Divalent Cation Impurities Research Articles

Model for F-center production in alkali halides doped with divalent cation impurities that change their valence state by irradiation.

A model for the radiation-induced F-center coloring in alkali halides doped with doubly valent metal impurity ions (${M}^{2+}$) that can change their valence state by irradiation is proposed in the present paper. The primary event is assumed to be the creation of F centers and interstitial halogen (H) pairs. The thermal- and radiation-induced processes associated with the dynamics of H defects and their trapping at defects leading to the stabilization of their counterpart F centers are taken into account during the three stages of the F-coloring curve. The fundamental traps for the interstitials are assumed to be the ${M}^{2+}$-cation vacancy dipoles. Therefore, a heterogeneous nucleation process around these complexes is one of the assumptions on which the model is based. It is proposed that stage I of the F-coloring curve is reached with the single process of the capturing of one H defect by a dipole producing an ${S}_{1}$ center. The trapping of one additional interstitial by ${S}_{1}$ generates a less stable center, ${S}_{2}$. The trapping of one or more interstitials by ${S}_{2}$ gives a stable aggregate center denoted by A, which is responsible for stage III. On the other hand, another way in which these dipoles may vary is through the capture of free electrons during the irradiation process to become either ${M}^{+}$ or ${M}^{0}$ states. It is assumed that these latter states have different capabilities for the trapping of interstitials than those of ${M}^{2+}$. The possibility of hole capture by these reduced states, which would allow them to return to the original ${M}^{2+}$ state, is also included in the model. These basic assumptions are used to postulate a set of ten differential equations which we have solved numerically using a Runge-Kutta iteration program. The numerical solutions of the equations are in good qualitative agreement with the more important features of the experimentally determined effects of room-temperature x-ray irradiation in europium- and lead-doped alkali halides. In some cases, semiquantitative agreement with experiment is obtained.

Effects of divalent anionic impurities on space charge in AgBr microspheres

The model of the space-charge layer in AgBr microcrystals containing divalent cationic impurities has been extended to divalent anionic dopants. Results of calculations for both mobile and immobile impurities are reported. Increasing the impurity concentration leads to a change in the potential difference between the interior and the surface, which in some cases may reverse the sign of the space-charge layer and the compensating surface charge. The point at which this occurs is dependent on the binding energy of the impurity-interstitial complex and on the temperature.

Computer simulation of the induced Snoek effect of impurity-vacancy pairs in NaCl-type crystals

At room temperature and above, the solution hardening of NaCl:M 2+ systems (where M 2+ is a divalent cation impurity) is controlled by impurity (I)-vacancy (V) dipoles through the induced Snoek effect. As the movement of vacancies is essentially a diffusion, the rotation of I–V dipoles can be considered as a Markov process that can be simulated on a computer. In spite of the simplicity of the model and computation presented, they give fairly good numerical results for calcium-doped NaCl.

Point-defect mobility in thallous chloride doped with divalent cation and anion impurities

A detailed study of the temperature dependence of the ionic conductance of single crystals of thallous chloride has been made. Twenty crystals were grown from the melt and detailed results are presented for ten of these: three doped with divalent cations, four doped with divalent anions and three which were nominally pure. Precautions were taken during the conductivity measurements to minimise the effects of sublimation at elevated temperatures. An analysis of the data using computer fitting techniques has provided values for the thermodynamic parameters which govern the formation, migration and interaction of the point defects in the host lattice. The conventional Schottky model with transport on both sublattices and taking account of both nearest-neighbour and long-range (Debye-Huckel) defect interactions was employed. The value for the enthalpy of formation of a Schottky defect was found to be 1.247 eV and that for the migration of the cation and anion vacancies to be 0.379 eV and 0.097 eV respectively. The defect parameters derived are compared with previous experimental and theoretical results and the effect of the changes of the dopant species on the overall conductivity, particularly at lower temperatures, is discussed. The anion vacancy is found to be more mobile than the cation vacancy with the transport number of the former being virtually unity at low temperatures except in the case of the cation-doped crystals. The overall conductivity is interpreted in terms of the concentrations and mobilities of the defects and the extent of the vacancy-impurity association.

Diffusion Model for Charge Transport by Moving Dislocations in Simple Ionic Crystals

AbstractA diffusion model for the calculation of the charge transport caused by moving dislocations in simple ionic crystals is developed. The model is based on the assumption that the charge transport originates from the diffusional motion of vacancies under the influence of the potential field of moving dislocations. The numerical results obtained for LiF and NaCl doped with divalent cation impurities show that the assumed mechanism is suitable for the explanation of the main features of the experimental results.

Aggregation pathways and Suzuki phase formation in doped alkali halides

A model about Suzuki phase formation in Alkali Halides doped with divalent cationic impurities is presented. It considers rearrangement of small aggregates as dimers and trimers. Experimental data of Suzuki phase nucleation at room temperature in NaCl:Cd 2+ corroborate this model.

Slip structures and cross-slip of screw dislocations during the deformation of NaCl single crystals at low temperatures

NaCl single crystals were deformed at liquid helium temperature and at liquid nitrogen temperature. The slip structure was investigated by the gold decoration technique. At liquid helium temperature, relatively pure crystals show a predominance of sharp narrow slip bands with a high internal strain. Localized displacements of the bands are frequently indicated by microcracks. At liquid nitrogen temperature the slip is homogeneous in most cases. In the work reported in this paper, the slip steps of individual dislocations are imaged, and the cross-slip processes of screw dislocations were investigated as a function of the concentration of divalent cationic impurities. Both the cross-glide events were almost identical with the corresponding room temperature data. The results suggest that cross-slip is initiated by long-range internal stresses.

Electron Spin Resonance Study of HD‐Type BrCl− Defects in KCl Doped with Divalent Cation Impurities

AbstractHD‐type BrCl− defects produced by X‐irradiation at low temperatures in KCl containing both Br− and Sr2+ or Ba2+ impurities are studied by electron spin resonance. In KCl:Sr2+ two HD‐type BrCl− defects, both with their molecular axes tilted in a {100} plane, are observed. The models that are proposed consist of a BrCl− molecule ion, on an anion site, next to a Sr2+ and an associated positive ion vacancy which are nearest neighbors of each other. In one center the bromine is on a more interstitial site and in the other on a more substitutional site. The two HD‐type BrCl− defects observed in KCl:Ba2+ are tilted in a {110} plane. The models consist of a BrCl− on an anion site in the immediate vicinity of a Ba2+ configuration. Again the bromine is either more substitutionally or more interstitially situated. A so‐called HDD<100>‐type BrCl− defect, involving a dimer of impurityvacancy complexes, is also observed.

Model of first-stageF-center production in alkali halides containing divalent cation impurities

A system of equations, describing the formation of $F$ centers and their complements in irradiated alkali halides doped with divalent impurities, has been developed. These incorporate the stabilization of $H$ centers and di-interstitial halogen defects at impurity-vacancy dipoles or aggregates of such dipoles, as well as radiation-induced interstitial detrapping mechanisms. Numerical and approximate analytical solutions of the equations reproduce the dynamic, temperature-dependent features of $F$-center growth found in experimental studies. They provide a new explanation of the experimentally observed relations between the growth of defects and the divalent-cation-impurity concentration; i.e., the square-root relation between first-stage $F$ centers and such impurities, and the relation between the saturation concentration of ${H}_{D}$ centers and these impurities.

Pressure and temperature dependences of the ionic conductivity of CsCl

Measurements of the pressure and temperature dependences of the ionic conductivity of CsCl combined with earlier diffusion and/or ionic conductivity measurements at atmospheric pressure have allowed determination of energies and volume relaxations associated with the formation and motion of Schottky defects in this crystal. An important result is that the lattice relaxation associated with vacancy formation is large and outward, the formation volume for Schottky pairs being 1.8-2.0 times the molar volume. The association energy of ${\mathrm{Cs}}^{+}$ vacancies and divalent cation impurities is estimated to be 0.36 eV.

EPR Spectra of Cr5+ Ions in KCl Crystals

AbstractEPR, optical, and IR absorption spectra of X‐irradiated KCl crystals doped with K2CrO4 and K2Cr2O7 are studied. It is ascertained that the impurity enters as chromate in both cases and there are two centre types — isolated CrO ions and CrO ions coupled with divalent cation impurities. The X‐irradiation results in the formation of paramagnetic CrO3−4 centres with monoclinic local symmetry Cs whose excessive negative charge is compensated by an anion vacancy. The EPR spectrum due to these centres is described by a spin‐Hamiltonian for S = 1/2 with gx = 1.948, gy = 1.953, gz = 1.967. The x‐axis of the g‐tensor is oriented along 〈001〉 and two other principal axes along the directions of 〈110〉 type.

First stage f centre formation in pure and doped KCl crystals by room temperaure X-irradiation

The electrical conductivity and optical absorption of potassium chloride crystals, pure and doped with divalent cation impurities, have been measured before and after X-irradiation at room temperature. The concentration of free positive ion vacancies at room temperature has been calculated from conductivity for each crystal before irradiation and is found to be much less than the first stage F centre concentration. This shows that both free and associated positive ion vacancies are the latent source of F centres in the first stage colouration. Pb+ + ions trap electrons producing Pb+ + and Pb0 centres and making free the associated cation vacancies. Such centres are not produced in Ca-doped crystals where impurity-vacancy complexes trap F centres producing Z centres. The higher ratio of F centres to positive ion vacancies in Pb-doped crystals indicates that free cation vacancies are more effective in producing F centres. However, the concentration of divalent cations is found to decrease in both the crystals after irradiation, the decrease being more in Ca-doped crystals.

Influence of divalent cationic impurities on plasticity of AgCl

Influence of divalent cationic impurities on plasticity of AgCl

Energy parameters of cation motion in alkali halides as determined by dielectric relaxation measurements

Energy parameters for the motion of a cation into an adjacent vacancy when both are in the vicinity of a divalent cation impurity have been determined more accurately than previously from dielectric measurements on three of the more commonly investigated alkali halides, LiF, NaCl and KCl. There is good agreement with the best recent determinations of the parameters for the motion of a cation into an adjacent cation vacancy as determined by DC electric conductivity measurements. Differences in activation enthalpies between the two methods of measurement are less than 2%. Measurements on NaI:Ca2+ have been included also in this programme because of a relatively large discrepancy between the enthalpy determined earlier by dielectric absorption and that determined more recently by other methods.

Effect of quenching on the electrical conductivity of doped KCl crystals

Electrical conductivity and optical absorption of divalent cation and anion doped KCl crystals have been measured before and after quenching from 750 °C. The conductivity after quenching is found to decrease in undoped and cation doped crystals while it increases in the anion doped crystals. This is due to higher coagulation rate of cation impurities as compared to anion impurities. Since the divalent impurities diffuse in the form of impurity-vacancy complexs, the higher mobility of cation vacancies (which form dipoles with divalent cation impurities) accounts for the higher aggregation rate of the cation impurities. The aggregation rate, during annealing after quenching, has also been found to depend on concentration.

The Solid State Displacement Reaction CsCl + Nal → Csl + NaCl

Abstract The solid state reaction CsCl+Nal → CsI+NaCl was investigated, employing diffusion couples with two different contents of divalent cation impurities, in the temperature range 350-419 °C. From kinetic determinations, inert marker experiments and X-ray diffraction analysis it was possible to deduce that the rate − determining step is the diffusion of I- in CsI via VI.

Electrical Properties of Lithium Hydride

Ionic conductivity of LiH crystal has been measured. The energy for Schottky vacancy pair formation (2.3±0.3 eV) and energies of a positive ion vacancy for migration and for the association with a divalent cation impurity (0.54 eV and 0.50 eV, respectively) were obtained from the ionic conductivity curve. The energy for the rotation of a vacancy around divalent cation was 0.38±0.02 eV from the measurements of ionic thermocurrent (ITC). Photoconductivity peaks at 5.05 eV and 6.7 eV are ascribed to the exciton and to the photoemission state, respectively. It is suggested that the defect properties of LiH are similar to those of LiCl and LiF and that the electronic properties are close to those of LiI.

Association energy of divalent and monovalent impurities with edge dislocations in NaCl

In the present paper calculations have been made of the association and interaction elastic energies for monovalent cation impurities and divalent cation impurities unassociated and associated with cation vacancies in the form of dipoles with dislocationsa/2 〈110〉. {110} anda/2〈110〉 {100} in NaCl. The results obtained for the chosen positions in dislocation cores show that a) among the unassociated divalent impurities considered the increase in barium concentration around dislocations takes place in the extended region of dislocations and the increase in calcium and other impurity concentration with\(r_{ + + }< r_{Ca^{ + + } } \) in the compressed region of dislocation, mainly on the dislocationa/2〈110〉 {100}. In the case of divalent impurities associated with cation vacancies in the form of dipoles the increase in impurity concentration can take place at all impurities as in the above mentioned case and additionally Sr++ b) the increase in monovalent impurity concentration (r+>164-01) around dislocations takes place, excepting lithium, in the extended region of the dislocation of both types. Our calculations are compared with the published experimental data.

Kinetics of aggregation of divalent cation impurities in alkali halides: Comments on a paper by Unger and Perlman

Deductions made in a recent paper by Unger and Perlman about the kinetics of aggregation in doped alkali halides are discussed and the experimental basis for deciding on the order of the aggregation process is restated.

Surface impurities in silver halide films

Auger spectroscopy has shown that the divalent cation impurities Zn++, Cd++, and Pb++ in the concentration range above 0.1% segregate to the surface of silver halide films. Sputtering of the films shows a layer approximately 10 Å thick at the film-vacuum interface where the impurities accumulate. The surface ratio of impurity to silver ion increases with the total amount of impurity present.