Relaxation kinetics of correlated neutral Frenkel pairs of defects in alkali-halide crystals

A modelling method is developed that permits qualitative investigation of the evolution of genetic pairs of Frenkel defects in alkali-halide crystals (AHC). The relaxation kinetics and the form of the low-temperature annealing curves of F-, H-pairs are determined by initial mutual distribution functions (IDF) of the pairs being formed up to the time of excess energy dissipation by an H-center. It is shown that, because of discreteness of the lattice for AHC, interactions between the F- and H-centers are characteristic of IDF with a nonuniform spacing distribution of the F- and H-pairs. Fundamental regularities of Frenkel defect relaxation after pulse exposure and of F-, H-pair annealing after low-temperature AHC exposure are discussed on the basis of modelling results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 16–21, March, 1989.     

Effects of lattice polarity on interfacial space charges and defect disorder in ionically conducting AgI heterostructures

Heterostructured (β/γ)-AgI exhibits a spontaneous lattice polarization not accounted for in standard space-charge models. This polarization field dominates the positional variation of energies of isolated defects, and Ag(+) vacancies and interstitials are stabilized at alternate [β/γ] interfaces. This suggests enhanced Frenkel pair separation, analogous to electronic charge separation in polar semiconductor heterostructures. Stoichiometric systems are, however, characterized by associated Frenkel pairs due to strong V(Ag)-Ag(i) interactions and show no enhancement of defect numbers. In nonstoichiometric systems, lattice polarization does direct the distribution of the excess defect species, and defect-defect interactions enhance local Frenkel pair concentrations at interfaces, suggesting that nonstoichiometry is critical to the extreme room-temperature ionic conductivities observed in heterostructured AgI nanoplates.     

Changes in the photosensitivity and dark conductivity of CdS single crystals after illumination at high temperature

The increase of photoconductivity after previous illumination in the temperature range 320–420 K has been investigated. An activation energy of about 0.8 eV for this process, and two energies around 0.4 and 0.7 eV for the recovery process, have been found. In part of the crystals high residual conductivity has been observed as a co-appearing phenomenon. At 300 K the enhanced photosensitivity persists for a period of longer than 10⁵ s. It is shown that the enhanced photosensitivity is not connected with the destruction of donor-acceptor pairs. As a possible explanation the mechanism of generation of Frenkel pairs in the presence of free carriers is suggested.     

On the temperature dependence of the F-coloring efficiency in KCl

A simple model is presented to account for the major features of the temperature dependence of the primary F-coloring efficiency in KCl. It assumes that irradiation creates Frenkel pairs in three basic configurations followed by some correlated diffusion, leading either to annihilation of the pair or formation of a “surviving” primary pair. Steady irradiation as well as fast pulse data are analyzed in terms of the model.     

Ionic conductivity of mixed silver halide crystals

To evaluate the effects of mixed halides on the lattice defect parameters of the silver halides, we have measured the ionic conductivity both of the entire range of mixed AgBr-AgCl single crystals, aftd also of several iodide-doped crystals. For the AgBr-AgCl system, the intrinsic conductivity at a given temperature decreases monotonically from pure AgBr to pure AgCl. The deduced Frenkel defect formation energy varies only a little from 0 to 50 mole % AgCl, and then increases rapidly with further increase in AgCl content, closely paralleling the ratio of bulk modulus to dielectric constant. The defect formation energy in these crystals hence reflects the average macroscopic properties of the solid solution. For the iodide-doped crystals, however, the results are quite different. Small amounts-of iodide cause large increases in the conductivity of AgBr andiAgCl, especially in the latter. These results suggest that the elastic strain introduced by the oversized iodide ion exerts an appreciable local effect on the Frenkel defect formation, in contrast to the crystal-averaged response found for the AgBr-AgCl solid solutions. Furthermore, the Arrhenius plots for the conductivitiei of the AgBr: I specimens show curvature which suggests a temperature-dependent pairing of the solute.     

Quantum-chemical simulation of Frenkel pairs separation in a LiF crystal

Semiempirical quantum-chemical (INDO) simulation of the creation of the primary Frenkel pairs of defects in a LiF crystal based on the Pooley-Smoluchowski mechanism of the self-trapped exciton annihilation has been undertaken. The conclusion has been drawn that this mechanism can be operative from the viewpoints of both the time and the energy needed to create the F, H pair in terms of this mechanism. Unlike the Toyozawa's model an initial vibrational excitation is not bottleneck of the decay process (cf. [8]).     

Relaxation kinetics of primary pairs of radiation defects in ionic crystals

The relaxation kinetics of primary pairs of radiation defects in ionic crystals with a face-centered lattice is investigated using the Monte Carlo method. The dependence of the relaxation kinetics of an F-H pair on the parameters of the interaction potential between the components of the pair is studied. The obtained kinetic dependences are analyzed to determine the factors responsible for the relaxation processes.     

Irradiation induced radiative centres in ZnTe

High energy electron irradiation of ZnTe crystals at 4 K gives rise to a new luminescence line at 2.361 eV and to a broad band at 1.578 eV. These features disappear on annealing at T? 180 K. The defect responsible for these radiative transitions is tentatively identified with Frenkel type close pair. Another broad line at 2.065 eV appears after a 77 irradiation. Its annealing temperature is near 300 K.     

Volumenänderung von Alkalihalogenidkristallen bei der Bildung von Gitterdefekten durch niederenergetische Strahlung

Defects were created in the alkali halides KBr and KCl by X-irradiation. The type of disorder was determined by combined lattice parameter and length change measurements. The change of the lattice parameter was determined by the shift of a Bragg reflex, and the length change was detected by an inductive transducer. To omit thermal annealing and conversion of defects the crystals were irradiated below 10 °K. Defects are created as Frenkel defects. They anneal in distinct annealing stages, and this Frenkel disorder remains unchanged during annealing. Volume change and lattice distortion due to different defects are obtained by combined absorption and length change measurements. In KBr the volume change of two different Frenkel pairs was detected. A pair consisting ofα- andI-center causes the volume change (Δ)_{α +I} =3.2 atomic volumes and a pair ofF- andH-center(Δv) _F+H =1.2 atomic volumes.     

Proton N.M.R. of trismethylenemethaneiron tricarbonyl in a nematic liquid crystal

Single crystals of the cycloadduct of 1-(9-anthryl)-3-(1-naphthyl)propane (CANP) have been prepared, enabling a photochemical and photophysical study to be made of the anthracene-naphthalene sandwich pair in this material. The low temperature fluorescence spectrum is broad, exciplex in nature, and has a decay time of about 80 ns. This emission is quenched as the temperature is raised by a process which involves photocyloaddition of the pair. Arrhenius plots give an activation energy of about 600 cm^-1 for the photochemical process. The analogous photophysical properties have also been studied in methylcyclohexane glasses at 77 K and 60 K. Formation of the sandwich pairs at 77 K gives rise to a ‘ partially relaxed ’ pair having some structure in its emission spectrum and a shorter lifetime. Possible reasons for the differences in properties of sandwich pairs in methylcyclohexane glasses and the parent cycloadduct crystals are discussed. Very weak emission from the naphthalene moiety of monomer ANP has been observed at 77 K.     

Charged Frenkel biexcitons in organic molecular crystals

It is known that the energy of the lowest electronic transition in the neutral molecules of anthracene, tetracene, and other polyacenes is blue-shifted in comparison with the corresponding transition energy in univalent molecular ions. This effect in a molecular crystal may be responsible for the attraction between a molecular (Frenkel) exciton and a charge carrier. Due to this attraction, a bound state of Frenkel exciton and free charge (charged Frenkel exciton) may be formed [5]. As we demonstrate below, the same mechanism can be responsible for the formation of a charged biexciton (bound state of two Frenkel excitons and a charge carrier). A one-dimensional lattice model is used which corresponds to J aggregates and is also a good approximation for quasi-one-dimensional crystals. Calculations are performed for molecular crystals like tetracene, where the exciton band at low temperature is much narrower than the band of the charge carrier.     

Computer modelling of intrinsic defects and migration processes in KY3F10

Abstract

The computer modelling technique was used in the present work to study the intrinsic defects and the migration processes in KY₃F₁₀. The main intrinsic disorder was found to be F Frenkel pairs but at higher temperatures it is possible that KF pseudo-Schottky defects could also be present in the material. The ionic conduction process is mainly due to the fluorine ions migrating via both vacancy and interstitialcy mechanisms.     

Radiation resistance of LaPO4 (monazite structure) and YbPO4 (zircon structure) from data of computer simulation

The radiation resistance of the monazite LaPO₄ and the compound YbPO₄ (zircon structure type) has been investigated using the computer simulation. The number of Frenkel pairs, which are formed in the structure of these minerals after the passage of a primary knock-on thorium atom with an energy of 30 keV, has been calculated by the molecular dynamics method. The formation of Frenkel pairs and their recombination in the motion of recoil nuclei in the structure of the studied minerals have been discussed. It has been shown that the probability of the “survival” of Frenkel pairs in the LaPO₄ monazite is significantly lower than in the YbPO₄ compound. The tendency of these minerals toward amorphization under radiation damage has been described numerically. The obtained results have demonstrated that one of the main factors determining the radiation resistance of orthophosphates LnPO₄ is the type of crystal structure, and the compounds with the monazite structure are more radiation resistant than the compounds with the zircon structure.     

Temperature and impurity concentration dependences of the efficiency of Frenkel defect accumulation in alkali halide crystals

A simple phenomenological model of the efficiency of accumulation of F centers in alkali halide crystals as affected by temperature and homologous cation impurity is considered. The conclusion is drawn that the temperature dependence is probably due to elastic interaction of primary Frenkel defects — F , H centres affecting their diffusion-controlled separation (similar to metals) whereas the impurity concentration dependence is due to the tunneling recombination of F , H_A pairs.     

Non-luminescent disexcitation of F-center pairs by exchange-softened vibrational modes

The non-luminescent and spin-dependent disexcitation process observed in F-center pairs in alkali halides at low temperature is explained by a covalent bond within the pair. Exchange effects give a negative contribution to the lattice potential energy. If the pair separation is small, local modes become unstable and spontaneous lattice distortions bring back the pair in its ground state.     

Effect of recoil atoms on radiation-defect formation in semiconductors under 1–10-MeV proton irradiation

The formation of radiation defects in Si under 1–10-MeV proton bombardment is analyzed. Numerical simulation is carried out, and histograms of the distribution of the energy transferred to recoil atoms are obtained. Two energy ranges are considered when analyzing the histograms. Single Frenkel pairs with closely located components are produced in the first range (small energies). Recoil atoms of the second range have an energy sufficient for the production of a displacement cascade. As a result, nanoscale regions with high densities of vacancies and different types of their complexes appear. In addition, as the energy of the primary knocked-out atoms increases, the average distance between genetically related Frenkel pairs increases, and, as a consequence, the fraction of pairs that are not recombined under bombardment increases.     

Enrichment centers in sodium bromide crystals

The primary products of irradiation of alkali halide crystals are anion and cation Frenkel pairs [1–4]. Interaction of these products among themselves and with structural defects existing before irradiation leads to the appearance of various structural defects, the majority of which are optically active. The present study will treat enrichment centers which can be formed from the components of Frenkel cation pairs. Studies of this type are important in providing information on states in which radiation-created cation defects can exist stably in the lattice.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 119–121, August, 1979.     

Formation and diffusion of self-interstitial atoms in silicon crystals under hydrostatic pressure: Quantum-chemical simulation

A theoretical modeling of the formation of Frenkel pairs and the diffusion of a self-interstitial atom in silicon crystals at normal and high (hydrostatic) pressures has been performed using molecular dynamics, semiempirical quantum-chemical (NDDO-PM5, PM6), and ab initio (SIESTA) methods. It is shown that, in a silicon crystal, the most stable configuration of a self-interstitial atom in the neutral charge state (I ⁰) is the split configuration 〈110〉. The shifted tetrahedral configuration (T ₁) is stable in the singlet and triplet excited states, as well as in the charge state Z = +2. The split 〈110〉 interstitial configuration remains stable under hydrostatic pressure (P ≤ 80 kbar). The activation barriers for diffusion of self-interstitial atoms in silicon crystals are determined to be as follows: ΔE _a (Si)(〈110〉 → T ₁) = 0.59 eV, ΔE _a (Si)(T ₁ → T′₁) = 0.1 eV, and ΔE _a (Si)(T ₁ → 〈110〉) = 0.23 eV. The hydrostatic pressure (P ≤ 80 kbar) increases the activation barrier for diffusion of self-interstitial atoms in silicon crystals. The energies of the formation of a separate Frenkel pair, a self-interstitial atom, and a vacancy are determined. It is demonstrated that the hydrostatic pressure decreases the energy of the formation of Frenkel pairs.     

离子配对动力学不一定遵循Eigen-Tamm机理

Eigen-Tamm模型认为水溶液中的离子对从单水分子桥链离子对到紧密离子对是离子对溶剂化平衡的动力学控制性步骤,两态之间自由能垒最高,相互转化速率最慢. 设计了两类带有单位电荷的离子对模型(2.0:x和x:2.0,固定离子对中的阳离子或阴离子的半径为2 ?,另一个为不同半径的阴阳离子),通过计算离子对不同距离的平均力势来获取自由能曲线,发现2.0:x系列离子对由于溶剂水的作用,从单水分子桥链离子对到紧密离子对转化的自由能垒有明显减低,并不是离子成对动力学平衡中的最慢步骤,与Eigen-Tamm 模型描述存在偏差.     

Probability calculations on the auto-annihilation of radiation damage in LiF

Simplified models are presented, simulating the irradiation of lithium fluoride single crystals. In these models the evolution of the primary defect concentration versus the number of Frenkel pairs, induced at random in the lattice, is studied from a statistical point of view. The maximum defect concentration follows as a consequence of postulated annihilation volumes in which a newly created primary recombines with one of its antidefects. The fact whether single or di-atomic interstitials are considered has only a minor effect on the maximum defect retention.