Aggregation Of Dipoles Research Articles

Dipole aggregation in LiF doped with Be 2+

The thermally stimulated depolarization currents technique (TSDC) has been employed for the study of the kinetics of the aggregation of impurity-vacancy ( I– V) dipoles. This study is based on the time variation of a complex band which peaks around 200 K. It is found that for ageing times up to 150min the clustering seems to follow second-order kinetics.

ITC and theoretical studies of divalently doped RbCl

Experimental (ITC) and theoretical studies on RbCl doped with Ca, Sr, Pb and Ba impurities have been made. For all the impurities, single ITC peaks have been found and their activation parameters obtained. The binding and activation energies for reorientation have been calculated by using an extended Mott-Littleton approach. It has been found that reorientation proceeds via cation jumps in next-nearest-neighbour positions, the calculated activation energies agreeing with the experimental values. The calculated values for impurity-vacancy exchange can also explain the observed experimental trends for dipole aggregation.

Theoretical Studies of the Configuration and Association Energies of Point Defect Aggregates in Alkali Halide Crystals

AbstractThe aggregation of dipoles consisting of a divalent metal impurity and a cation vacancy in alkali halide crystals is determined mainly by the association energy connected with the formation of the special types of aggregates. It is shown that also larger aggregates (complexes of four and more dipoles) are important to characterize the equilibrium state. Therefore a simple computational method will be presented to estimate atomistically the association energy of such aggregates. A comparison with results from the literature valid for little aggregates (mainly dimers and trimers) confirms the applicability of the proposed procedure.

A theoretical study of vacancy-impurity association and of defect aggregation processes in divalently-doped silver halide crystals

Abstract Defect structures in AgCl and AgBr doped with Zn2+, Mn2+, Cd2+, Sr2+ and Pb2+ have been investigated using computer simulation methods, and the forces between the ions have been described using two-body shell-model potentials. The [110] nearest-neighbour dipole is found to be more stable than the [100] next-nearest-neighbour dipole. For both of these complexes lattice relaxation contributes to the binding energy, but for the third-neighbour complex the binding is due to Coulomb forces only. The additional binding energy that results from the aggregation of dipoles to form dimers, trimers and tetramers has been calculated and possible aggregation pathways are discussed. The cohesive energy of the Suzuki phase has been calculated; this phase was found to be stable relative to its isolated constituents but it can only form if a stable sequence of precursors can be established.

A theoretical study of the formation and aggregation of vacancy-impurity dipoles in divalently doped alkali halide crystals

Abstract Computer simulation techniques based on generalized Mott-Littleton methods have been used to investigate defect structures in systems resulting from the incorporation of Mn2 +, Cd2+ and Pb2+ ions into NaCl, KC1 and KBr crystals. Two-body interionic potentials based on the simple shell model were used in the calculations. The (110) nearest-neighbour (n.n.) and (200) next-nearest neighbour (n.n.n.) simple impurity-vacancy dipoles were found to be of comparable stability, although definite trends with ion size are evident, whereas the (211) complex invariably is found to be less stable. The additional binding energies which arise when the n.n. and n.n.n. dipoles aggregate into a variety of dimers, trimers and tetramers have been calculated, as has the stability of the Suzuki phases. In the present work the results are compared with available experimental data and possible pathways for the formation of the Suzuki phase are discussed.

Hardening of NaCl single crystals induced by Eu obstacles

Abstract The hardening of NaCl induced by the presence of europium ions has been systematically investigated as a function of its aggregation-precipitation state. The latter was characterized by means of optical and EPR techniques. The contribution to the hardening of free dipoles, aggregates and precipitates could be separated by using samples subjected to different suitable thermal treatments. For free dipoles a thermally activated hardening stage is found between 77 and 235 K, followed by a region where the hardening is essentially athermal. Above room temperature a slight increase in the value for the yield stress (σ) is observed. The thermally activated hardening has been analysed in terms of the elastic interaction between edge dislocations and dipoles, which leads to a linear relationship between σ2/3 and T 1/2. The thermal and athermal contributions to the yield stress of dipole aggregates were found to be similar to those of the individual dipoles; therefore, the dipoles forming part of the small...

Transition-dipole coupling in the Raman modes of dipolar aggregates of liquid sulfur dioxide

Raman spectra have been obtained for liquid SO/sub 2/ and S/sup 18/O/sub 2/ over a wide range of temperatures and for solutions of SO/sub 2/ in CCl/sub 4/. Evidence is presented which shows the existence of transition-dipole coupling between pairs of neighboring SO/sub 2/ molecules with perpendicular molecular planes and parallel C/sub 2/ axes. This evidence takes the form of noncoincident peak frequencies in the polarized and depolarized spectra and anomalous frequency shifts of the coupled components of the major species (/sup 32/S/sup 16/O/sub 2/)/sub 2/ from the uncoupled frequencies of the dilute isotopic species /sup 34/S/sup 16/O/sub 2/, /sup 32/S/sup 16/O/sup 18/O. The coupling has been confirmed through the use of dilution of SO/sub 2/ in CCl/sub 4/ to remove the effects of the coupling. Increased temperature also decreased the effects of intermolecular coupling. The results for liquid SO/sub 2/ have been contrasted to recent results for solid SO/sub 2/. There is no evidence to suggest that liquid SO/sub 2/ can support transverse optical (TO)-longitudinal optical (LO) modes as is possible for solid SO/sub 2/. The correlation field effects for the nu/sub 1/ and nu/sub 2/ regions of liquid and solid SO/sub 2/ are similar but the nu/sub 3/more » region of liquid SO/sub 2/ shows almost no effects of correlation field coupling compared to large effects for solid SO/sub 2/.« less

Deformation induced thermoluminescence in NaCl:Ca(T)

The deformation induced thermoluminescence of NaCl:Ca(T) exhibits marked changed in its pattern when subjected to thermal cycling. This is suggested to be due to the break-up of the impurity — vacancy dipole aggregates and subsequent rearrangement of the free dipoles.

Optical absorption and luminescence in lithium fluoride TLD-100

The isothermal decay at 25 °C of the optical absorption bands in irradiated LiF TLD-100 single crystals has been compared with the decay, under the same conditions, of thermoluminescence (TL) and dielectric loss. The decay of the 380-nm band can be described by a third-order process suggesting that this band is directly related to free Mg-vacancy dipoles, which cluster during aging to form trimers. When compared with recent measurements which indicate that the TL peaks 2 and 3 are not related to the Mg dipoles [G. C. Taylor and E. Lilley, J. Phys. D 15, 1243, 1253 (1982)] the present results infer that the 380-nm band is not directly related to these TL peaks either, in contrast to earlier conclusions [J. H. Jackson and A. M. Harris, Phys. Lett. 29A, 423 (1969)]. This assertion receives some support from pulse annealing measurements and from x-ray-induced luminescence spectra. In particular, the latter results reveal that the spectrum consists of several overlapping bands, the relative intensities of which depend upon the aggregation state of the Mg within the LiF lattice. An emission peaking at ∼460 nm is increased if the concentration of Mg-vacancy dipoles is increased, and an emission at ∼420 nm is related to the number of dipole aggregates (Mg trimers) present. As Mg is precipitated from solid solution the main emission approaches ∼410 nm, which is that seen in Mg-free, Ti-doped LiF. A comparison of these data with recent thermoluminescence spectra measurements leads to the suggestion that TL peak 2 is caused by Mg dipole/Ti complexes, and that TL peak 5 is caused by Mg trimer/Ti complexes.

Aggregation and precipitation stages in NaCl:Pb 2+ studied by UV absorption

UV absorption measurements during isochronal annealing of homogenised and as-grown single crystals show two distinct stages in NaCl:Pb 2+ where number density of free impurities reduce. With the help of our earlier light scattering results the low temperature stage has been attributed to aggregation of impurity-vacancy dipoles whereas the stage appearing at the higher temperature is assigned to the precipitation process.

Influence of the variation of the association energy on the equilibrium concentration of dipole aggregates in ionic crystals

Association energies of dipoles and dipole aggregates calculated theoretically by different methods or determined experimentally by an analysis of kinetic processes of dipole aggregation show a wide scattering. But surprisingly the equilibrium concentration of these aggregates calculated by minimizing the free energy of the crystal and computed with these mentioned energies are lying in a very narrow range. It is shown, that this phenomenon can be attributed to the fact, that the absolute values of the energies are indeed very different, but that the energy differences per defect site are approximately equal comparing two data sets. Only additional deviations can influence the equilibrium concentrations. Assoziationsenergien von Dipolen oder Dipolaggregaten, die mittels verschiedener Methoden theoretisch berechnet oder durch Analyse des kinetischen Prozesses der Dipolaggregation experimentell bestimmt werden, zeigen eine breite Streuung. Uberraschenderweise liegen jedoch die Gleichgewichtskonzentrationen dieser Aggregate, die durch Minimierung der freien Energie des Kristalls und mittels der erwahnten Energien berechnet werden, in einem sehr engen Bereich. Es wird gezeigt, das dies auf die Tatsache zuruckzufuhren ist, das die absoluten Werte der Energien zwar sehr verschieden sind, das aber die Energiedifferenzen pro Defektplatz ungefahr gleich sind, wenn zwei Datensatze verglichen werden. Nur zusatzliche Abweichungen konnen die Gleichgewichtskonzentration beeinflussen.

Effects of pre-irradiation and quenching on the low-temperature thermoluminescence of KCl:Sr

The effect of quenching from 650 degrees C to room temperature as well as that of heavy predoses at room temperature on the thermoluminescence induced by X-irradiation at temperatures between 80 and 300K in KCl:Sr (80 ppm) samples have been investigated. These heavy predoses strongly reduce the glow peaks in the 170-300K range which are clearly related to interstitial centres involving impurity-vacancy dipoles. This result gives additional evidence to support the idea that such dipoles are also involved in the thermoluminescent processes operative above room temperature. The study of quenched samples has revealed the existence of a dipole aggregation process which is induced by irradiation. It is suggested that the energy transfer mechanism is the recombination of the radiation-induced interstitials and vacancies close to the dipoles, either during irradiation or in the thermoluminescence run.

Effects of X-irradiation in europium-doped NaCl

The first stage of F- and M-centre colouring has been investigated at room temperature in quenched samples of europium-doped NaCl as a function of the impurity concentration in the range 30-500 PPM. Attention has been paid to determine whether a correlation exists between the change in the concentration of Eu2+-cation vacancy dipoles and the colour centre formation in order to get a deeper insight into the nature of the interstitial halogen traps. The effects of x irradiation are (a) to reduce the number of (I-V) dipoles, (b) to increase the number of trapped electrons, (c) to convert the doubly valent europium ions to the monovalent, as well as to the tripositive state, and (d) to increase the rate of aggregation of dipoles. The number of dipoles destroyed by irradiation was found to be equal to or to exceed the sum of the number of europium ions which had changed their valence state and the number of F centres produced. The results presented in this paper suggest that the (I-V) dipoles act as the dominant traps for the radiation-induced interstitials. It was also ascertained that the M-centre density is proportional to the square of the F-centre density, the proportionality constant being clearly dependent on the inverse of the impurity concentration.

Influence of concentration and aggregation-precipitation state of divalent europium in the room-temperature coloring of KCl

The first stage of the room-temperature $F$-center coloring in europium-doped KCl has been investigated as a function of the impurity concentration as well as of the aggregation-precipitation state of the europium ions in the crystal with the use of optical-absorption, photoluminescence, electron-paramagnetic-resonance (EPR), and ionic-thermocurrent (ITC) techniques. For the range of europium concentration employed in this work (50-600 ppm), the initial $F$ coloring increases with concentration monotonically. The effects of x irradiation are (a) to reduce the number of isolated ${\mathrm{Eu}}^{2+}$-cation vacancy dipoles, (b) to increase the number of trapped electrons, and (c) to convert the doubly valent impurity ions to the monovalent state. It is reported that the amount of the first-stage coloration is proportional to the square root of the total europium concentration. Evidence has been gathered to show for the first time, that the x irradiation greatly increases the rate of aggregation of dipoles as previously suggested by Muccillo and Rolfe. The results presented in this paper suggest that the ${\mathrm{Eu}}^{2+}$-cation vacancy dipoles act as the dominant traps for the radiation-induced halogen interstitials. The influence of the aggregation-precipitation state of the ${\mathrm{Eu}}^{2+}$ ions on the initial $F$-coloring efficiency has also been studied in considerable detail. It was found that the metastable precipitated phase which has been tentatively associated with the Suzuki phase shows a slightly higher coloring rate than the isolated dipoles or the first products of aggregation. However, the $F$-coloring efficiency decreases as a function of the Eu precipitation into the metastable and stable phases possessing the Eu${\mathrm{Cl}}_{2}$ structure.

Effect of γ‐Irradiation on Eu2+ Ions in KCl Crystals

AbstractOptical absorption and emission, EPR, ITC, and thermoluminescence spectra of RT γ‐irradiated KCl:Eu2+ crystals are examined. ITC and EPR measurements of heavily irradiated samples point to the lack of any I.V. Eu2+ dipoles. Examination of optical properties permits to exclude the presence of Eu+ and Eu3+ ions. On the other hand, the examination of thermal annealing effects in heavily irradiated samples reveals the lack of any correlation between the recovery of original europium dipoles and the decay of colour centres. The results obtained can be understood by assuming radiation field induced aggregation of europium dipoles.

Dopant aggregation and precipitation in alkali halides doped with divalent ions

We have used reliable computer simulation techniques and accurate lattice and defect interionic potentials to calculate the binding energies of a range of possible structures for clusters involving two or more dopant cations for NaCI, KC1 and KBr containing Mg2+ and for the NaCl : BaZt system. Aggregation mecha- nisms involving both nn and nnn dipoles have been considered and the relative importance of alternative pathways examined. We also report results obtained using Born-Model lattice energy techniques on the energetics of meta- stable Suzuki-phase precipitation in these systems. 1. Introduction. - We present results of a theore- tical survey of the aggregation of impurity-vacancy dipoles in alkali-halides doped with divalent cations. This has been undertaken to help resolve difficulties in the analyses of conductivity data (l-31 and thermal depolarization experiments (4, 51 and because of the inherent interest in the kinetics of decay of these dipoles (6-101 and in precipitation in these systems (I 1- 141. Our study includes calculations of the energy of the metastable Suzuki phase precipitates. A theoretical survey of this type is necessary in view of the complexity of these aggregation processes and because previous theoretical studies in this field (8, 15-19) used methods and lattice potentials which have since been shown to be inadequate. 2. Method and potential. - The energies of the defects were calculated using the HADES pro- gram (20) which employs a generalized and automated Mott-Littleton procedure (21). The effect of changing the number of ions in region I, which immediately surrounds the defect, was investigated and it was found to be adequate, even for the largest aggregate studied, if 171 sites were treMed explicitly. The lattice

Comparison of Experimentally and Theoretically Determined Dipole Concentrations of NaCl:Ca++ Considering Aggregation Phenomena

AbstractThe equilibrium state of impurity–vacancy dipoles at RT is calculated by means of experimental determined association energies of the different dipole aggregates and of the point charge approximation, respectively. A comparison with experimental results originating in dielectric loss measurements shows a good agreement, especially with the results computed by the experimentally determined energies. It is possible to quench‐in dipole concentrations higher than the equilibrium value by cooling down the samples from 600 °C. But at higher impurity content partially aggregates or precipitates are formed during this cooling process, as a comparison with the Stasiw‐Teltow association theory shows.

Random motion of IV‐dipoles in alkali halide crystals forming stable aggregates

AbstractThe formation of higher aggregates of IV‐dipoles consisting of a divalent impurity ion and a cation vacancy is investigated by the simulation of their random motion in a model lattice at different annealing temperatures. An interaction potential between the sink for the dipoles and the dipole in the lattice is assumed. Additionally a decay of the sink is allowed. The results show, that already two dipoles (i.e. a dimer) from a stable aggregate. This is in agreement with theoretical calculations of the equilibrium concentrations of higher aggregates and the results of kinetic measurements of dipole aggregation. The reason is the relative high binding energy of dipoles in alkali halide crystals.

X-ray effects on cation impurity-vacancy pairs in KCl:Eu2+

X-ray irradiation of KCl: ${\mathrm{Eu}}^{2+}$ at room temperature destroyed impurity-vacancy ($I\ensuremath{-}V$) dipoles, converted the doubly valent impurity ion to a singly valent state, and created defect centers, e.g., $F$ centers and a 220-nm optical-absorption band. ${\mathrm{Eu}}^{+}$ produced by the irradiation was observed to grow in proportion to the destruction of ${\mathrm{Eu}}^{2+}$. The 220-m\ensuremath{\mu} band was also observed to grow approximately in proportion to the height of the $F$ band, in agreement with previous workers. The number of dipoles destroyed was equal to or exceeded the sum of the number of converted ions and the number of $F$ centers produced. A possible explanation of these results is that x rays increase the rate of aggregation of dipoles. It is likely that it is the cation vacancies bound in $I\ensuremath{-}V$ dipoles that are converted to anion vacancies which later form $F$ centers.

Chiorosulfonated block copolymers

AbstractRubber‐phase chlorosulforiated polystyrene‐poly‐(ethylene‐butene‐1)‐polystyrene block copolymers were prepared by reaction of the block copolymer in solution with SO2 and C12. Sample compositions ranged up to 7 weight percent sulfur and 29 weight percent chlorine contents as determined by X‐ray fluorescence and infrared spectroscopy. A series of chlorosulfonated derivatives of an analogue of the elastomeric midblock were prepared and analyzed in similar fashion. A comparison of dynamic mechanical and dielectric behavior of solvent cast films of the block copolymer and midblock polymer materials indicates the existence of sulfonyl chloride dipole‐dipole complexes in the solid state. Further evidence for the presence of such aggregates was obtained from analysis by Fourier‐transform infrared spectroscopy. It appears that polystyrene domains in the block copolymer materials hinder this aggregation up to moderate concentrations of the sulfonyl chloride group as compared with the relatively unrestrained aggregate formation in the analogue midblock materials. At high levels of substitution, the formation of dipole aggregates disrupts the integrity of the polystyrene domain structure in the block copolymer materials, resulting in a reduction in mechanical strength.