Ionic Thermocurrent Research Articles

A Method for Polarizing More Number of Impurity-Vacancy Dipoles

Free rotating impurity-vacancy (IV) dipoles in an alkali halide matrix are polarized to the extent of 1/3 of the total number of IV dipoles. An experimental procedure is suggested in this article which will help in the polarization of IV dipoles to the extent of 2/3 of the total number of IV dipoles. In the suggested experimental procedure, the electric field will be applied at first in one direction and then will be applied in succession in opposite direction. Ionic thermocurrent technique is employed to ascertain the increase in polarization of IV dipoles.

A model for the occurrence and analysis of ionic thermocurrent spectrum involving different orders of kinetics

Ionic thermocurrent (ITC) spectrum is much similar to a thermoluminescence (TL) glow curve involving monomolecular kinetics. It has already been reported that different orders of kinetics are involved in TL processes, which depend specifically on the extent of recombination and simultaneous retrapping. It is found that the involvement of different orders of kinetics in ITC spectrum depends on the experimental conditions of polarization and rate of rapid cooling. Consequently, order of kinetics involved in the ITC spectrum does not represent any specific feature of the system under investigation. An equation is developed which explains the occurrence of ITC spectrum involving any order of kinetics. Dielectric relaxation parameters, order of kinetics and approximate number of dipoles per unit volume are evaluated conveniently and easily following the proposed model.

Equivalent electric circuit analyses of ionic thermocurrent in sodium-doped CaF 2 crystals

The characteristics of high-temperature ionic thermocurrent (HT ITC) in CaF 2 doped with different sodium concentrations were studied by the Teflon-insulated electrode ITC method. It was shown that, with increasing sodium concentration, the HT ITC band moved toward a Na +–F V dipole band with a peak at 162 K. The results of analyses of the HT ITC spectra using an equivalent electric circuit proved that the activation energy of space charge migration related to HT ITC was also strongly dependent on the doped sodium concentrations if varied from 0.94 to 0.46 eV with increasing sodium concentration in our ITC study. In addition, the broadening of the Na +–F V dipole band was observed in 3 nominal mole% NaF-doped CaF 2, which was accompanied by a considerable decrease of the activation energy from 0.46 to 0.29 eV without showing marked temperature shifts of the peak ITC bands.

Properties of cesium iodide prepared by different purification methods

The effect of the raw material purification on crystal quality was studied for the cesium iodide single crystals prepared by the Stockbarger method. Purification of the raw cesium iodide using an inorganic absorbent was found to be the most favorable. A brief description of the purification procedure was provided as well as the results of characterization of the obtained crystals by the ionic thermocurrent, electroconductivity and IR-spectroscopy methods.

The Role of Oxygen Containing Impurities in Defects Formation in Cesium Halide Crystals

The dependence of defect formation efficiency in CsI single crystals both on the type of oxygen containing impurities and the value of the absorbed irradiation dose was studied. Correlative results were obtained under investigation by methods of ionic thermocurrent (ITC), thermostimulated exoemission (TSEE), electrical conductivity and optical spectroscopy. The peculiarities of defect formation in γ-irradiated CsI-CO3(SO4, OH) and X-irradiated CsI-OH single crystals are discussed.

Analysis of space charge relaxation by equivalent electric circuit for yttrium doped CaF 2

Characteristic behaviours of high temperature ionic thermocurrent (HT ITC) in CaF 2:Y were studied by a usual ITC method with Teflon insulated electrodes in the nominal yttrium concentration range from n=0.002 to 30 mol%. An equivalent electric circuit was used to analyze HT ITC by assuming a space charge flow model with a specific resistivity in the form of ρ( T)= ρ syso T exp( E a / kT). The analysis revealed that a relaxation of the space charges occurred in the specific resistivity from insulating to semi-insulating change-over of CaF 2:Y crystal. In addition, dependences of the peak temperature shift and total charge of HT ITC on the crystal thickness could be explained by taking into account the capacitance of the crystal. And the yttrium concentration dependence of the activation energy for the space charge flow could be well approximated by E a ( n)= E ao − γn 1/3.

Precipitation of dopant ions in NaCl:Eu+2 crystals

The aim of the work presented was to study the effect of the annealing temperature upon the formation of second phases in the NaCl matrix by using the optical absorption and ionic thermocurrent measurements. The 60, 125, 235 and 350°C isotherms were followed mainly for crystals containing 120 mole ppm of divalent europium. The three lower annealing temperatures result in a decay of the impurity-vacancy dipole concentration up to ∼5% of the initial value being accompanied by the growth of metastable phases with their characteristic optical absorption spectra previously observed for as received samples, only. At 350°C the decay of the impurity-vacancy dipole concentration reaches ∼20% of the initial value, and the newly appeared optical absorption spectrum has been ascribed to the stable EuCl2 precipitates with the PbCl2 type of structure.

ITC study of Ga+-, Ge2+-, and Sn2+-doped alkali halides

The search of the off-centre displacement of an impourity iron from the crystal lattice site in the ground electronic state has been performed by the ionic thermocurrent (ITC) method for Ga+, Ge2+ v− c, and Sn2+ v− c centres in some alkali halides. No ITC peak has been found at T>5 K for Ga+ ions. One ITC peak at the temperatures 187–214 K has been observed for Ge2+ v− c and Sn2+ v− c centres. It has been concluded that due to the presence of a cation vacancy v− c near a Ge2+ ion, the latter is shifted towards v− c. As a result, only one off-centre position of the Ge2+ ion with a deep potential minimum is possible. The reorientations of the off-centre Ge2+ ion can occur only due to v− c jumps around Ge2+ at the temperatures near 200 K where reorientations of Ge2+ v− c dipoles become possible. The same may also be true for Sn2+ v− c centres. The position of v− c with respect to Ge2+ and Sn2+ ions in potassium halide is discussed.

The Effect of Three Different 4-hydroxy-1,3,3a,7-tetraazaindenes on the Melt pAg, Ionic Activation Energies and Photocarrier Responses of an AgBr Emulsion

Melts and coatings of a 0.3-μm octahedral AgBr emulsion were prepared with four concentrations of each of three different 4-hydroxy-1,3,3a,7-tetraazaindenes (TAIs). pAg measurements were made on the melt samples and ionic thermocurrent and radiofrequency photoconductivity measurements were made on the coated samples. The melt pAg (i.e., a measure of the Br− desorbed from the grain surface) increased as the pKsp and the concentration of the TAI added to the sample increased. The concentration of desorbed Br− measured as a function of TAI concentration was consistent with the proposal that TAI anions compete with Br− for adsorption to the surface of the grain. The ionic activation energy for formation of interstitial silver ions and the photoelectron lifetime increased as the pKsp and the concentration of TAI− increased. A linear correlation between desorbed Br− and ionic activation energy was observed. The log of the rate constant for electron trapping decreased linearly as the ionic activation energy increased. The presence of TAI did not affect the photohole response. For TAIs with pKa<5.0, these effects were independent of pH for pH values between 4.5 and 7.0, whereas when pKa > 6, these effects decreased with decreasing pH.

Space charge relaxation and clustering in yttrium doped BaF 2

Characteristics of high temperature ionic thermocurrent (HT ITC) in BaF 2:Y were studied by varying yttrium concentration. A thermal sampling ITC (TS ITC) method was used to decompose the HT ITC band and revealed that the high temperature part of the band consists of a single and intense ITC one. The HT ITC spectra could be analyzed by a space charge flow model with a relaxation time of τ( T) = τ 00 T exp ( E a / kT). With the aid of a usual ITC method, a relation between the temperature of maximum ITC ( T m ) and yttrium concentration ( n) could be expressed in a form of T m = T mo −αn 1 3 . Furthermore, the activation energy ( E a ) follows E a ( n) = E ao − γn 1 3 and was explained by the Lidiard-Debye-Hückel (LDH) model.

Defect dipole relaxation in polycrystalline dolomite [CaMg(CO3)2

The thermally stimulated depolarization current or ionic thermocurrent technique detects two low-temperature bands with maxima at 140 and 188 K, respectively. The choice of proper polarizing conditions, to inhibit the formation of space-charge polarization, permits the accurate study of the low-temperature mechanisms. The extensive dielectric characterization shows that the relaxation mechanisms are related to homogeneous polarization processes and most likely originate from dipolar centers. The thermal treatments indicate that the centers are probably defect dipole agglomerates. The energy parameters for dielectric relaxation are also evaluated. \textcopyright{} 1996 The American Physical Society.

Studies of high temperature ionic thermocurrents observed in CaF 2 and CaF 2:Eu

A strong and thermally stable ionic thermocurrent (ITC) band peaked at 316 K was observed in europium-free CaF 2 crystals. The experimental ITC spectrum can be described by an ionic conduction kinetic expression of the type τ = τ 0 exp( E a kT ) , with τ 0 = 1.90 × 10 −13 s and E a = 0.93 eV. This new band was very sensitive to europium doping, the ITC peak intensity being drastically reduced with the addition of only 0.002 mol.% europium. At the same time, a high-temperature typical ionic thermocurrent (HT ITC) band due to mobile interstitial fluorine ions appeared at 416 K. The decrease of the new band at 316 K is explained by assuming that mobile fluorine vacancies are compensated with fluorine ions introduced into CaF 2 as charge compensators for the trivalent europium ions. The dependence of the temperature T m on the ITC peak in the higher-temperature ITC band on europium concentration is determined and the kinetic parameters are estimated.

Sensitivity of ionic thermocurrent spectra to the presence of divalent nickel in annealed NaCl crystals

Ionic thermocurrent measurements were performed for solution-treated NaCl:Ni crystals additionally annealed in a wide temperature range. It has been shown that the decay of the impurity-vacancy (IV) dipoles at temperatures in the range 333 K ⩽ T an ⩽ 463 K is accompanied by the formation of some vacancy-rich clusters which are of metastable character. These clusters transform into the nuclei of the stable NiCl 2 phase. The decay of IV dipoles has been described in terms of an Avrami approach for precipitation of the dopant within stress fields of dislocations.

Thermal depolarization spectroscopy for probing the contribution of CaCO3to the dielectric relaxation of dolomite (CaMg(CO3)2)

The Ionic Thermocurrent (ITC) technique probes a relaxation mechanism in both polycrystalline and monocrystal dolomite, at 189 K. Dielectric characterization shows that it is dipolar and is probably related to the defect structure of the calcium sublattice.

Observation of ionic thermocurrents in zirconia-based solid electrolytes

In ionic conductors, long range-migrating charges are a main cause of polarization processes. This has complicated, up to date, the study of ionic thermocurrents (ITC) in solid electrolytes. However, the method is appealing, as it probes directly charge-formation phenomena that are important both from a scientific point of view and for applications.

On the analysis of the ionic thermocurrent data

A detailed analysis of the method of Prakash and coworkers for the calculation of ionic thermocurrent (ITC) peak parameters is presented. It is shown that this method leads to a shift of the ITC peak position and a change in the magnitude of the ITC peak height. Accordingly the expression for ITC as a function of temperature requires a modification.

Radiation-induced Schottky defects in NaCl

It has been found in nominally pure NaCl single-crystal samples that have been either gamma irradiated, plastically deformed or quenched into liquid nitrogen from 700 degrees C, that an additional quenching with a short residence time at the quenching temperature reveals the presence of dipolar defects by means of ionic thermocurrent (ITC) measurements. This procedure does not induce ITC signals in untreated samples. Since the occurrence of divacancies and divacancy clusters in plastically deformed or quenched samples is expected. It is pointed out that gamma irradiation may also form divacancies. The fact that all the ITC spectra of the samples treated in the above-indicated ways exhibit peaks at similar temperatures also supports this view. The parallelism observed between the stored energy spectra of these samples and the evolution of the ITC areas against the quenching temperature indicates that the two phenomena are related. The occurrence of dipolar defects is consistent with a previous proposal: that the stored energy release is due to the recombination of divacancies with alkali halide molecules.

Drift of impurities toward dislocations in NaCl:Ni 2+ crystals

The effect of thermal and mechanical treatment upon dielectric characteristics was investigated for NaCl crystals doped with divalent nickel. The behaviour of two ionic thermocurrent (ITC) bands was followed during room temperature (RT) storage of the pretreated samples. The isothermal annealing curves are of the t an − m type, with m characteristic of segregation of the impurities near dislocations accompanied by Ostwald ripening. The activation energy of this process is equal to the migration activation energy for cation vacancies in NaCl crystals, i.e., the migration of impurities is controlled by the diffusion of vacancies. Prolonged annealing of supersaturated solid solution (ST) samples at RT results in the appearance of a third ITC band, probably related to the formation of secondary-phase particles rich in cation vacancies.

Fourier analysis of ionic thermocurrents observed at high temperature in BaF 2:Sm crystals

In BaF 2 crystals doped with Sm 3+ impurities, a single ionic thermocurrent (ITC) band was observed in a temperature region higher than the usual ITC bands for depolarization of electric dipoles. The single ITC band has already been ascribed to a depolarization process of the space charges accumulated near a positive electrode. Fourier analysis was applied to the high temperature ITC with blocking electrodes to investigate the effects of sample thickness, impurity concentration and heating rate on the peak temperature shift and intensity of ITC. Combined Fourier analyses with a least square fitting revealed that the concentration of mobile interstitial fluorine ions at 283K is 3.9 ± 0.2 × 10 12 cm −3 and the diffusion parameters D 0 and E d are 3.3 ± 0.2 × 10 4 cm 2 s −1 and E d = 0.805 ± 0.02 eV for BaF 2 doped with 0.5 mole% (nominal) SmCl 3 impurities.

Electrical and EPR studies of heterovalent solid solutions based on superionic β-PbF 2

Ionic thermocurrent (ITC), ac-conductivity, and electron paramagnetic resonance (EPR) experiments have been performed on solid solutions Pb 1− x RE x F 2+ x . These hetrovalent solid solutions were prepared as crystals and powders by different te chniques. The differences in the dependence on concentration of the electrical properties are explained by the nature of defect structures. Using the ITC method we were able to distinguish different types of dipole defects created by the RE 3+ cation and the charge compensating interstitial fluoride. The activation energies for all these complexes were determined and correlated with structural aspects. EPR-spectra (0.1 mol% GdF 3 as a probe) for both polycrystalline and single crystals show three different RE-sites and are compared to spectra obtained from other Gd-doped fluorite-structured solid solutions.