Correlation between deformation bleaching and mechanoluminescence in coloured alkali halide crystals

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, E _a and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.     

Lyoluminescence mechanism of gamma and additively coloured alkali halides in pure water

The light emitted on dissolution of gamma and additively coloured crystals of NaCl, NaBr, KCl, KBr, CsCl and CsBr in pure water is studied. Experiments on gamma irradiated crystals have proved that the light emission originates from the recombination of released F-centres with trapped holes (V₂-centres) at the water-solid interface. Exceptionally pure additively coloured crystals having only F-centres produced no light when dissolved in pure water. However, relatively impure crystals emit light on dissolution due to the recombination of F-centres with traces of impurity ion centres in these crystals.     

Mechanoluminescence response to the plastic flow of coloured alkali halide crystals

The present paper reports the luminescence induced by plastic deformation of coloured alkali halide crystals using pressure steps. When pressure is applied onto a γ-irradiated alkali halide crystal, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value and later on it decreases with time. The ML of diminished intensity also appears during the release of applied pressure. The intensity I_m corresponding to the peak of ML intensity versus time curve and the total ML intensity I_T increase with increase in value of the applied pressure. The time t_m corresponding to the ML peak slightly decreases with the applied pressure. After t_m, initially the ML intensity decreases at a fast rate and later on it decreases at a slow rate. The decay time of the fast decrease in the ML intensity is equal to the pinning time of dislocations and the decay time for the slow decrease of ML intensity is equal to the diffusion time of holes towards the F-centres. The ML intensity increases with the density of F-centres and it is optimum for a particular temperature of the crystals. The ML spectra of coloured alkali halide crystals are similar to the thermoluminescence and afterglow spectra. The peak ML intensity and the total ML intensity increase drastically with the applied pressure following power law, whereby the pressure dependence of the ML intensity is related to the work-hardening exponent of the crystals. The ML also appears during the release of the applied pressure because of the movement of dislocation segments and movements of dislocation lines blocked under pressed condition. On the basis of the model based on the mechanical interaction between dislocation and F-centres, expressions are derived for the ML intensity, which are able to explain different characteristics of the ML. From the measurements of the plastico ML induced by the application of loads on γ-irradiated alkali halide crystals, the pinning time of dislocations, diffusion time of holes towards F-centres, the energy gap E_a between the bottom of acceptor dislocation band and the energy level of interacting F-centres, and work-hardening exponent of the crystals can be determined. As in the elastic region the strain increases linearly with stress, the ML intensity also increases linearly with stress, however, as in the plastic region, the strain increases drastically with stress and follows power law, the ML intensity also increases drastically with stress and follows power law. Thus, the ML is intimately related to the plastic flow of alkali halide crystals.     

Mobile interstitial model and mobile electron model of mechano-induced luminescence in coloured alkali halide crystals

A theoretical study is made on the mobile interstitial and mobile electron models of mechano-induced luminescence in coloured alkali halide crystals. Equations derived indicate that the mechanoluminescence intensity should depend on several factors like strain rate, applied stress, temperature, density of F-centres and volume of crystal. The equations also involve the efficiency and decay time of mechanoluminescence. Results of mobile interstitial and mobile electron models are compared with the experimental observations, which indicated that the latter is more suitable as compared to the former. From the temperature dependence of ML, the energy gaps between the dislocation band and ground state of F-centre is calculated which are 0.08, 0.072 and 0.09 eV for KCl, KBr and NaCl crystals, respectively. The theory predicts that the decay of ML intensity is related to the process of stress relaxation in crystals.     

Mechanoluminescence induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates

Mechanoluminescence (ML) emission from coloured alkali halide crystals takes place during their elastic and plastic deformation. The ML emission during the elastic deformation occurs due to the mechanical interaction between dislocation segments and F-centres, and the ML emission during the plastic deformation takes place due to the mechanical interaction between the moving dislocations and F-centres. In the elastic region, the ML intensity increases linearly with the strain or deformation time, and in this case, the saturation region could not be observed because of the beginning of the plastic deformation before the start of the saturation in the ML intensity. In the plastic region, initially the ML intensity also increases linearly with the strain or deformation time, and later on, it attains a saturation value for large deformation. When the deformation is stopped, initially the ML intensity decreases at a fast rate; later on, it decreases at a slow rate. The decay time for the fast decrease of the ML intensity gives the relaxation time of dislocation segments or pinning time of the dislocations, and the decay time of the slow decrease of the ML intensity gives the diffusion time of holes in the crystals. The saturation value of the ML intensity increases linearly with the strain rate and also with the density of F-centres in the crystals. Initially, the saturation value of the ML intensity increases with increasing temperature, and for higher temperatures the ML intensity decreases with increasing temperature. Therefore, the ML intensity is optimum for a particular temperature of the crystals. From the ML measurements, the relaxation time of dislocation segments, pinning time of dislocations, diffusion time of holes and the energy gap between the bottom of the acceptor dislocation band and interacting F-centre level can be determined. Expressions derived for the ML induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates indicates that the ML intensity depends on the strain, strain rate, density of colour centres, size of crystals, temperature, luminescence efficiency, etc. A good agreement is found between the theoretical and experimental results.     

Isothermal decay of colour centres in microcrystalline powders of alkali bromides

The bleaching of F-centres on storing in darkness of electrolytically coloured single crystals and microcrystalline powders of solid solutions of KBr with other alkali bromides is reported. The colouration of the microcrystalline powders decays isothermally while that of single crystals is comparatively stable. The decay proceeds slower in the powders of mixed crystals than in the pure materials. A tentative explanation is suggested.     

Dislocation spectroscopy of crystals

A new method of studying the energy characteristics of dislocations is proposed, which is based on the investigation of the interaction of moving dislocations with purposefully introduced electronic and hole centers. A study has been made of KCl, NaCl, KBr, LiF, and KI alkali halide crystals containing electronic F and hole V _K and Me ⁺⁺ (Cu⁺⁺, Ag⁺⁺, Tl⁺⁺, In⁺⁺) centers. Investigation of the temperature dependence of the dislocation interaction with the F centers permitted determination of the position of the dislocation-induced electronic band (DEB) in the band diagram of the crystal. In KCl, the DEB is separated by ≈2.2 eV from the conduction-band minimum. It is shown that dislocations transport holes from the centers lying below the dislocation-induced hole band (DHB) (X ⁺, In⁺⁺, Tl⁺⁺, V _K) to those above the DHB (the Cu⁺ and Ag⁺ centers). Such a process is temperature independent. The DHB position in the crystal band diagram has been determined; in KCl it is separated by ≈1.6 eV from the valence-band top. The effective radii of the dislocation interaction with the electronic F and hole X ⁺, V _K, and Tl⁺⁺ centers have been found. Fiz. Tverd. Tela (St. Petersburg) 41, 2139–2146 (December 1999)     

Mechanoluminescence induced by elastic deformation of coloured alkali halide crystals using pressure steps

During the elastic deformation of coloured alkali halide crystals, the bending segments of dislocations capture F-centre electrons lying in the expansion region of edge dislocations, to the states of dislocation band. After the separation from interacting F-centres, the captured electrons move together with the bending segments of dislocations and also drift along the axis of dislocations and subsequently the radiative electron–hole recombinations, owing to both the processes of captured-electron movement, give rise to the light emission. The generation rate of electrons in the dislocation band and the mechanoluminescence (ML) intensity initially increase with time, attain maximum value at a particular time, and then they decrease with time. The intensity I_m corresponding to the peak of ML intensity versus time curve and the total intensity I_T of ML increase with the applied pressure and also with the density of F-centres in the crystals. At low temperature, both I_m and I_T increase with temperature and at higher temperature they decrease with increasing temperature due to the thermal bleaching of F-centres and also due to the decrease in luminescence efficiency. Thus, both I_m and I_T are optimum for a particular temperature of the crystals. For longer time duration, the ML intensity decreases exponentially with time in which the decay time is equal to the lifetime of interacting F-centres. Expressions derived for the different characteristics of ML are able to explain the experimental results. It is shown that the time constant for rise of pressure, lifetime of the interacting F-centres or damping time of dislocation segments, and the activation energy can be determined from the ML measurements.     

Energy transfer due to excitons and holes under host-sensitization in KBr:I

Energy transfer due to excitons and holes to iodine ions in KBr:I crystals has been investigated. It is confirmed that, when the exciting photon energy is increased beyond the Γ-band edges of KBr, probability of the energy transfer due to exciton diffusion rapidly decreases with the complementary enhancement of that due to hole migration followed by electron capture.     

Semiempirical pseudopotential surfaces for chemical reactions: The (AB)+ X - dialkali halide systems

A generalization of the Roach-Child semiempirical pseudopotential calculation for K + NaCl to several analogous dialkali halide systems has been used to elucidate the chemical interactions governing the reaction dynamics. The Li + LiF ground-state potential surface, which exhibits a ～ 20 kcal/mole basin for isosceles Li₂F, is qualitatively similar to one obtained in a recent configurational interaction calculation. It is shown that regions of the Na₂Cl ground-state surface corresponding to Na₂ ⁺ interacting with Cl^- can be described in terms of an ion-pair Rittner potential model similar to that employed for the alkali halides. Chemical trends in the triangular complex well depths satisfactorily account for the experimentally observed transition between the collision complex mechanism (Rb + KCl) and the osculating complex model (Li + KBr) for the alkali-alkali halide exchange reactions at thermal energies. For collinear configurations with the alkalis on opposite ends, avoided intersections between the lowest two potential surfaces are characterized in terms of diabatic surfaces computed from truncated basis sets. Crossings of these surfaces account for the vibrational-electronic energy transfer between alkali atoms and vibrationally excited alkali halides. The ionic X ^- + A ₂ ⁺ potential surfaces are used to predict the product electronic excitation and partitioning of exoergicity in reactions of halogen atoms with alkali dimer molecules.     

Electroluminescence of alkali halide crystals activated with europium ions

The prebreakdown electroluminescence of NaCl-Eu, KCl-Eu, and KBr-Eu crystals has been studied. The activator concentration in the crystals was on the order of 10¹⁸ ions per cubic centimeter. The emission spectra, the brightness wave during pulsed excitation, and the concentration dependence of the quantum yield were measured. The experimental results support the suggestion of an impact mechanism for the excitation of the electroluminescence of alkali halide crystals. Recombination and exciton mechanisms for energy migration play an insignificant role. There is no significant contribution from impurity-vacancy dipoles, which act as emission centers, in electric fields up to 10⁶ V/cm.Tomsk Academy of Control and Electronic Systems. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 105–109, February, 1994.     

Behavior of block boundaries during ultrasonic vibration in alkali halide crystals at temperatures 20–300°C

The behavior of block boundaries was investigated in alkali halide crystals of various orientations deformed by ultrasound at frequencies 40–73 kHz in the temperature interval 20–300°C. The specimen orientation was defined by the angle between the fourfold axis and the propagation direction of the ultrasonic wave. The initial stage of plastic deformation at room temperature is due to the generation of sources at the block boundaries in KCl and KBr, and by the heterogeneous nucleation of dislocation in NaCl and LiF. In the temperature interval 200–300°C, in NaCl and LiF dislocation multiplication begins already with the generation of sources at the block boundaries. In 30° orientation specimens the beginning of multiplication is preceded by the motion initiated by the ultrasound of sections of the block boundaries in the {1 0 0} <110> secondary glide system.Translated from Izvestiya Vysshika Uchebnykh Zavedenii, Fizika, No. 1, pp. 49–53, January, 1986.     

Dislocation models of mechanoluminescence in γ- and X-irradiated alkali halides crystals

The mechanoluminescence appears in the elastic, plastic and fracture regions of γ- and X-irradiated KCl and NaCl crystals. A linear relation is found between the mechanoluminescence intensity and the newly created dislocations. Four models are proposed for the mechanoluminescence excitation during the movement of dislocations. These models are: dislocation unpinning model, dislocation interaction model, dislocation defect stripping model, and dislocation innihilation model. The dislocation annihilation model seems to be a dominating process for the M.L. excitation in γ- and X-irradiated alkali halide crystals.     

Calculation of energies of radiative tunneling transitions between defects in alkali halides

The procedure for calculating the energies of radiative tunneling transition between spatially separated electron and hole centres in ionic crystals based on semiempirical INDO calculation of a pair of close defects and on a further account of electronic polarization of a crystal due to defects has been presented. Pairs of an electron, F, and hole (H, V_k, V₂) centres in KCl and LiF crystals are studied. The influence of close defect interaction and of polarization upon emitted photon energy has also been considered. It is shown that due to polarization this energy varies with a distance between defects in a more complicated way than $\text{e}{}^2\text{?R}$ shown in semiconductors.     

Studies of defects in photonic materials

Indium tin oxide (ITO) films have high optical transmission and infrared reflectance, good electrical conductivity, excellent substrate adherence, hardness and chemical inertness. These properties lead to many applications in the area of photonics. Bombardment of ITO films with 1 MeV protons has been carried out resulting in an observed darkening. Insights into the darkening mechanism that consists of three growth stages as a function of fluence are provided by a study of the optical absorption and X-ray lattice parameter. A new interpretation is provided for the darkening mechanism in terms of the production of defect clusters resulting from the atomic displacements during implantation.CsI crystals are very effective scintillator materials for particle detectors in high energy physics. Although radiation hard, radiation damage produces colour centres in CsI that reduce light emission and can negatively affect the luminescent centres. Using a combination of Raman and optical absorption spectroscopy applied to CsI crystals bombarded with 1 MeV protons at 300 K, the resulting defects are shown to be F-type centres and interstitial V-centres having the I₃⁻ structure and being responsible for absorption bands at 2.7 and 3.4 eV. Isochronal and isothermal annealing experiments show a mutual decay of the F and V-centres. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides.     

Lyoluminescence mechanism of additively and gamma coloured alkali halides in fluorescent and chemiluminescent solutions

Lyoluminescence of gamma irradiated and additively coloured NaCl, NaBr, KCl and KBr crystals when dissolved in fluorescent and chemiluminescent aqueous solutions is investigated. Spectral analysis of the emitted light has shown that lyoluminescence spectra are similar to the fluorescence spectra of these solutions. Luminescence takes place when a fluorescent acceptor is directly excited by a liberated F-centre (hydrated electron e^-_aq) or indirectly by an energy transfer from a released excited halide ion ∥X^-∥¹ (solvated hole after recombination with hydrated electron). Oxygen, in general, has a marked quenching effect on luminescence. In chemiluminescent systems, its presence is essential to provoke luminescence in a sequence of events leading to a fluorescent end product.     

Photoleitfähigkeit additiv verfärbtet SrF2-Kristalle

Additively coloured SrF₂ crystals proved to be photoconducting in the region of the coloration bands from 700 to 200 nm, the photocurrents showing some characteristics of photoelectric primary currents. Without applying an external voltage the same photoresponse is measured in the polarizing field of the crystals. Photoelectric excitation (analogous to theF′-centres in crystals of the NaCl-type) could not positively be established. The assignment of the relative spectral photosensitivity to the absorption spectra of the coloured crystals has not yet been possible. In interpreting the photoconduction effects it cannot completely ruled out whether chemical impurities play some part in the photoconduction process.     

Multiplication of anion and cation electronic excitations in luminescent wide-gap ionic crystals

The spectra of intrinsic luminescence excitation by synchrotron radiation (6–32 eV) at 8 K have been analyzed for NaCl, KCl, RbCl, KBr, RbBr, CsBr, MgO, CaO and YalO₃ crystals. In all crystalsv (except MgO and CaO) the process of multiplication of electronic excitations (MEE) causes a sharp increase of the intensity of self-trapped exciton emission, but leads (in KBr and NaCl) to the decrease of intra-band luminescence efficiency. The analysis of the intensity ratio spectra for two components of exciton emission allows us to separate the process of secondary exciton creation by hot photolectrons (NaCl, KBr, YAlO₃). The threshold energies of excitonic and electron-hole mechanisms of MEE are compared for a number of alkali halides.     

Specific features of the temperature quenching of luminescence of self-trapped excitons in alkali halide crystals under low-temperature deformation

The activation energy of temperature quenching of luminescence of self-trapped excitons in alkali halide crystals subjected to low-temperature uniaxial deformation is evaluated experimentally. It is found that an increase in the activation energy is observed in the following series of crystals: KBr → NaCl → KI → NaBr → CsBr → RbI. The effect of enhancement of intrinsic luminescence of alkali halide crystals due to the lowering of the symmetry of the crystal lattice under low-temperature uniaxial deformation is interpreted by analyzing the observed increase in the activation energy that characterizes the height of the potential barrier separating channels of radiative and nonradiative decay (with the formation of radiation defects) of self-trapped excitons.     

Non-radiative ionization of perturbation centres by means of excitons in semi-conducting and dielectric crystals

Non-radiative ionization of a perturbation centre formed by an electron trapped in the Coulomb field of the perturbation by means of excitons of large radius in semi-conducting and dielectric crystals is theoretically investigated. It is shown that such non-radiative ionization may occur by two processes: the direct non-radiative transfer of the excitation energy from the exciton to the electron of the perturbation and by exchange non-radiative ionization whereby the electron on the perturbation recombines non-radiatively with the hole in the exciton and the electron of the exciton with the energy released during this recombination passes to the ionized state of the perturbation.The corresponding probabilities per sec and recombination coefficients are calculated as a function of the concentration of perturbation centres and the type of exciton according to its optical origin. The result is used for a theoretical estimate of the influence of perturbation centres of the given type on the optical life-time of the exciton.