Alkali Halide Crystals Research Articles

ЖЕГИЧ-ГАЛОИДДИК КРИСТАЛЛДАРДАГЫ БАШТАПКЫ НУРЛАНУУДА ПАЙДА БОЛГОН ДЕФЕКТИЛЕРДИН КАСИЕТТЕРИ ЖӨНҮНДӨ НЕГИЗГИ ИДЕЯЛАР

The impact of ionizing radiation on solids leads to the appearance of new structural defects in them and the transformation of defects that existed before irradiation. The creation of new defects in the structure of crystals by radiation manifests itself after the end of irradiation in a change in many physical and chemical properties. Questions related to the elucidation of the nature of radiation defects and the mechanisms of their transformation in various materials occupy one of the central places in the radiation physics of solids. This paper expresses modern ideas about the basic concepts of radiological defects of alkali-halide crystals and the mechanisms of defects under various active exposures. Any deviation from the periodic structure of the crystal is called a defect. The latter is usually divided into articles and expanded. Point defects are distinguished by the fact that the distortions of the crystal lattice are concentrated around the node, i.e. localized at the interatomic order distance.

Investigation of alkali halide crystals AX (A = Li, Na, K; X = F, Cl, Br) by elastic, mechanical and ultrasonic analysis

Abstract The ultrasonic attenuation studies of alkali halides single crystal AX (A = Li, Na, K and X = F, Cl, Br) have been investigated along <100>, <110> and <111> directions at room temperature. For this investigation, a most appropriate potential model for studying physical studies of rock-salt type alkali halides, the Coulomb and Born–Mayer model has been used to evaluate higher order elastic constants. The second order elastic constants are used to compute the mechanical properties and ultrasonic velocities of the chosen materials. Additionally, the thermal properties have also been evaluated along different orientations. Thermoelastic loss is very small in comparison to the attenuation due to phonon-phonon interaction mechanism at room temperature. The achieved results of AX have been discussed and compared with available experimental and theoretical results.

Improved epitaxy of CsI(Tl) film on Si substrate buffered by graphene for X-ray detection

The growth of the CsI(Tl) scintillators deposited on glass substrates with proper wavelength matching the mainstream semiconductor circuits is crucial for X-ray imaging industry. But the coupling loss between the scintillators and silicon based photo sensors has limited the performance. So fabricating CsI(Tl) films on monocrystalline silicon is greatly important for next-generation high performance X-ray image devices. However, the lattice and thermal mismatch between CsI(Tl) and Si always deteriorates the morphology and structure, and thus degrades the property of the CsI(Tl) scintillation converters. Here, by utilizing crystalline graphene(Gr) as a buffer layer, the van der Waals epitaxial growth of CsI(Tl) film on Si(111) substrate is reported and its advanced applications are demonstrated. The introduced graphene buffer layer extremely frees the stress between CsI(Tl) and Si substrate. Therefore, the CsI(Tl) films show perfect micro-columnar and crystalline structures, and then, exhibit much higher light output and signal to noise ratio, and better modulation transfer functions and a bar pattern X-ray image compared to that on bare Si(111) substrate. This facile method not only lays the foundation for the epitaxy of the alkali halide crystals, including CsI(Tl), onto Si-based detectors and its applications in radiation detection, but also may broaden a new perspective of deposition of CsI(Tl) scintillators into flexible substrates.

Effect of the Anticaking Agent FeCN on the Creeping Properties of Alkali Halide Crystals

Fe(CN)64– (abbreviated as FeCN) is a well-known anticaking agent for NaCl that has a nearly perfect geometrical fit to the (001) surface. In this work, the investigation of the action of FeCN on nucleation and creeping is extended to a wide range of alkali halides of both the NaCl and CsCl structures. It was found that FeCN is a powerful nucleation inhibitor and enhances the creeping of alkali halides when FeCN geometrically fits well in the octahedral surface site of hosts with the NaCl crystal structure. The tested alkali halides that do not fall in this group or that have the CsCl structure show some reduction in creeping upon the addition of FeCN, while NaF and LiCl did not show a difference. For this category of crystals, there is a poor fit of the FeCN ion in the crystal surface. This study shows that a strong adsorption strength of FeCN enhances creeping, whereas a weak interaction reduces the creeping of alkali halide crystals.

Modeling the surface stress and solid–liquid interface energy of alkali halide crystals

In this paper, the surface stress f(T) and solid–liquid interface energy γsl(T) of alkali halides are re-examined by introducing a measurable physical quantity bulk evaporation entropy Sb instead of vibration entropy Svib. The predictions show agreement with previous theoretical work and available experimental results. In addition, the temperature effect is considered. We found that both f(T) and γsl(T) functions decrease following the drop of temperature T, and f(T) exhibits a larger reduction than γsl(T). We also found that f is proportional to melting enthalpy Hm and inversely proportional to the atomic diameter h and compressibility κ. All these results will help us to effectively estimate the values of f and γsl and understand the surface phenomenon.

The features of spectroscopic properties of alkali halide crystals exposed tolow-temperature uniaxial elastic deformation

The features of spectroscopic properties of alkali halide crystals exposed tolow-temperature uniaxial elastic deformation

Elastic-plastic invariant of autowave plasticity

It is shown in the work that plastic flow in solids develops always in localized manner. A macroscopic scale ~ 10-2 m characterizes the localization. The localized flow zones form the pattern of localized strain, which is the projection of the autowave processes of plastic flow, developing in the volume, on the observed surface of the tested specimen. One can observe the pattern with the help of speckle-photography method. The investigations of various materials allowed to establish that the pattern of localized deformation is the information source for a kinetics of deformation processes. A general characteristic of localized plastic flow in solids is the elastic-plastic invariant of deformation which couples the typical characteristics of localized plastic flow autowaves with the same for the elastic waves in crystal lattice. The quantity of the invariant ratio is defined for nearly forty various materials (BCC, FCC, HCP metals and alloys, alkali-halide crystals, ceramics, rocks) studied in the conditions of active elongation and compression at the temperature range 143-420 K. The physical considerations are presented to explain the invariant origination and its relation to other physical characteristics of crystal lattice, in particular, the Debye temperature. In the light of these considerations, it is possible to explain the meaning and the origin of the invariant, and to derive numerous consequences from them. In fact, the set of these consequences comprehend all the regularities of developed plastic flow processes and allows to consider the elastic-plastic invariant of deformation as the master equation for developing this day autowave approach to physical theory of plastic deformation.

Recording of carbon clusters and (–СН2–)n radicals in a vacuum at the site of irradiation (in situ) of polyethylene

For the first time, absorption bands of C atoms and carbon clusters C2, C3, C4, C5, and C6 were found in irradiated polyethylene. The relationship between the type and mass of –CH2–, –C2H4–, –C3H6–, –C4H8–, – C5H10–, – C6H12 – free radicals and clusters of C, C2, C3, C4, C5, and C6 with their ionization energy in electron-irradiated polyethylene has been established. The results are based on the measured transmission spectra of the films and calculated values of the absorption bands after each dose of electron exposure in a vacuum at the site of irradiation (in situ). A linear dependence of the energy position of absorption bands on the type and mass of free radicals and clusters has been obtained. By the areas of the absorption bands of free radicals and carbon clusters the contribution of each defect to their total concentration has been determined depending on the absorbed dose in the range of 104–107Gy. The established dependences correlate with the known regularities in the formation of colloidal and color centers during irradiation of alkali-halide crystals, photographic materials, and oxide compounds.

A digital technology for scanning the luminescent characteristics of alkali halide crystals

A digital technology for scanning the X-ray luminescence, tunnel luminescence, and thermostimulated luminescence spectra in the spectral range from 200 nm to 850 nm is experimentally demonstrated using the example of a NaCl crystal, which is part of the group of alkali halide crystals. This is done with the specified spectrum intervals and scanning speed using a high-aperture monochromator MSD-2 and a photoelectronic multiplier type H 8259 of the company “Hamamatsu”, operating in the photon counting mode, controlled by special programs SpectraScan and ThermoScan. The obtained experimental data are digitally converted for further processing of the graphical representation of the results using the OriginPro software package, which allows you to draw up graphical dependencies of the spectra and perform their mathematical processing.

Probing lattice vibration of alkali halide crystals by broadband terahertz spectroscopy

Terahertz spectral features of alkali halide crystals were studied with the combination of broadband terahertz time-domain spectroscopy and the solid-state-based density functional theory calculations. To understand the particular modes of the observed terahertz features of the alkali halide crystals, the resonant modes of KCl and CsCl were analyzed using face-centered cubic and body-centered cubic lattice models, respectively. The results show that the characteristic terahertz absorption peaks could be assigned to the lattice vibration of the ionic crystals. Furthermore, the terahertz responses of a series of alkali halides were recorded, and obvious absorption peaks were observed in each salt in the frequency region below 8.5 THz. What is more interestingly is that the frequencies of these observed peaks are red-shifted with the increases of the mass and radius of the ions. This correlation between the resonant frequency of the lattice vibration, the reduced atomic mass, and the equilibrium distance between the ions agrees well with the harmonic oscillator model.

Efficiency of H center stabilization in alkali halide crystals at low-temperature uniaxial deformation

The efficiency of stabilization of H centers as well as its dependence on the degree of uniaxial deformation are considered within the framework of the modified geometric model of alkali halides. It is shown that stabilization of H centers is difficult in KI and RbI crystals, while in other NaCl-type crystals it becomes quite probable. Under uniaxial deformation, the interstitial space, in which the defect will be located, decreases, and the efficiency decrease. In the case of cesium halides, the orientation of the H centers takes place predominantly in the <100> direction; therefore, the criteria for their stabilization differ from the NaCl-type alkali halide crystals. According to calculations, in CsI, large displacements of the nearest neighbors are required to stabilize H centers; while in CsBr their stabilization becomes sensitive to the effect of uniaxial deformation. The efficiency of H center stabilization obtained by the geometric model is compared with the corresponding efficiencies obtained from the theory of local excitations and the calculated total probability of accumulation of H centers under the influence of uniaxial deformation.

On the Transparency of Alkali-Halide Crystal in the Terahertz Spectral Range

The transmission of THz (to 3000 μm) radiation by alkali halide single crystals of sodium chloride (NaCl), potassium chloride (KCl), potassium bromide (KBr), and rubidium iodide (RbI), which are widely used in the IR spectral region, is studied. The spectral dependences of the absorption coefficients of these materials in the range of 0.9–3000 μm are determined. These materials are transparent in the range of 1000‒3000 μm, which makes it possible to use them in terahertz (millimeter) devices.

Fabrication of metal-dielectric nanocomposites using a table-top ion implanter

A miniature low-voltage vacuum spark is an effective facility for creating composite layers in a dielectric matrix, including metal clusters, which have numerous optical applications. The results of studies of the optical characteristics of alkali-halide crystals (LiF and KCl) irradiated with beams of accelerated metal ions, which are emitted by a small-sized, low-voltage (2.5 kV) and low-energy (10 J) vacuum spark and a high-voltage (50 kV) pulsed ion implanter are presented. In both cases, composite layers of metal nanoparticles and/or molecular clusters are formed at submicrometer depth when irradiating these crystals. The low voltage implant method produces similar optical effects as seen in crystals processes at much higher energies. This result shows the possibility of creating a desktop metal ion implanter for various applications.

Effect of Gamma Irradiation on Structural and Optical Properties of Zirconium Incorporated Sodium Borate Glasses

ABSTRACT The effects of gamma irradiation in the dose rate of 25 kGy on the structural and optical properties of ZrO2 doped sodium borate glasses were studied. Glassy nature confirmed by XRD and all the physical parameters were calculated. IR spectra revealed the characteristic vibrational and stretching bands due to trigonal and tetrahedral borate network. Slight changes were caused by gamma irradiation in the intensities of the characteristic infrared borate bands and zirconium bands. Irradiated samples developed a noticeable absorption band similar to the F-band familiar in alkali halide crystals which clearly indicated that successive irradiation produced defects in the glass material. Substitution of ZrO2 to the glass matrix suppressed the formation of visible absorption band formed by irradiation whereas no extra peak was observed in the spectral regions. A nonlinear trend was ascribed in the energy band gap values due to the creation of more number of non-bridging oxygen atoms. Emission spectra showed five emission bands corresponding to the zirconium ions which decreased to a very low intensity with a red shift after irradiation. Matching the colour co-ordinates bright blue emission was found for 5 mol% ZrO2 in both irradiated and non irradiated sample.

Accounting for the Local Field When Determining the Dielectric Loss Spectra of Metals in the Region of the Frequencies of Volume, Surface and Localized Plasmon Oscillations.

The optical constant of bulk metal is used to determine the dispersion of the local field under one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) confinement. 3D confinement, expressed as , corresponds to the dielectric loss spectra of spherical particles with a diameter, d, much less than the wavelength of the beam used to measure the spectrum (d << λ). Excellent agreement with the results of Mie theory and experimental data for solid colloids within alkali halide crystals was observed. The function expressed as allows the measurement of spectral micro-characteristics in the frequency range of the longitudinal collective motion of the free electrons. This corresponds to the spectrum of dielectric losses of bulk plasma oscillations. The function describes the spectra of the dielectric losses of surface plasma oscillations in thin metal films. It is shown that the peak positions of , and spectra for simple metals, viz. alkali metals as well as Al, Be, Mg, Ga, In, Sn and Si, are in agreement with experimental results from electron-energy-loss spectroscopy and various optical techniques.

Luminescence of self-trapped excitons in alkali halide crystals at low temperature uniaxial deformation

The processes of temperature quenching of the luminescence of the self-trapped excitons (STEs) from 93 to 200 K in the conditions of applied elastic low-temperature deformation for several alkali-halide crystals (AHCs) are presented in the paper. The theoretical explanation of the influence of low-temperature uniaxial deformation on the formation of the luminescence of the STEs was done. The increase in activation energy of the temperature quenching of the luminescence of the STEs, narrowing of the emission bands, increase in the frequency of the active oscillations of the STEs and the decrease in the Huang-Rhys parameter are shown as the results. These changes indicate a weakening of the exciton-phonon interaction in elastically deformed AHCs, which, in turn, leads to an increase in the probability of radiative annihilation of excitons.

Optical Properties of Alkali Halides in Ultraviolet Spectral Regions

The optical reflectance spectra of alkali halide crystals KI and RbI were measured over the energy range of 4.14 to 6.91 eV. Both single crystal and poly-crystal samples were used to accomplish this task. The phase θ ( ω ) was computed using the Kramers-Kronig relation between the real and imaginary parts of the complex function, ln r = ln | r | + i θ ( ω ) . Subsequently, the optical constants n and κ were determined from the Fresnel reflectivity equation. The real and imaginary parts of dielectric constants ε 1 and ε 2 were then calculated using n and κ. The optical absorption spectra of the crystal have also been measured in these spectral regions. The spectra agree reasonably well with the current theory concerning exciton peaks. In addition, a shoulder was found in the spectra similar to those previously seen and associated with the band-to-band transition in the alkali iodides.

Alkali halide force fields: A search for an acceptable compromise

We used three sets of individual interaction potentials for alkali and halide ions. The first was the model where a charge on spring dipole (CAS) expressed the polarization of the ion fitted to the hydration free energy, energy and geometry of ion-water clusters, and energy and density of ambient crystals (P.T. Kiss and A. Baranyai, J. Chem. Phys., 141, 114501 (2014)). The halide ions have considerable polarization but a mere dipole is unable to improve the quality of estimations for all possible states and transitions. In symmetric environment like in alkali halide crystals the dipole moment of ions is close to zero and not very different in melt. Thus, we can also test the nonpolarizable part of this model as a second version. A third model was a modified nonpolarizable force field, also producing correct ion-water clusters, but devised to estimate the melting temperature of alkali halide crystals. Using the third model we calculated the melting temperature and the density of melts. However, the need for higher order polarization is indicated by the difficulty to produce accurate melting temperatures for each pair of the 20 combinations of ions. The role of polarization was studied in ambient aqueous solutions of NaCl and RbI. All three interaction models were investigated at three different concentrations in BK3 water. While in the ion – single BK3 water clusters the contribution of the induced dipole of the ion is at most 5% of the energy, in aqueous solutions of RbI the polarizable model produces very different structures than its nonpolarizable counterparts. We evaluate the results and discuss problems towards a polarizable potential of individual alkali and halide ions.

On the Abnormal Conductivity of the Surface (001) of Alkali Halide Crystals

Surfaces (001) of alkali halide crystals (KCl, NaCl), which were obtained by cleavage in air and ultrahigh vacuum, have been studied by Auger electron spectroscopy. It is established that the surface obtained by cleavage in ultrahigh vacuum has an anomalously high conductivity, which is broken down during heating and depositing an atomic layer of metal (Au).

Nonlinear interaction of macrosteps on vicinal surfaces at crystal growth from vapor

The profile dynamics of a macroscopic curved vicinal surface of a crystal, during its growth from the vapor phase, has been investigated. The averaging method on large intervals of spatial coordinate change, which nevertheless remained infinitely small, was used. The expressions for average values of adatom concentration and velocity of elementary steps were obtained. With a phenomenological account of the boundary curvature, the nonlinear Korteweg-de Vries-Burgers equation describing the concentration of adatoms on a macroscopically curved vicinal surface of a crystal was obtained. A monotone shock wave without oscillation was considered and its parameters were determined. The obtained theoretical results have been confirmed by experimental data for some alkali-halide crystals.