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.     

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.     

Possibility of elastico-mechanoluminescence dosimetry using alkali halides and other crystals

The elastico-mechanoluminescence (EML) intensity of X or γ-irradiated alkali halide crystals can be used in radiation dosimetry. The EML intensity of X or γ-irradiated alkali halide crystals increases linearly with the strain of the crystals, and when the crosshead of the testing machine deforming an X or γ-irradiated crystal is stopped, then the EML intensity decreases with time. The semilog plot of the EML intensity versus (t − t_c) (where t_c is the time where the crosshead of the testing machine is stopped) indicates that, in the post-deformation region, the EML intensity initially decreases exponentially at a fast rate and later on it decreases exponentially at a slow rate. The EML intensity increases linearly with the density of the F-centres in the crystals. This fact indicates that elastico-ML can suitably be used for the radiation dosimetry. The EML spectra of X or γ-irradiated alkali halide crystals are similar to their thermoluminescence spectra. Based on the detrapping of electrons during the mechanical interaction between the dislocation segments and F-centres, an expression is derived, which indicates that the EML intensity should increase linearly with the density of F-centres in the crystals. The expression derived for the decay of EML indicates that the decay time for the fast decrease of EML should gives the pinning time of dislocation segments (lifetime of interacting F-centres), and the decay time for the slow decrease of EML intensity should gives the lifetime of electrons in the shallow traps. As the elastic deformation is non-destructive phenomenon and the EML intensity depends on the radiation dosage given to the alkali halide crystals, similar to the thermoluminescence and photo-stimulated luminescence, the EML of alkali halide crystals and other crystals may be used for the radiation dosimetry. In EML dosimetry, the same crystal can be used number of times because the elastic deformation does not cause permanent deformation in the crystals, and moreover, comparatively the devices needed for the EML measurements are of low cost and very simple. In recent years, a large number of elastico mechanoluminescent materials have been investigated, and the study of their suitability for the radiation dosimetry may be interesting.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Strong mechanoluminescence induced by elastic deformation of rare-earth-doped strontium aluminate phosphors

When rare-earth-doped strontium aluminate phosphor mixed in an epoxy resin, is deformed elastically by applying a uniaxial pressure, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value I_m at a particular time t_m, and later on it decreases with time. After t_m, initially, the ML intensity decreases exponentially at a fast rate and then it decreases exponentially at a slow rate. The ML appears after a threshold pressure and then, initially at low pressure, the peak intensity I_m of ML increases linearly with the magnitude of applied pressure, and for high pressure, I_m increases exponentially with the magnitude of applied pressure. The value of I_m increases linearly with the density of filled hole traps. The ML emission also takes place during the release of applied pressure. There should be a significant effect of temperature on the ML intensity of rare-earth-doped strontium aluminate phosphors. The ML intensity of rare-earth-doped strontium aluminates decreases with successive number of the applications of pressure and the diminished ML intensity can be recovered with the exposure of the samples to UV-radiation. The ML spectra of rare-earth-doped strontium aluminate phosphors are similar to their photoluminescence spectra. As only the piezoelectric-phase of the strontium aluminate phosphors exhibit ML during their elastic deformation, the ML emission can be attributed to the piezoelectrification of the crystals. Considering that the piezoelectric field causes decrease in the trap-depth of the hole traps and, therefore, the holes transferred from traps to the valence band recombine with (Eu¹⁺)^* ions, whereby the Eu²⁺ ions are excited, expressions are derived for different parameters of ML, which are able to satisfactorily explain the experimental results. It is shown that the lifetimes of holes in the shallow traps in stressed and unstressed materials, and the threshold pressure P_t for the ML emission, and other parameters of the ML, can be determined from the ML measurements. Finally, the criteria for tailoring strong elasico-mechanoluminescent materials are explored.     

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.     

Transient behaviour of the mechanoluminescence induced by impulsive deformation of fluorescent and phosphorescent crystals

When a crystal is fractured impulsively by the impact of a moving piston, then initially the mechanoluminescence (ML) intensity increases quadratically with time, attains a peak value and later on it decreases with time. Considering that the solid state ML and gas discharge ML are excited due to the charging and subsequent production of electric field near the tip of moving cracks, expressions are derived for the transient ML intensity I, time t_m and intensity I_m corresponding to the peak of ML intensity versus time curve, respectively, the total ML intensity I_T, and for fast and slow decays of the ML intensity. It is shown that the decay time for the fast decrease of the ML intensity after t_m, is related to the decay time of the strain rate of crystals, and the decay time of slow decay of ML, only observed in phosphorescent crystals, is equal to the decay time of phosphorescence. The value of t_m decreases with the increasing impact velocity, I_m increases with the increasing impact velocity, and I_T initially increases and then it tends to attain a saturation value for higher values of the impact velocity. The values of t_m, I_m and I_T increase linearly with the thickness, area of cross-section and volume of the crystals, respectively. So far as the rise, attainment of ML peak, and fast decay of ML are concerned, there is no any significant difference in the time-evolution of solid state ML, gas discharge ML, and the ML emission consisting of both the solid state ML and gas discharge ML. From the time-dependence of ML, the values of the time-constant for decrease of the surface area created by the movement of a single crack, the time-constant for the decrease of strain rate of crystals, and the decay time of phosphorescence of crystals can be determined. A good agreement is found between the theoretical and experimental results. The importance of fracto ML induced by impulsive deformation of crystals is discussed.     

Mechanoluminescence and thermoluminescence of BaFCl:Sm 2+ and BaFBr:Sm 2+ crystals

The alkaline-earth fluorohalide crystals MFX, where M=Ca, Sr, Ba, Pb and X=Cl, Br, I, form an important class of materials crystallizing in the PbFCl-type tetragonal structure which is also called the matlockite structure. These compounds have long been of interest because of the various defect species which can be detected by spin resonance and associated techniques. The crystals were prepared by slow cooling of the melt of a stoichiometric mixture of BaF ₂ and the corresponding chloride or bromide under 0.2 bar of ultrapure argon (5N5), often slightly fluorinated. We have studied the mechanoluminescence (ML) of BaFBr:Sm ²⁺ and BaFCl:Sm ²⁺ crystals. It is seen that after the impact of a moving piston, initially the ML intensity increases with time, attains a maximum value and then it decreases with time up to a particular minimum value, and then it increases again, attaining a peak value and finally disappears. The first peak lies in the deformation region and the second peak lies in the post-deformation region. The ML intensity of the BaFCl:Sm ²⁺ crystal is much higher than the ML intensity of the BaFBr:Sm ²⁺ crystal. For different impact velocities, the ML intensity increases with velocity; and the total ML intensity attains a saturation value for higher impact velocities. The total ML intensity increases with the increase in the applied load. It is suggested that the moving dislocation produced during deformation of crystals captures holes from hole-trapped centers (like H centers), and the subsequent radiative recombination of the dislocation holes with electron gives rise to ML. Thermoluminescence (TL) of BaFBr:Sm ²⁺ and BaFCl:Sm ²⁺ crystals was studied after exposure to ultraviolet rays with the help of a TLD reader. The peak of TL for the BaFBr:Sm ²⁺ crystal is found at ～247°C and for BaFCl:Sm ²⁺ crystals at 283°C. The TL intensity initially increases with increase in the UV radiation and then it attains saturation for higher values of UV exposure. The absorption spectrum was recorded with the help of a UV–visible spectrophotometer (Shimadzu). The band found at 275 nm was attributed to H centers.     

Fracto-mechanoluminescence and mechanics of fracture of solids

The present paper explores the correlation between fracto-mechanoluminescence and fracture of solids and thereby provides a clear understanding of the physics of fracto-mechanoluminescence. When a fluorescent or non-photoluminescent crystal is fractured impulsively by dropping a load on it, then initially the mechanoluminescence (ML) intensity increases linearly with time, attains a maximum value I_m at a particular time t_m and later on it decreases exponentially with time. However, when a phosphorescent crystal is fractured impulsively by dropping a load on it, then initially the ML intensity increases linearly with time, attains a maximum value I_m at a particular time t_m and later on it decreases initially at a fast rate and then at a slow rate. For low impact velocity the value of t_m is constant, however, for higher impact velocity t_m decreases logarithmally with the increasing impact velocity. Whereas the peak ML intensity I_m increases linearly with the impact velocity, the total ML intensity I_T, initially increases linearly with the impact velocity and then it tends to attain a saturation value for higher values of the impact velocity. The value of t_m increases logarithmally with the thickness of crystals, I_m increases linearly with the area of cross-section of crystals and I_T increases linearly with the volume of crystals. Generally, the ML of non-irradiated crystals decreases with increasing temperature of crystals. Depending on the prevailing conditions the ML spectra consist of either gas discharge spectra or solid state luminescence spectra or combination of the both. On the basis of the rate of generation of cracks and the rate of creation of new surface area of crystals, expressions are derived for the ML intensity and they are found to explain satisfactorily the temporal, spectral, thermal, crystal-size, impact velocity, surface area, and other characteristics of ML. The present investigation may be useful in designing of damage sensors, fracture sensors, ML-based safety management monitoring system, fuse-system for army warheads, milling machine, etc. The present study may be helpful in understanding the processes involved in earthquakes, earthquake lights and mine-failure as they basically involve fracture of solids.     

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.     

Luminescence induced by elastic deformation of ZnS:Mn nanoparticles

When the thin film of ZnS:Mn nanoparticles deposited on a glass substrate is elastically deformed by applying a load, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value I_m at a particular time t_m, and later on it decreases with time. The rise and decay characteristics of the ML produced during release of the load are also similar to those produced during the application of load. Similar rise, occurrence of peak and then decrease in ML intensity are also found, when the film is deformed impulsively by dropping a steel ball of small mass from a low height; however, in this case, the time durations for the occurrence of ML and decay time of ML are very short. In the cases of loading and impulsive deformation ,after t_m, initially the ML intensity decreases at a fast rate and then at a slow rate, in which the decay time of fast decrease is equal to the time-constant for rise of pressure and the decay time for slow decrease is equal to the relaxation time of the surface charges. In the case of loading, the peak intensity I_m and the total intensity I_T of ML increase quadratically with the magnitude of applied pressure; however, in the case of impulsive deformation, both the I_m and I_T increase linearly with the height through which the ball is dropped on to the sample. In the case of deformation of the samples at a fixed strain rate, I_m should increase linearly with the applied pressure. The elastico ML in ZnS:Mn nanoparticles can be understood on the basis of the piezoelectrically-induced electron detrapping model, in which the local piezoelectric field near the Mn²⁺ centres reduces the trap-depth, and therefore, the detrapping of filled electron traps takes place, and subsequently the energy released non-radiatively during the electron-hole recombination excites the Mn²⁺ centres and de-excitation gives rise to the ML. The equal number of photons emitted during the application of pressure, release of pressure, and during the successive applications of pressure, indicates that the detrapped electron-traps get filled during the relaxation of the surface charges induced by the application and release of pressure because the charge carriers move to reduce the surface charges. On the basis of the piezoelectrically-induced electron detrapping model, expressions are derived for different characteristics of the ML of ZnS:Mn nanoparticles and a good agreement is found between the theoretical and experimental results. The expressions explored for the dependence of ML intensity on several parameters may be useful in tailoring the suitable nanomaterials capable of exhibiting ML during their elastic deformation. The values of the relaxation time of surface charges, time-constant for the rise of pressure, and the threshold pressure can be determined from the measurement of the time-dependence of ML. It seems that the trapping and detrapping of charge carriers in materials can be studied using ML.     

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.     

Persistent mechanoluminescence induced by elastic deformation of ZrO2:Ti phosphors

ZrO₂:Ti phosphors show such a strong mechanoluminescence (ML) that it can be seen in day light with naked eye. When a pellet of ZrO₂:Ti phosphor mixed in epoxy resin is deformed in the elastic region at a fixed strain rate using a testing machine, ML intensity increases linearly with time, and when the deformation is stopped, ML intensity decreases exponentially with time. For a given strain rate, ML intensity increases linearly with pressure, and for a given pressure, ML intensity increases linearly with the strain rate. The total ML intensity, in the deformation region, increases quadratically with pressure; however, the total ML intensity in the post-deformation region increases linearly with pressure. ML intensity decreases with successive number of pressings, whereby the reduced ML intensity can be recovered by UV-irradiation of the sample. ML intensity increases linearly with density of filled electron traps and it is optimum for a particular concentration of Ti in ZrO₂. ML intensity should change with increasing temperature of the phosphors. Although ZrO₂ is non-piezoelectric as a whole, it seems that the local structures near the Ti ions in ZrO₂ crystals are in the piezoelectric phase. The elastico ML in ZrO₂ phosphors can be understood on the basis of the localized piezoelectrification-induced detrapping model. According to this model, the localized piezoelectric field near Ti ions causes detrapping of electrons and subsequently the detrapped electrons moving in the conduction band are captured by the energy state of excited Ti⁴⁺ ions, whereby excited Ti⁴⁺ ions are produced and consequently the decay of excited Ti⁴⁺ ions gives rise to the light emission. The expressions derived on the basis of this model are able to explain satisfactorily the characteristics of ML. The relaxation time of localized piezoelectric charges and the threshold pressure for the ML emission can be determined from ML measurements. The long decay of elastico ML indicates the possibility of exploring persistent elastico ML, which may be useful for the fabrication of dim light sources capable of operating without any external power.     

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.     

Mechanoluminescence by impulsive deformation of γ-irradiated Er-doped CaF2 crystals

An impulsive technique has been used for mechanoluminescence (ML) measurements in γ-irradiated Er doped CaF₂ crystals. When the ML is excited impulsively by the impact of moving piston on to γ-irradiated CaF₂:Er crystals, two peaks are observed in ML intensity with time and it is seen that the peak intensities of first and second peaks (I_m1 and I_m2) increase with increasing impact velocity. However the time corresponding to first and second peaks (t_m1 and t_m2) shifts towards shorter time values with increasing impact velocity. It is also seen that the total ML intensity I_Total initially increases with the impact velocity and then it attains a saturation value for higher values of the impact velocity. We have presented a theoretical explanation for the observed results.     

Mechanoluminescence Study of Europium Doped CaZrO<Subscript>3</Subscript> Phosphor

Behaviour displayed by mechanoluminescence (ML) in CaZrO₃:Eu³⁺ doped phosphors with variable concentration of europium ions are described. When the ML is excited impulsively by the impact of a load on the phosphors the ML intensity increases with time, attains a maximum value and then it decreases. In the ML intensity versus time curve, the peak increases and shifts towards shorter time values with increasing impact velocities. Sample was synthesized by combustion synthesis method with variable concentration of Eu³⁺ ions (0.1, 0.2, 0.5, 1, 1.5 mol%) and characterized by X-ray diffraction technique. The total ML intensity I_T is defined as the area below the ML intensity versus time curve. Initially I_T increases with impact velocity V₀ of the load and then it attains a saturation value for higher values of impact velocities which follow the relation I_T = I_T ⁰ exp.(?V_c/V₀) where I_T ⁰ and V_c are constants. Total ML intensity increases linearly with the mass of the phosphors for higher impact velocities. The ML intensity I_m, corresponding to the peak of ML intensity versus time curve increases linearly with the impact velocities. The time t_m, is found to be linearly related to 1000/V₀. The mechanoluminescence induced by impulsive excitation in europium doped CaZrO₃ phosphors plays a significance role in the understanding of biological sensors and display device application.     

Effect of dislocation velocity on the nuclear spin relaxation

Using the Jeener pulse sequence, dislocation motion at various velocities and temperatures in alkali halide single crystals was studied by measuring the resultant variation in the zero-field relaxation time T₁₀, which is known to be influenced by slow atomic movements.It was found that T₁₀ is strongly dependent on the plastic deformation rate g?3 — although not on the plastic strain ε. The theory of Rowland and Fradin for atomic diffusion is used as a basis for an explanation of the results of this new type of experiment.     

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.