KCl Crystals Research Articles

Collective effects during crystal growth in the presence of mobile nonreactive impurities: experiments and simulations

The growth of a solid phase in a liquid medium containing randomly dispersed mesoscopic impurities is discussed. Particles represent hindrances for the growth front. They can remain static or can be pushed by the advancing interface. The physics of the collective particle motion is outlined. Experiments with Al 2O 3–SiO 2 impurities in KCl crystal serve as illustrative examples. The collective effects due to the particle concentration are investigated through a kinetic growth model.

The radical-cation of hydrogen sulfide

A centre formed in KCl crystals doped with KSH has been identified as the H2S•- radical-anion (A. Hausmann, Z. Phys., 1966, 192, 313). The EPR parameters for the centre are examined herein and it is suggested that this assignment is incorrect. Also, comparison with the results for authentic H2S•- anions in an argon matrix (J. E. Bennett, B. Mile and P. Thomas, J. Chem. Soc., Chem. Commun., 1966, 192) shows that the centre in KCl crystals cannot be H2S•-. Analysis of the EPR parameters leads firmly to the conclusion that this centre is the previously unknown radical-cation, H2S•+.

Calculations of hyper-Raman tensor components of alkali halides

Calculations of hyper-Raman scattering by the zone-centre optical phonons in NaCl, KCl and KBr crystals based on the bond polarizability approach are presented. The results obtained for the hyper-Raman tensor components are in agreement with experimental data reported previously in literature. A weak cubic anisotropy of the hyper-Raman tensor is found. For the LO mode our results predict the lattice and electro-optic contributions to hyper-Raman scattering to be of the same order of magnitude.

A re-investigation of the quadratic electrostriction of alkali halidesof rocksalt type

In order to clarify the reasons for the contradictory results regarding quadratic electrostriction in alkali halides reported by various authors, we developed an improved capacitive dilatometer. Employing the frequency-modulation technique, the resolution of the experimental set-up reaches l 2×10-13 m. Various possible sources of errors could be detected, including sample mounting, calibration faults and, in particular, electro-mechanical resonances caused by strong Coulomb interactions between certain components of the dilatometer. From frequency-dependent measurements in the range 60-1200 Hz we determined the complete tensors of the quadratic electrostriction of LiF, NaCl, NaBr, KCl, KBr, KI, RbCl, RbBr and RbI at room temperature. The results are in reasonable agreement with values derived from sophisticated lattice-theoretical models. Furthermore, we studied the influence of the real structure of Li-doped KCl crystals on their electrostrictive properties.

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 1013–1015 cm−2 and at a flux density of 3×1011 cm−2 s−1. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone.

On the Kinetics of Elementary Processes of Pressure Solution

—Pressure solution is a widespread deformation mechanism in crustal materials, in particular in polyphase aggregates. The kinetics of the elementary processes of pressure solution, that is, the convergence rate of two crystals separated by a liquid interlayer under non-hydrostatic stress, are fundamental for the prediction of strain rates. Based on the concept of disjoining pressure, the problem is solved for the film diffusion mechanism. The geometry is that of a sphere in contact with a plane interface. Solutions are obtained for two cases, higher solubility of the initially spherical crystal and higher solubility of the crystal with the initially planar interface. The problem is solved in a general form with expressions in closed form for two limiting cases, interface-controlled and diffusion-controlled kinetics. An assumption with respect to the rate-controlling step is not required for the calculation of the convergence rate. Only a characteristic parameter must be calculated, which includes the constants of the system. The value of this parameter indicates which step limits the rate of the process. The expression for the convergence rate includes the constant of disjoining pressure, but not the true thickness of the interlayer, which is not constant over the contact area and is difficult to determine experimentally. Using the equations derived in this work, the calculated convergence rates between an NaCl or KCl crystal and a less soluble solid (quartz) in a saturated salt solution are in satisfactory agreement with published results of experimental studies.

Evidence of STE emission in low temperature thermoluminescence of pure KCl UV-irradiated at 15 K

Thermoluminescence spectral measurements in the 20-250 K range were made in nominally pure KCl crystals irradiated at 15 K with 203, 230 and 270 nm UV light. Two spectral regions are clearly distinguished for the irradiation wavelengths used. The luminescence in the 300-400 nm interval (region I) has been suggested to be intrinsic impurity related emission bands. The emission in the 400-600 nm interval (region II) shows characteristics (position, band width and temperature dependence) very similar to the π-polarized perturbed self-trapped exciton luminescence band or to the electron-VK center recombination luminescence. Additionally, for irradiation with 203 nm light the spectral emission shows a deep valley with minimum around 540 nm in the 85-100 K temperature range, which has been previously associated to the self-absorption due to F centers in the crystal.

ESR of an Mn2+–S2-Complex Center in KCl Crystals

Electron spin resonance of MnS-doped KCl crystals has been investigated. It was found that as-grown samples show a simple spectrum looking like being constructed from eighteen resonance lines. The spectrum was very stable at room temperature and the same spectrum was also obtained after annealing of quenched samples which exibit only well-known spectra due to two types of Mn 2+ -vacancy complex centers. Such a spectrum characteristic of as-grown samples was analysed using a model of an Mn 2+ –S 2- complex center in an orthorhombic crystalline field basically with principal axes along <100> cubic crystal axes, two of which are tilted by 10° from <100> crystal axes. When as-grown samples were cooled down to about 150 K, the tilt angle decreased to zero and the crystalline field surrounding the center changed from orthorhombic to nearly tetragonal. It was also found that the Mn 2+ –S 2- complex center responsible for the 18-line spectra disappears by uv irradiation.

Phase diagram of the ternary system CeCl 3–KCl–LiCl

The phase diagram of the ternary system CeCl 3–KCl–LiCl has been investigated by means of differential thermal analysis (DTA). It was found that the ternary system can be subdivided into two pseudoternary systems and no ternary compound was found in the whole ternary system. It was determined that there are six surfaces corresponding to the primary crystallization of KCl, LiCl, CeCl 3, K 3Ce 5Cl 18, K 2CeCl 5 and K 3CeCl 6. There are two ternary eutectic points, E 1 (15.6 wt% KCl, 19.3 wt% LiCl, 65.1 wt% CeCl 3; 395°C) and E 2 (13.6 wt% CeCl 3, 50.8 wt% KCl, 35.6 wt% LiCl; 340°C), and one ternary peritectic point, P (68.1 wt% CeCl 3, 13.5 wt% KCl, 18.4 wt% LiCl; 398°C), in the system CeCl 3–KCl–LiCl.

Influence of surface charge and surface structure on the molecular and ionic sublimation of KCl single crystal

A mass spectrometric method was used to study a kinetics of potassium chloride single crystal sublimation in the form of neutral molecules and positive and negative ions. In electron-impact-ionization mass spectrum K +, KCl + and K 2Cl + ions originated from KCl and K 2Cl 2 molecular precursors were detected in the temperature range 780–900 K. The curvature exposed by the temperature dependencies of ion currents, ln I i T−1/ T, was interpreted as a result of surface charge influence on the rate of sublimation. Theoretical and experimental retardation factors were analyzed for free sublimation of KCl and NaCl crystals [J.E. Lester, G.A. Somorjai, J. Chem. Phys. 49 (1968) 2940]. In thermal ionization mass spectrum K +, K 2 +, K 2Cl +, K 3Cl 2 + positive ions and Cl −, Cl 2 −, and KCl 2 − negative ions were identified in the ranges 710–1020 K and 950–1020 K, respectively; the negative ion currents were much weaker than positive ones. During an induction period, a gradual decrease of isothermal ion currents was observed. The temperature dependence, ln I(K +)−1/ T, measured at successive stages of vaporization revealed (i) the existence of knee temperature, above and below which the ion emission is characterized by different activation energies for ion desorption, E +, (ii) the values of E + were found to be dependent on the time elapsed from the vaporization start, i.e., on the developing surface morphology. The comparison of mass spectra of both types obtained in this work with those measured under equilibrium conditions showed the similarity of the electron-impact-ionization mass spectra and the significant difference between the thermal ionization ones. All the results are discussed in light of terrace-ledge-kink model of vaporization and surface charge concept that are a corollary of defect theory.

High-pressure solid-state spectroscopy at UVSOR by Infrared Synchrotron Radiation

Solid-state spectroscopy by infrared synchrotron radiation (IRSR) has been in use since 1985 at the beamline BL6A1 of UVSOR, the electron storage ring facility of the Institute for Molecular Science at Okazaki in Japan. The wavelength region covered by BL6A1 extends from the near-infrared to the millimeter wave region (1 µm to 3 mm). Up to now, many types of experiments have been done there. In the present article, after a description of the optics of BL6A1 and of the time-resolved infrared spectrum of the source, I discuss the use of that beamline for spectroscopy with diamond anvil cells (DAC) in the far-infrared region. Spectroscopy under high pressure, based either on transmission and on reflection measurements, has been carried out in order to show the advantage of the utilization of the infrared synchrotron radiation with respect to conventional sources. In the former type of experiment, the bulk TO phonon spectra of alkali halide thin films have been measured in order to observe the change in the phonon spectrum due to the phase transition under pressure. The transmission spectrum on the surface phonon state of CuBr microcrystals has also been measured in order to study the response of the surface phonon state to a pressure up to 20 GPa. The reflectivity spectra under pressure have been done instead on a single crystal of KCl, where a change in the rest-strahlen band of KCl due to the phase transition has been observed. Through these measurements one has confirmed that infrared synchrotron radiation is a powerful light source, particularly for those experiments where the acceptance angle of the sample surface with respect to the incident light beam is very small. This is indeed the case of spectroscopy under high pressure in a diamond anvil cell. Finally, a proposal is presented for realizing an even more intense infrared radiation source on a storage ring, with particular regard to the spectroscopic investigation of solid specimens.

Analysis of thermal expansivity of NACL and KCL crystals

We have studied the volume-temperature relationship and thermal expansivity of NaCl and KCL crystals at high temperatures using the formulation recently presented by Shanker et al. The results are found to present close agreement with the experimental data. It has also been possible to produce the P- V- T data for NaCl crystal up to a pressure of 3.5 GPa and in the temperature range 298–773 K, in agreement with the measurements reported by Boehler and Kennedy.

Growth and perfection of KCl crystals grown from aqueous solution

KCl crystals were grown from aqueous solution by the evaporation method at the growth temperature of 40°C, the Pb ion concentration was 0–1/2000 in molar fraction, the pH was 2.0–4.0, the evaporation rate 0.4–1.1 g/day and the duration of growth 5–32 days. The results obtained are summarized as follows. (1) Pb ion addition played the most important role for growing larger KCl crystals, since it reduced the generation of parasite crystals. (2) Growth shape changed from cubic to octahedral with increasing Pb ion concentration. (3) Dislocation density in the crystals grown from seeds was always much higher than that in the crystals grown from spontaneous nuclei. Some crystals grown from spontaneous nuclei were nearly free of dislocations. (4) Growth striations were always observed on the etched {100} surfaces of KCl crystals grown in this work. A growth history could be seen from the change of striation pattern with the growth.

Influence of uniaxial compression at 80 K on the formation of radiation defects in KCl, KBr, and KI crystals

The influence of uniaxial compression at 80 K on the efficiency of formation of stable radiation defects in KCl, KBr and KI crystals is investigated by absorption spectroscopy. It is found that compression along the 〈100〉 directions does not alter the efficiency of radiation defect formation in KCl and KBr crystals, but in KI the efficiency drops by more than an order of magnitude. It is concluded from a semiquantitative analysis that the observed difference is attributable to the impossibility of H centers fitting into the compression-reduced interstitial voids in KI, whereas several multiples of ten-percent compression is required to produce the analogous effect in KCl and KBr.

Optical Absorption Study on the Annealing Behaviour of Metallic Colloids in KCl: K Crystals

The results of optical absorption measurements on potassium colloids in KCl crystals have been presented in this paper. The samples were annealed at various temperatures with different time. It is found that the colloids band is dependent on the annealing temperature and annealing time. The transformation process between F-centers and colloidal centers were observed. The colloids band shows complex structures for the samples which did not attained equilibrium, indicating that there are different kinds of colloidal centers in such samples. Also we found that in these samples(which is in the nonequilibrium state), the transformation process from F-centers to colloidal centers does not stop even at room temperature.

Two-dimensional angular correlation study of delocalized positronium in alkali halides

The two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) from KCl and KI single crystals at low temperatures has been measured. The intensities of the narrow peaks resulting from the Bloch modulation of the delocalized positronium (Ps) are studied. The intensities of the peaks except for those at the momenta corresponding to the (000) and (200) reciprocal-lattice vectors are very low for both KCl and KI. The experimental results for KCl are compared with the calculations made by Zhang and Song. Calculations with simpler approximations are also performed for both KCl and KI.

Deformation Bleaching of Coloration in Alkali Halide Crystals

The present paper reports the phenomenological theory of deformation bleaching of coloration in alkali halide crystals. Expressions are derived for the dislocation capture probability of F-centre electrons in alkali halide crystals, and for the rate of generation of electrons in the dislocation band. On the basis of the rate of generation of dislocation electrons and their subsequent recombination with hole centres, deep traps and other compatible traps (including deformation generated traps), an expression is derived for the deformation bleaching, which indicates the exponential decrease of the density of F-centres with deformation of the crystal. A good agreement is found between the theoretical and experimental results. The value of the effective radius re of the interaction of dislocations with F-centres, is estimated and it is found to be 1.95b, 2.00b, 1.33b and 1.70b for KCl, KBr, NaCl and NaBr crystals, respectively (where b is the Burgers vector).

Radiation induced conductivity in α Al2O3 crystals

Radiation induced conductivity in α Al2O3 crystals is studied during excitation by subnanosecond pulses from an electron accelerator. At least three components are observed in the relaxation of the conduction current. A component of the conduction current which follows the exciting pulse without delay is not observed, as, for example, in NaCl and KCl crystals. This is followed by a subnanosecond decay that does not fit a simple pattern and then, to within the limits of the sensitivity of the apparatus, by at least two other relaxation components for the conduction current in the nanosecond range. It is assumed that the observed components of the conduction current relaxation, including the fastest, are caused by thermal release of carriers from traps and their subsequent redistribution, while the second and third components of the conduction current pulse appear to the associated with electrons.

Modeling of harmonic generation and shock formation in nonlinear surface acoustic waves in several real crystals

Harmonic generation and shock formation in nonlinear surface acoustic waves that propagate in anisotropic crystals were studied numerically on the basis of a new theoretical model presented earlier [Hamilton et al., Nonlinear Acoustics in Perspective, edited by R. J. Wei (Nanjing University Press, Nanjing, 1996), pp. 64–69]. The theory applies for arbitrary elastic materials, surface cuts, and propagation directions. Numerical simulations were performed for initially sinusoidal signals propagating across the surface of KCl, Ni, and Si crystals for the (001), (110), and (111) cuts, and over the appropriate range of directions for each cut. Waveforms are shown to exhibit asymmetric distortion, well-defined shocks with cusped spikes in the horizontal waveform, and a phase shift in the zero crossings. Solutions for propagation in the (001) plane and in the 〈100〉 direction of KCl are shown to exhibit atypical trapping of energy in the lowest order harmonics. Analytical solutions derived for the fundamental and second-harmonic components for this particular case are in good agreement with the numerical solutions close to the source. Although the phenomenon resembles one observed in nonlinear optics resulting from dispersion, it is due instead to properties of the nonlinearity coefficient matrix. [Work supported by ONR.]

Alteration of the lattice constant and the composition of KCl and KBr crystals under macrogravity

In KCl and KBr crystals grown from supersaturated aqueous solutions under centrifuging (3g, 7g, and 12g) the lattice constant and the halogen content are altered significantly in comparison with original compounds, and the microhardness is increased by 1.5–2 fold.