B. P. Chandra

Cleavage Mechanoluminescence in Crystals

When a crystal is cleaved, initially the mechanoluminescence (ML) intensity increases linearly with time, attains an optimum-value I m at a particular value of time t m , and then decays exponentially with time. Cleavage ML provides a new tool to determine the velocity, v of cracks in crystals, and it may be given by v = H/t m , where H is the thickness of the crystal. Both, the peak ML intensity I m and total ML intensity I T increase linearly with the area of newly created surfaces A as well as with the surface charge density γ. The ML intensity decreases with temperature primarily due to the decrease in the surface charge density. Beyond a particular temperature, the surface charge density may decrease to such a value where the breakdown of gases and solids may not be possible and thereby the ML may not appear. Depending on the prevailing conditions either the ML emission resembling gas discharge or other types of the luminescence of solids, or that having these two characters may be otained. There exists a good correlation between the theoretical and experimental results obtained for cleavage ML in crystals.

Tetrahedral manganese (II) complexes—intense and unique type of mechanoluminophores

Intense and unique type of mechanoluminescence (ML) is found in tetrahedral manganese (II) complexes. During the excitation of ML by the impact of a piston onto the crystal, the ML intensity initially increases with time, attains a maximum value and then decreases. After retardation of the piston, the decay rate of ML is faster during crystal deformation; however, its value decreases after cessation of the deformation and becomes equal to the decay rate of phosphorescence. The ML disappears below the melting point. Since the crystals of tetrahedral manganese (II) complexes are centrosymmetric, the local non-centrosymmetric sites near the defects are attributed to be responsible for the mechanoluminescence excitation.

Fracto-mechanoluminescence induced by impulsive deformation of II-VI semiconductors: FML induced by implusive deformation of II-VI semicondutor

When II-VI semiconductors are fractured, initially the mechanoluminescence (ML) intensity increases with time, attains a maximum value Im at a time tm, at which the fracture is completed. After tm, the ML intensity decreases with time, Im increase linearly with the impact velocity v0 and IT initially increase linearly with v0 and then it attains a saturation value for a higher value of v0. For photoluminescence, the temperature dependence comes mainly from luminescence efficiency, ηo; however, for the ML excitation, there is an additional factor, rt dependent on temperature. During fracture, charged dislocations moving near the tip of moving cracks produce intense electric field, causes band bending. Consequently, tunneling of electrons from filled electron traps to the conduction band takes place, whereby the radiative electron-hole recombination give rise to the luminescence. In the proposed mechanism, expressions are derived for the rise, the time tm corresponding to the ML intensity versus time curve, the ML intensity Im corresponding to the peak of ML intensity versus time curve, the total fracto-mechanoluminescence (FML) intensity IT, and fast and slow decay of FML intensity of II-VI semiconductors. The FML plays a significant role in understanding the processes involved in biological detection, earthquake lights and mine failure.

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.

A search for mechanoluminophors capable of pressure-induced thermal population of excited states

During the process of deforming a crystal, a high pressure is developed near the tip of mobile cracks, which may in turn produce a new ground state by thermal electron transfer. Upon sudden release of pressure, the electron can either relax to one atmosphere ground state or remain in the excited state potential well long enough to relax to one atmosphere and radiatively transfer back to the ground state. For analysing the pressure induced thermal population of the excited state, the mechanoluminescence(ML) and high pressure photoluminescence(PL) of several organic and inorganic crystals were measured. The study indicated that usual pressure coefficient of energy shift of the order of 50–100 cm−1/kbar and the stress at the crack-tip of the order of 5–10 kbar, are not sufficient to cause the thermal population of the excited state. If by any means the product of pressure coefficient and stress at the mobile crack-tip can be increased by 50 to 100 times, then the thermal population of the excited states may take place. Using the pressure coefficient of energy shift and the difference in ML and PL spectra, and using independently the change in relative intensities of the vibronic peaks, the pressure at the emitting mechanoluminescent crystal sites is evaluated and it is found to be of the order of several kbar which varies from crystal to crystal.

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 temperatureTc at whichML disappears and the temperatureTs 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.

Fracto‐mechanoluminescence induced by impulsive deformation of II–VI semiconductors

When II–VI semiconductors are fractured, initially the mechanoluminescence (ML) intensity increases with time, attains a maximum value Im at a time tm, at which the fracture is completed. After tm, the ML intensity decreases with time, Im increase linearly with the impact velocity v0 and IT initially increase linearly with v0 and then it attains a saturation value for a higher value of v0. For photoluminescence, the temperature dependence comes mainly from luminescence efficiency, ηo; however, for the ML excitation, there is an additional factor, rt dependent on temperature. During fracture, charged dislocations moving near the tip of moving cracks produce intense electric field, causes band bending. Consequently, tunneling of electrons from filled electron traps to the conduction band takes place, whereby the radiative electron–hole recombination give rise to the luminescence. In the proposed mechanism, expressions are derived for the rise, the time tm corresponding to the ML intensity versus time curve, the ML intensity Im corresponding to the peak of ML intensity versus time curve, the total fracto-mechanoluminescence (FML) intensity IT, and fast and slow decay of FML intensity of II–VI semiconductors. The FML plays a significant role in understanding the processes involved in biological detection, earthquake lights and mine failure. Copyright