ferrimagnetic Curie-temperature lying in the range 15–27 K. Evaluated magneton numbers agree fairly well with free tripositive rare-earth ions. The results are discussed on the basis of a new proposed model. Except for Tb all the evaluated parameters have systematic variation over the entire rare-earth series.     

Dielectric investigation of relaxation in some heterocyclic compounds

The dielectric relaxation mechanism in five heterocyclic compounds in dilute solutions using microwave technique is reported. Measurements have been made at different temperatures in order to calculate dipole moments and free energies of activation for the process of dipole orientation and viscous flow. It has been observed that the relaxation time is very closely related with the molecular parameters, such as size, shape and nature of the solute molecule.     

Dielectric properties of superionic lead fluoride

Recent experimental studies show that the dielectric constant of lead fluoride increases rapidly as the superionic transition temperatureT _c is approached. In this work the static dielectric constant (ε) is calculated theoretically for low and high temperature limits. Assuming conduction to take place through a percolation mechanism,ε is found to diverge at the threshold temperatureT _th where ionic conduction starts.     

Dielectric properties of polyacrylamide polymer films

Summary Polyacrylamide polymer films of thickness 4.15 μm were prepared by isothermal immersion technique. The dielectric capacitance and dielectric loss of the films were studied as functions of frequency and temperature in the range 100 Hz|10 MHz and (300|450)K, respectively. Two dielectric loss peaks were observed in the dielectric loss spectra and were identified as β and α peaks. The β peak was attributed to the reorientation of dipoles and the α-peak was attributed to the deformations accompanied by large changes in the directions or locations of the dipoles.     

Dielectric properties of nickel-doped NaCl crystals

The influence of nickel impurities upon dielectric losses in natrium chloride crystals is studied. Experiments have shown that in non-heated samples, stored over prolonged periods of time, the majority of nickel is present in the form of aggregates and higher complexes. Their decomposition and the building up of dipoles depends in the first instance on sample heating and cooling methods. The aggregation of dipoles in time at room temperature is studied.It is shown that with nickel concentrations reaching approximately up to 2. 10^–3 mol% in heated and quickly cooled samples, all nickel builds itself into the NaCl lattice substitutionally in the form of dipoles Ni²⁺ + cation vacancy, while at higher concentrations some nickel precipitates or builds itself in in another non-substitutional way.The author wishes to express his thanks to Dr. Funk for making the polarographic analysis of the samples and to Dr. Bohun for valuable discussions of the results obtained.     

Dielectric properties of ferroelectric diisopropylammonium iodide

Temperature dependences of dielectric constant, amplitude of the third harmonic and heat capacity for the organic ferroelectric of diisopropylammonium iodide (C₆H₁₆NI) have been investigated. The measurements were carried out through heating and cooling cycles in the range of 300–400?K. It was found that upon the first heating, only one phase transition occurred without the presence of the ferroelectric phase. For samples preheated over 420?K, two phase transitions at 363 and 378?K appeared in the heating process, and the ferroelectric state was also observed between them. Upon cooling, the ferroelectric phase was detected in the range of lower 361?K and persisted up to room temperature.