Alkali Halide Crystals Research Articles

The Effect of Radiation “Memory” in Alkali-Halide Crystals

The exposure of the alkali-halide crystals to ionizing radiation leads to the destruction of their structure, the emergence of radiation defects, and the formation of the electron and hole color centers. Destruction of the color centers upon heating is accompanied by the crystal bleaching, luminescence, and radio-frequency electromagnetic emission (REME). After complete thermal bleaching of the crystal, radiation defects are not completely annealed, as the electrons and holes released from the color centers by heating leave charged and locally uncompensated defects. Clusters of these “pre centers” lead to electric microheterogeneity of the crystal, the formation of a quasi-electret state, and the emergence of micro-discharges accompanied by radio emission. The generation of REME associated with residual defectiveness, is a manifestation of the effect of radiation “memory” in dielectrics.

Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9.

We report a study on the optical properties of the layered polymorph of vacancy-ordered triple perovskite Cs3Bi2Br9. The electronic structure, determined from density functional theory calculations, shows the top of the valence band and bottom of the conduction band minima are, unusually, dominated by Bi s and p states, respectively. This produces a sharp exciton peak in the absorption spectra with a binding energy that was approximated to be 940 meV, which is substantially stronger than values found in other halide perovskites and, instead, more closely reflects values seen in alkali halide crystals. This large binding energy is indicative of a strongly localized character and results in a highly structured emission at room temperature as the exciton couples to vibrations in the lattice.

A CsI low-temperature detector for dark matter search

Cryogenic detectors have a long history of success in the field of rare event searches. In particular scintillating calorimeters are very suitable detectors for this task since two signals are induced by a particle interaction in a scintillating crystal. The thermal signal provides a precise measurement of the deposited energy while the simultaneously measured scintillation light signal yields particle discrimination as the amount of produced scintillation light depends on the nature of the interacting particle. We investigate the calorimetric properties and background rejection capabilities of two large CsI (undoped) crystals (∼122 g each) operated as scintillating calorimeters at milli-Kelvin temperatures. Furthermore, we discuss the feasibility of this detection approach towards a future background-free dark matter experiment based on alkali halide crystals, with active particle discrimination via the two-channel detection.

Theoretical analysis of the kinetics of low-temperature defect recombination in alkali halide crystals

We analyzed carefully the experimental kinetics of the low-temperature diffusion-controlled F, H center recombination in a series of irradiated alkali halides and extracted the migration energies and pre-exponential parameters for the hole H centers. The migration energy for the complementary electronic F centers in NaCl was obtained from the colloid formation kinetics observed above room temperature. The obtained parameters were compared with data available from the literature.

Graphitic cage transformation by electron-beam-induced catalysis with alkali-halide nanocrystals

We found that alkali-halide nanocrystals, such as KCl and NaCl, have strong catalytic capability to form graphitic carbon cages from amorphous carbon shells under electron beam irradiation. In addition to the electron beam irradiation strongly inducing the decomposition of alkali-halide nanocrystals, graphene fragments were formed and linked together to form the final product of thin graphitic carbon cages after the evaporation of alkali-halide nanocrystals. The required electron dose was approximately 1 to 20 C/cm2 at 120 keV at room temperature, which was about two orders of magnitude smaller than that required for conventional beam-induced graphitization. The “knock-on” effect of primary electrons strongly induced the decomposition of the alkali-halide crystal inside the amorphous carbon shell. However, the strong ionic cohesion quickly reformed the crystal into thin layers inside the amorphous shell. The bond excitation induced by the electron beam irradiation seemed to enhance strongly the graphitization at the interface between the outer amorphous carbon shell and the inner alkali-halide crystal.

Decay kinetics of high- and low-energy emission in the A band of KCl:Tl

Within a revival of interest in the emission properties of Tl+-like impurity centers in alkali-halide crystals, the problem of a complete interpretation of high-energy and low-energy emissions in the A-band of KCl:Tl remains without a satisfactory explanation. Two previous works reported experimental results relative to spectral and decay-time measurements for low-energy (weak) emission at 475nm, and for high-energy emission at 300nm. In the latter case, however, a confirmation of the existence of an anomalous type of behavior in the slow decay time needed to be clarified. The results reported here demonstrate that this anomaly is not present in high-energy emission. On the basis of these more recent experimental results, an attempt is made to obtain a unifying model, one always based on the Jahn–Teller effect, which could serve in interpreting both high- and low-energy emissions.

Growth and Characterization of Single Crystals of Alkali-Halides

Growth and Characterization of Single Crystals of Alkali-Halides

Ab initioperspective on the Mollwo-Ivey relation forFcenters in alkali halides

The authors revisited the Mollwo--Ivey relation of $F$ centers in alkali halide crystals, a prototypical color center, based on post-density-functional-theory and post-Hartree-Fock methods. In contrast to earlier interpretations, which stress the importance of the Madelung potential, they find ion-size effects to be the predominant mechanism. These determine the shape of the defect-electron wave function and are responsible for the fractional Mollwo--Ivey exponent of 1.8.

Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations.

We present a study of the electronic and optical properties of a series of alkali halide crystals AX, with A = Li, Na, K, Rb and X = F, Cl, Br based on a recent implementation of hybrid-exchange time-dependent density functional theory (TD-DFT) (TD-B3LYP) in the all-electron Gaussian basis set code CRYSTAL. We examine, in particular, the impact of basis set size and quality on the prediction of the optical gap and exciton binding energy. The formation of bound excitons by photoexcitation is observed in all the studied systems and this is shown to be correlated to specific features of the Hartree-Fock exchange component of the TD-DFT response kernel. All computed optical gaps and exciton binding energies are however markedly below estimated experimental and, where available, 2-particle Green's function (GW-Bethe-Salpeter equation, GW-BSE) values. We attribute this reduced exciton binding to the incorrect asymptotics of the B3LYP exchange correlation ground state functional and of the TD-B3LYP response kernel, which lead to a large underestimation of the Coulomb interaction between the excited electron and hole wavefunctions. Considering LiF as an example, we correlate the asymptotic behaviour of the TD-B3LYP kernel to the fraction of Fock exchange admixed in the ground state functional cHF and show that there exists one value of cHF (∼0.32) that reproduces at least semi-quantitatively the optical gap of this material.

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 − tc) (where tc 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.

Discrete breathers in 2D and 3D crystals

Discrete breathers (DB) are spatially localized, large‐amplitude vibrational modes in defect‐free nonlinear lattices. Recent numerical and experimental studies have confirmed that DB exist in crystals. In the present work, we briefly describe the well‐known existence conditions of DB in crystals and present our recent results on DB in 2D crystals such as graphene and graphane and in 3D crystals such as alkali halide crystals and pure metals. The possible role of DB in solid state physics and materials science is discussed.Stroboscopic picture of atomic motions in the vicinity of discrete breather excited in graphene homogeneously strained with ϵxx=0.3, ϵyy=−0.1. Displacements of atoms from the equilibrium lattice positions are multiplied by the factor 4 for clarity. The discrete breather is of high spatial localization, so that only two atoms oscillate with a large amplitude.

Influence of an initial chemical state of bivalent cation impurities on their diffusion activity in alkali-halide crystals

Influence of an initial chemical state of bivalent cation impurities on their diffusion activity in alkali-halide crystals is investigated. The effect of an intensive electron beam on diffusion of magnesium impurity in crystals of fluoride of lithium is studied.

Nanodefect substructures in crystal phosphors

We investigated spectral and kinetic characteristics of the luminescence in alkali halide crystals with polyvalent impurities, crystal phosphors based on metal tungstates in the form of nano-, micro- and bulk crystals. The possibility of the existence of complex defects (nanodefects) in materials containing high defect concentration is shown.

Application of secondary ion mass spectrometer for measuring the diffusion profiles in alkali-halide crystals

Depth profiles of magnesium, fluorine and oxygen impurities was examined in the surface layers of alkali-halide KBr crystals using method of secondary ion mass spectrometry. Samples of potassium bromide, coated with a surface film of magnesium fluoride were subjected to isothermal diffusion annealing in air at various times. It is shown that the diffusion of O ions occurs from the ambient atmosphere besides the diffusion of Mg and F ions during annealing of KBr crystals. Accurate estimation of the diffusion coefficients of cationic impurity Mg requires taking into account the possible interaction of this impurity and oxygen.

Epifluorescence microscopy: a sensitive tool for studying the morphology and oriented growth of europium precipitates in KI single-crystal hosts.

The morphology and oriented growth of europium precipitates in well-annealed Eu²⁺-doped KI single crystals are investigated by epifluorescence microscopy using the proper doping ions as fluorochromes. To make this, electronic spatial reconstructions of some fields of precipitates and of some individual precipitates were built from epifluorescence microscope images of different optical cross-sections of these objects. The building procedures are carefully explained. Previously, the KI:Eu²⁺ system was characterized by fluorescence spectrophotometry and the KI-host long-range translational order was tested by single-plate X-ray diffraction. Precipitates are shaped as plates, with their broad faces being parallel to host lattice planes of either {100}- or {110}-forms (the {100}- or {110}-plates, respectively) and as rods lying along host lattice <100>-directions. The {100}-plates have rhomboidal broad faces with a side lying along a <100>-direction, an internal angle of about 45°, as measured on the corresponding {100}-plane, and, consequently, another side (the {100}<110>-side) lying along a <110> direction on this plane. The {110}-plates have rectangular broad faces with a side lying along a <100>-direction and with another side (the {110}<110>-side) lying along a <110>-direction on the corresponding {110}-plane. Spatial reconstructions of a typical precipitate field, a typical {100}-plate, a typical {110}-plate and a typical rod are described in detail. Precipitates were measured in their different dimensions and the measuring procedures are explained. The plate thicknesses and rod diameters are into a common narrow range of values (0.5-0.2 μm) which contains also the inferior limits of the obtained length ranges for the {100}<110>- and {110}<110>-sides (5.1-0.3 and 4.9-0.3 μm, respectively). It is discussed that that three different europium precipitation states are responsible for the studied precipitation and that plates grew from rods during annealing.

Elastoplastic Invariant Relation for Deformation of Alkali-Halide Crystals

Plastic strain localization patterns in compression-strained alkali halide (NaCl, KCl, and LiF) crystals have been studied using a double-exposure speckle photography technique. The main parameters of strain localization autowaves at the linear stages of deformation hardening in alkali halide crystals have been determined. A quantitative relationship between the macroscopic parameters of plastic flow localization and microscopic parameters of strained alkali halide crystals has been established.

Mechanoluminescence of Coloured Alkali Halide Crystals

The present paper reports both the experimental and mathematical aspects of elastico-mechanoluminescence (EML), plastico-mechanoluminescence (PML) and fracto-mechanoluminescence (FML) of coloured alkali halide crystals in detail, and thereby provides a deep understanding of the related phenomena. The additively coloured alkali halide crystals do not show ML during their elastic and plastic deformation. The ML emission during the elastic deformation takes place due to the mechanical interaction between bending dislocation segments and F-centres, and the ML emission during plastic deformation takes place due to the mechanical interaction between the moving dislocations and F-centres. The ML emission during fracture is also caused by the mechanical interaction between the moving dislocations and F-centres; however, in certain hard crystals like LiF, NaCl, NaF, etc., fracto ML also occurs due to the gas discharge caused by the creation of oppositely charged walls of cracks. The EML, PML, and solid state FML spectra of coloured alkali halide crystals are similar to their thermoluminescence spectra and afterglow spectra. However, the fracto ML spectra of certain hard crystals like LiF, NaCl, NaF, etc., also contain gas discharge spectra. The solid state ML spectra of coloured alkali halide crystals can be assigned to deformation-induced excitation of halide ions inV2-centres or in other hole-centres. Whereas, the intensity of EML and FML increases linearly with the applied pressure and the impact velocity, the intensity of PML increases quardratically with the applied pressure and the impact velocity because of the plastic flow of the crystals. Both Imand ITincrease with the density of F-centres in the crystals and strain rate of the crystals; however, they are optimum for a particular temperature of the crystals. The ML of diminished intensity also appears during the release of applied pressure. Expressions are derived for the elastico ML, plastico ML and fracto ML of coloured alkali halide crystals, in which a good agreement is found between the experimental and theoretical results. Many parameters of crystals such as band gap between the dislocation band and interacting F-centre energy level, radius of interaction between dislocations and F-centres, pinning time of dislocations, work hardening exponent, velocity of cracks, rise time of applied pressure, lifetime of electrons in the dislocation band, lifetime of electrons in shallow traps, diffusion time of holes, critical velocity of impact, etc., can be determined from the ML measurements. The ML of coloured alkali halide crystals has potential for self-indicating method of monitoring the microscopic and macroscopic processes; mechanoluminescence dosimetry; understanding dislocation bands in crystals; interaction between the dislocations and F-centres; dynamics of dislocations; deformation bleaching of coloration, etc. The ML of coloured alkali halide crystals has also the potential for photography, ML memory, and it gives information about slip planes, compression of crystals, fragmentation of crystals, etc.Contents of Paper

A new polarizable force field for alkali and halide ions.

We developed transferable potentials for alkali and halide ions which are consistent with our recent model of water [P. T. Kiss and A. Baranyai, J. Chem. Phys. 138, 204507 (2013)]. Following the approach used for the water potential, we applied Gaussian charge distributions, exponential repulsion, and r(-6) attraction. One of the two charges of the ions is fixed to the center of the particle, while the other is connected to this charge by a harmonic spring to express polarization. Polarizability is taken from quantum chemical calculations. The repulsion between different species is expressed by the combining rule of Kong [J. Chem. Phys. 59, 2464 (1972)]. Our primary target was the hydration free energy of ions which is correct within the error of calculations. We calculated water-ion clusters up to 6 water molecules, and, as a crosscheck, we determined the density and internal energy of alkali-halide crystals at ambient conditions with acceptable accuracy. The structure of hydrated ions was also discussed.

Structured AT-emission of KCl:Tl

The experimental evidence of an anomalous behavior in the decay times of Tl+-like impurity centers in alkali-halide crystals has generated a revival of interest with respect to these materials. We discuss here about the possibility that the AT emission of KCl:Tl can be considered as a double emission, namely as a superposition of the AT and AX emission bands. The plausibility of this hypothesis is discussed in the light of the available, old and recent, experimental features. A quantitative test of this assumption is performed in the framework of a model which demonstrates to be suitable for interpreting other similar cases.

Measurement of the index of refraction of μm crystals by a confocal laser microscope--potential application for the refractive index mapping of μm scale.

A conventional laser microscope can be used to derive the index of refractivity by the ratio of geometrical height of the transparent platelet to the apparent height of the normal incident light for very small crystals in the wide size range. We demonstrate that the simple method is effective for the samples from 100 μm to 16 μm in size using alkali halide crystals as a model system. The method is also applied for the surface fractured micro-crystals and an inclined crystal with microscopic size regime. Furthermore, we present two-dimensional refractive index mapping as well as two-dimensional height profile for the mixture of three alkali halides, KCl, KI, and NaCl, all are μm in size.