Precipitation and dissolution of KCaCl3 impurity in NaCl crystals

Impurity precipitation in NaCl crystals containing CaCl₂ and KCl in various proportions is studied by means of flotation density measurements. The paper is concerned with the determination and dissolution, the kinetics and activation energy, and the mechanism of nucleation. Two temperature regions for solid solution decomposition were revealed. The characteristic stages in density variation upon isochronal annealing are shown to be correlated with radical changes in the X-ray diffraction patterns. Evidence is given that the nucleation is heterogeneous for the stable phase and homogeneous for the metastable one. – The activation energy in the low-temperature region has been found to be 0.8 ± 0.1 eV, which suggests precipitation to be diffusion-limited. From the kinetics studies it follows that the composition of the metastable phase corresponds to KCaCl₃ and the rate-controlling stage in stoichiometric crystals is a K⁺ diffusion.     

The role of impurity dispersion and hardness anisotropy studies in NaCl:Sm crystals

The effect of the distributed impurities and their state of dispersion on hardness of crystal is studied from microhardness measurements on NaCl crystals doped with different concentrations of samarium both in as-grown state as well as after quenching them from various elevated temperatures. It is found that the small additions of impurity decrease the hardness. This has been explained on the basis of formation and dissolution of precipitates     

Effect of change of state of aggregation of impurity on tl and microhardness of KClxBr1–x mixed crystals doped with Ca2+

Microhardness and Thermoluminescence (TL) measurements have been made in KCl_xBr_1–x mixed crystals doped with Ca²⁺ impurity. The variation of hardness in undoped and Ca²⁺ doped KCl_xBr_1–x crystals with quenching temperature is investigated. TL studied of KCl_xBr_1–x crystals doped with Ca²⁺, both in as-grown state and after quenching them from various elevated temperatures indicate that the positions of the glow peak which has been attributed to F-centers is found to depend upon the state of dispersion of impurity.     

Structure sensitivity of relaxation phenomena in NaCl and KCl crystals doped with some divalent impurities [I] ITC-spectra characteristic of the as-cleaved state

ITC spectra of NaCl and KCl crystals doped with europium, lead, and nickel (in a wide concentration range) have been measured at temperatures between 120 and 400 K. It has been shown that the spectra taken for as cleaved samples are usually very rich. The origin and behaviour of the high-temperature peaks (above the IV-dipole peak) have been discussed in frames of the SILLARS theory of Maxwell/Wagner-type polarization effects in heterogeneous dielectrics. It is suggested that the peak positioned near room temperature is related with any type of vacancy rich precipitates formed within otherwise homogeneous crystal lattice. The second peak, positioned at still higher temperatures, is a Maxwell/Wagner effect due to dislocations surrounded by impurity-related imperfections in a fashion of the so called Cottrell atmosphere.     

The effect of impurity complexes on the shape of PbTe and Bi2Te3 crystals

The causes of strong dependences of the external shapes of lead and bismuth telluride crystals grown under conditions close to equilibrium on the deviation from stoichiometric composition have been investigated. Auger-analysis of lead telluride and its solid solutions indicates the change in the character of bonding in the subsurface layers connected with appearence of smooth faces not belonging to the equilibrium shapes of pure substances. The surface phase existance has been explained on basis of statistical thermodynamics within the framework of the Guttman model and due to the formation of „the crystalline”︁ impurity complexes in a layer medium in contact with the growing crystal surface. Particles attachment to the crystal under these conditions proceeds in two stages, which facilitates crystal growth.     

State of dispersion of impurity on formation of Z1-centres in NaCl:Sr2+ crystals

Thermoluminescence (TL) and Optical absorption studies are carried out to study the bleaching kinetics of F-centre and formation of Z₁-centres in X-irradiated strontium doped NaCl crystals, both in as-grown state and in quenched state. An additional TL peak is observed in this crystal system after F-bleaching of irradiated samples which is attributed to thermal annealing of Z₁-centres. Various models proposed for Z₁-centres are reviewed in the light of these results and it is concluded that Z₁-centres are associated with impurity-vacancy dipoles.     

Structure sensitivity of relaxation phenomena in NaCl and KCl crystals doped with some divalent impurities (II) effect of plastic deformation

By studying: 1/the role of dislocations in the origin of the C-ITC-peak in undeformed crystals, 2/ the effect of plastic deformation upon the behaviour of this peak, and 3/ the recovery of the deformation-induced changes of the basic characteristics of this peak (H^C, T_M^C) it has been confirmed that dislocation lines surrounded by the Cottrell-like atmospheres can be considered as polarizable entities related with the high-temperature dielectrical response of as cleaved NaCl and KCl crystals doped with divalent impurities. The Maxwell/Wagner model of polarization effects in heterogeneous dielectrics satisfactory accounts for the majority of the experimental data. The quantitative analysis of the results obtained yields some additional informations concerning the interaction- and motion-parameters of the defects considered.     

Deformation and microhardness studies on natural apophyllite crystals

Microhardness testing of three different planes of natural apophyllite crystals is reported. Variation of Vicker's microhardness value H_v with applied load ranging from 10–100 g indicates two distinct regions. In the low load region (10–70 g) H_v increases with load till it attains maximum value and then decreases thereafter. This variation is explained qualitatively in terms of depth of penetration of the indentor. Variation of orientation of the indentor w.r.t. each of the (100) and (111) planes indicates microhardness anisotropy.     

Anisotropy of the optical suppression of the magnetoplastic effect in NaCl single crystals

Anisotropy of the plane-polarized light effect on magnetosensitive point defects in NaCl crystals has been observed. The data obtained may yield the information on the symmetry of the electronic states of these defects.     

Luminescence of crystals of divalent tungstates

Crystals of divalent tungstates are characterized by two main luminescence spectral ranges: a short-wavelength (blue) luminescence band in the range 390–420 nm and a group (often two groups) of longer wavelength (green) bands in the range 480–520 nm. For crystals of calcium, strontium, barium, cadmium, magnesium, zinc, and lead tungstates, it is shown that the wavelength corresponding to the maximum of the blue luminescence band (λ_max) correlates with the melting temperature (T_m) of these compounds. The position of the blue luminescence band is the same (in the range 510–530 nm) for crystals with different divalent cations. Annealing in vacuum and electron irradiation decrease the intensity of both blue and green luminescence bands but do not change the ratio of their maximum intensities. This circumstance suggests that vacancies serve as luminescence quenchers to a greater extent rather than facilitate the formation of emission centers responsible for a particular luminescence band.     

Deformation and microhardness studies on KClO4 single crystals

The deformation characteristics of KClO₄ single crystals have been studied by the methods of static and dynamic indentation on (001) plane. The cracks produced by the dynamic indentation have been interpreted in terms of slip-twin interactions. Also, the load dependence of Vickers microhardness and its anisotropy in this crystal are reported. The observed hardness anisotropy has been used to confirm the indices of the slip system 〈101〉 〈111〉 operative in this crystal at ordinary temperatures.     

Influence of impurity atmosphere on the deformation of silicon crystals

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.     

Evaporation of doped NaCl crystals

In the present paper an investigation of the influence of impurity ions on the evaporation of NaCl crystals is reported. Monovalent impurities little influence the evaporation rate. The evaporating structures have changed only when there were extremal differences in the values of ionic radii. However, the bivalent impurities substantially influence the evaporation rate. This unfluence depends upon the fact whether the impurities are in non-associated or associated state with cation vacancies, respectively. This fact also affects the shape of evaporating structure.     

Aging-induced shift of the optical quenching maximum of the magnetoplastic effect in NaCl crystals

The shift of the optical quenching maximum due to the magnetoplastic effect in NaCl crystals caused by one-year aging at T = 293 K after heat treatment has been found. It was established that the spectral characteristics of crystal sensitivity to light were changed and, at the same time, the crystals became insensitive to magnetic fields.     

Effect of impurity adsorption on the growth of Rochelle salt crystals

The influence of impurities of copper compounds on the growth of Rochelle salt crystals of the composition KNaC₄H₄O₄ · 4H₂O is studied. The growth rates of the faces of various simple forms experimentally measured as functions of the CuCO₃ concentration in solution at a constant supersaturation and temperature are compared with the theoretical models of impurity adsorption on the faces of a growing crystal. Deceleration of the growth of various faces of a Rochelle salt crystal is satisfactorily described by the Bliznakov equation with the use of the Langmuir, Frumkin-Fowler, and de Boor adsorption isotherms for all the faces except for {010}. However, such a comparison does not allow one to reveal the cause of adsorption or its type on different faces. Photometric scanning of Rochelle salt solutions with copper-compound impurity showed that a small addition of alkali (0.06–0.4 g/l) to the solution results in the appearance in the absorption spectra of both the solution nd the crystal grown from it of a maximum at the wavelength 660–670 nm. The intensity of this maximum increases with an increase in the copper concentration. The EPR data, the absorption spectra of the solution and the crystal, and the modified crystal shape showed that the addition of alkali to the solution results in the formation of new copper complexes that more actively decelerate the growth of Rochelle salt faces.     

Preparation of NaCl color center laser crystals and perturbation effect of ions

NaCl:(F₂⁺)_AH-type color center laser crystals were prepared by the crystal growth, additional coloration, annealing, and by light aggregation at 290 and 77 K. The formation process of color center in NaCl crystals were investigated, the influence of doped cation and anion ions on the stability and optical properties of color centers is discussed in this paper.     

Magnetostimulated changes of microhardness in potassium acid phthalate crystals

A decrease in microhardness along the (010) cleavage in potassium acid phthalate single crystals by 15–18% after the application of a permanent magnetic field is revealed for the first time. It is shown that the effect revealed is of the volume character. The role of interlayer water in the processes stimulated by a magnetic field is studied. Interlayer water does not cause the observed changes; it only plays the part of an indicator of these changes in potassium acid phthalate crystals in a magnetic field. It is established that microhardness in the (100) plane of the crystal in an applied a magnetic field first increases by 12–15% and then remains constant in time within the accuracy of the experiment. The possibility of varying the crystal structure of potassium acid phthalate crystals by applying magnetic fields inducing rearrangement in the system of hydrogen bonds or in the defect structure is discussed.     

The effect of crystal surface roughness on impurity adsorption

The effect of crystal surface roughness on impurity adsorption was investigated in a fluidized bed crystallizer and in a batch crystallizer. The crystallisation of sucrose in pure and impure systems was the study subject. Calcium chloride was utilized as impurity in this work. The results show that the impurity adsorption is growth rate dependent and is strongly influenced by the crystal surface properties. Crystals with high surface roughness have lower impurity adsorption. Based on experimental evidences, a new theoretical model is proposed to quantify the surface roughness influence on the impurity adsorption, allowing, by operating at the more adequate supersaturation, to control the impurity transfer into crystals. The used impurity does not have a significant influence on the growth rates at the studied temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)