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 excitation in alkali halide crystals and colouration decay in microcrystalline powders

The mechanoluminescence (ML) of NaCl, NaBr, NaF, LiCl and LiF crystals ceases at 105, 58, 170, 151 and 175°C respectively. Both the temperatureT _c at whichML disappears and the temperatureT _s required to induce a particular percentage of colouration decay in a given time, decreases with increasing nearest neighbour distance in alkali halide crystals. This perhaps suggests that similar processes cause the disappearance ofml in alkali halide crystals and the colouration decay in their microcrystalline powders. It is shown that mobile dislocations may cause the leakage of surface charge and the decay of colouration in microcrystalline powders.     

A light induced configurational change of FA centres in Li doped KCl-KBr crystals

Abstract

Spectroscopic properties of F_A centres in Li doped KCl-KBr mixed crystals were studied. At low temperature light induced spectral shifts, for the F_A1 hand towards lower energy and for the F_A2 band towards higher energy, were observed. The shifts are proposed to be due to a configurational change where the electron occupied vacancy finds a new location in relation to the neighbouring chlorine and bromine ions. The recovery to the original configuration, obtained in the F → F_A conversion, is a temperature activated process.     

Crystalline properties and colour centres in alkali halide thin films

Abstract

Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage.     

X-ray diffraction and colour centre studies on RbCl-RbBr mixed crystals

X-ray diffraction and colour centre studies have been carried out on RbCl-RbBr mixed crystals. The lattice constant closely follows the linear form of Vegard’s law. The mean Debye-Waller factor shows a highly nonlinear composition dependence. The composition dependence of the F-band peak position is slightly nonlinear but that of the F-band half-width is highly nonlinear. The Ivey-Mollow relation holds for this system with an index of 2.5. The ‘size effect’ is found to have a dominant effect on the F-band width.     

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.     

The fractional ionic character of alkali and silver halide crystals

The fractional ionic character of alkali and silver halide crystals is defined in terms of the deviations from the additivity rule for polarizabilities of ions. The electronic polarizabilities of ions are calculated using an empirical relationship according to which the electronic polarizability of an ion can be assumed to be directly proportional to the cube of its radius. The calculated ionicities indicate that the alkali halides are nearly or more than 90% ionic and silver halides are much less ionic which is also evident from the Phillips ionicity scale.     

Thermally stimulated luminescence origin in LiF crystals irradiated in a reactor at different temperatures

Thermally stimulated luminescence as well as optical absorption and emission spectra have been studied in LiF crystals irradiated in a reactor at different temperatures. It was shown that aggregate colour centres give rise to thermally stimulated luminescence peaks registered below 450°C. Peak at 470°C is observed only in crystals that have been irradiated at standard temperature of the reactor experimental channels. The peak is caused by interaction of dislocations and F centres.     

Pulse-induced mechanoluminescence of ultraviolet-irradiated SrAl2O4:Eu,Dy phosphors

The SrAl₂O₄:Eu,Dy phosphors prepared by solid state reaction technique in a reduced atmosphere of 95% Ar+5% H₂ exhibit very intense mechanoluminescence (ML) which can be seen in daylight with naked eye. When the phosphors are deformed by the impact of a low-power electric hammer, initially the ML intensity increases with time, attains a maximum value and then decreases with time. After the threshold pressure, the peak of ML intensity I_m and the total ML intensity I_T increase with the increasing value of the impact pressure. For the ML excited by the pressure pulse of short duration, two decay times of ML are observed; however, for the ML excited by the pressure pulse of long duration, only one decay time is observed. The ML intensity decreases with successive applications of pressure on SrAl₂O₄:Eu,Dy phosphors. For the low applied pressure in the range below the limit of elasticity recovery of ML intensity takes place when the sample is exposed to ultraviolet (UV) light. This fact indicates that the vacant traps produced during the application of pressure pulses get filled during the exposure of the sample to UV light. The ML in the elastic region of SrAl₂O₄:Eu,Dy phosphors can be understood on the basis of the piezoelectrically induced detrapping model. The non-irradiated SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors exhibit ML during the fracture of the compact mass of phosphors whose ML intensity is less when compared to that of the UV-irradiated compact masses. The ML induced by pressure pulses may be useful for determining the magnitude and rise time of unknown pressure pulses and to determine the lifetime of charge carriers in shallow traps.     

X-ray determination of mean Debye-Waller factors and Debye temperatures of the K x Rb(1−x)Br mixed crystal system

The integrated intensities of Bragg reflections have been measured for mixed crystals in the K _x Rb_(1−x)Br system with an x-ray powder diffractometer. From the intensities, the mean Debye-Waller factors are determined. The Debye-Waller factors are corrected for static contribution and Debye temperature values are determined for the entire composition range. The x-ray Debye temperatures follow the Kopp-Neumann equation closely.     

Field emission theory of dislocation-sensitized photo-stimulated exo-electron emission from coloured alkali halide crystals

A new field emission theory of dislocation-sensitized photo-stimulated exo-electron emission (DSPEE) is proposed, which shows that the increase in the intensity of photo emission fromF-centres during plastic deformation is caused by the appearance of an electric field which draws excited electrons out of the deeper layer and, therefore, increases the number of electrons which reach the surface. The theory of DSPEE shows that the variation of DSPEE flux intensity should obey the following relation

Dislocation unpinning model of acoustic emission from alkali halide crystals

The present paper reports the dislocation unpinning model of acoustic emission (AE) from alkali halide crystals. Equations are derived for the strain dependence of the transient AE pulse rate, peak value of the AE pulse rate and the total number of AE pulse emitted. It is found that the AE pulse rate should be maximum for a particular strain of the crystals. The peak value of the AE pulse rate should depend on the volume and strain rate of the crystals, and also on the pinning time of dislocations. Since the pinning time of dislocations decreases with increasing strain rate, the AE pulse rate should be weakly dependent on the strain rate of the crystals. The total number of AE should increase linearly with deformation and then it should attain a saturation value for the large deformation. By measuring the strain dependence of the AE pulse rate at a fixed strain rate, the time constantτ _s for surface annihilation of dislocations and the pinning timeτ _p of the dislocations can be determined. A good agreement is found between the theoretical and experimental results related to the AE from alkali halide crystals.     

Characteristics of the fast electron emission produced during the cleavage of crystals

The present paper reports the fast electron emission produced during the cleavage of alkali halide crystals and models the dynamics of the process. The mechano-emission arises as a result of the ionization of surface traps at the expense of the energy which is released in the annihilation of the defects which are formed during cleavage. The slow electrons which appear upon the ionization of surface traps are subsequently accelerated in the field of negatively charged segment of the freshly cleaved surface. Considering the basic mechanism of fast electron emission, expressions are derived which are able to explain satisfactorily the temporal, thermal, charge, surface, coloration, water adsorption and other characteristics of the fast electron emission produced during the cleavage of crystals. The decay time of the charges on the newly created surfaces, and the velocity of cracks can be determined from the measurements of fast electron emission produced during the cleavage of crystals. It is shown that two types of diffusing centres are responsible for the charge relaxation and thereby for the emission of fast electrons produced during the cleavage of alkali halide crystals.     

F bands in KCl-KBr mixed crystals

Gamma ray-induced F bands in melt grown mixed crystals of KCl and KBr are studied at room temperature. The effect of dislocations on F-centre density in mixed crystals has been discussed. In addition, the influence of composition of mixed crystals on F band parameters has been studied. The behaviour of observed F bands is correlated to local strained regions and to the configuration of F centre in mixed crystals.     

Refractive index changes due to F → M conversion in KBr single crystals and their applications in holography

Refractive index changes due to the F → M colour centre conversion in a KBr crystal are given as a function of angular frequency and temperature. The applicability of the results obtained for holographic recording is discussed.     

High-pressure spectroscopic studies of free and self-trapped excitons in alkali halides at low temperatures

Abstract

A brief survey of our studies of free and self-trapped excitons (FE and STE) in alkali halide crystals under hydrostatic pressure up to 12.5 kbar at 4.2–140 K is presented. Main attention is paid to the following effects observed: (1) the strong coupling of three energy levels of FE in CsI revealing itself as an exciton analog of pressure-scanned Fermi resonance; (2) emergence of a new emission band of STE in CsI under pressure; (3) a large pressure shift of the thermal quenching curve for STE emission in NaCl.