Potassium Chloride Crystals Research Articles

Optically detected zero field magnetic resonance of phosphorous activated alkali halide crystals

Optically detected zero field magnetic resonance (ODMR) has been utilized to obtain the fine structure parameters | D| and | E| of the phosphorescent color center in phosphorous activated potassium chloride and bromide single crystals. The dipole activity of the zero field spin-orbital states, the Franck-Condon analysis of the vibronic structure of the phosphorescence spectrum and the fine structure parameters are all consistent with the proposed identification of the impurity center as PO − 2 ion.

On the Polymorphic Phase Transition in Alkali Halides

AbstractEarlier calculations of the equation of state and phase transition data at 298 K for a potassium chloride crystal are extended to include the other alkali halides. It is shown that both the elastic and phase transition data are well reproduced by the three‐body potential model (TBPM). Since the calculations tend to become lengthy, a simpler procedure of calculating the equation of state and transition pressure with the help of the Murnaghan's equation is also suggested. The procedure gives reasonably good agreement with experimental results.

Growth of potassium chloride crystals

Growth rates of potassium chloride crystals in cooling solutions were measured. Systems with either linear (0.2°C/min) or non-linear profiles and spontaneous nucleation were investigated. The range of the average overall measured growth rates was 0.6−1.5 × 10 -7 m s -1. Most crystals were aggregates of several cubes. Crystals <0.5 mm apparently grew faster than those >0.5 mm. The decrease of growth rate could be attributed to attrition which is evident in rounding of corners and edges and presumably breakage of ill fitted aggregates. Crystals grown at initial slow rate of cooling (∽0.01°C min -1) with subsequent fast cooling, were spherical, built around single central cubes. The existence of an analogy was suggested between this mechanism of growth and the sub-microscopic processes in crystallizers where cycling phenomena occur.

The behaviour of alkali metal colloids in additively coloured potassium chloride crystals. II. Study by transmission electron microscopy — colloid nucleation and growth

Direct observation of potassium colloids in additively-coloured KCl crystals by transmission electron microscopy reveasl their appearance particularly in regions of high dislocation density and confirms a large distribution of sizes and their heterogeneous density. Observed precipitate contrasts, together with the absence of additional electron dirffraction inforamtion, suggest that the potassium colloids may have an f.c.c. structure coherent with the alkali halide matrix. L'observation directe de colloides de potassium dans des cristaux de KCl colores additivement par microscopie electronique a transmission confirme leur grande distribution de tailles et leur repartition tres heterogene. Les cooloides apparaissent particulierement dans les regions a forte densite de dislocations. Les contrastes de precipites coherents, en l' a absence de cliches de diffraction electronique, suggerent que le postassium aurait une strucutre c.f.c. comme la matrice de KCl.

The Behaviour of Alkali Metal Colloids in Additively Coloured Potassium Chloride Crystals (I). I. Study by Optical Absorption and Surface Replicas — Colloid Size Distribution

Observation of potassium colloids in additively coloured KCl crystals by replica electron microscopy reveals a large distribution of sizes and their heterogeneous density. The average colloid sizes observed experimentally agree with those deduced from optical spectra. The colloids appear spherical when small (50 to 300 A radius), but develop {111}, {100}, and {110} crystallographic facets when larger. L'observation des colloides de potassium par microscopie electronique permet de mettre en evidence une grande distribution de tailles et une repartition tres heterogene. La taille moyenne ainsi obtenue est en bon accord avec celle deduite des spectres optiques. Spheriques pour des rayons de l'ordre de 50 a 300 A, ces colloides presentent des formes cristallographiques nettes {111}, {100} et {110} pour des tailles superieures.

The back force effect and its influence on sublimation of alkali halide single crystal surfaces

Sublimation structures at the atomic level were studied on sodium and potassium chloride crystals by the gold decoration technique. Three stages of sublimation were revealed: hole nucleation, spiral-ledge motion, and macroscopic pit formation. The results are in good agreement with recent theories of crystal sublimation by the terrace-ledge-kink model, including those for the back force effect at ledge sources.

Effects of γ‐ray irradiation on the properties of Pb2+ doped potassium chloride crystals

AbstractCorrelations between various properties (e.g. ionic conductivity, dislocation density, colourability, thermoluminescence, thermal recovery) of as‐received and irradiated KCl crystals containing up to about 20 mole · ppm Pb2+ are revealed. On the basis of the above data and the model proposed by MATSUNAMI et al. some speculations regarding the final irradiation product are presented.

INVESTIGATION OF THE THERMAL STABILITY OF RADIATION DEFECTS IN NEUTRON IRRADIATED KCl WITH A NEW METHOD

A convenient method for the investigation of gas transport in alkali halides is offered by the possibility to load single crystals homogeneously with radioactive rare gas ; (1) in the case of potassium chloride with Ar-39 through the reaction K(n, p) Ar. The radiation with neutrons will, however, also produce defects of various kinds in the lattice. The experimental results readily show, that the gas atoms interact with radiation-defects, (2), (3) in that the coefficient of diffusion D can be put on the form D ~ 1/CT, CT being the concentration defects. This, the so called trapping-diffusion-model successfully could explain as a trapping of the gas atoms a the defects (4). Some further work, undertaken to account for certain deviations from expected isothermal degassing curves, did not succeed in bringing these anomalities in consistency with the predictions based on the original assumption mentioned above. The contradictions led to the suggestion that the defects are not stable, but anneal inhomogeneously during the experiment, which in effect means that a time-and-position-depending coefficient of diffusion will have to be used in the formal description. Due to the fact that the gas produced in potassium chloride crystals is almost completely bound at the defects, there is a method at hand for attaining information on the behaviour of the defects, if the gasprofile, or some function of it, can be determined at low enough temperatures. Thus if the irradiated specimen is heated at a constant temperature until a certain amount of gas has left the crystal, the resulting gasprofile will hold information on the defect profile. Any difference from a gas-profile produced in a system with a constant or only time-depending coefficient of diffusion, will mean that the defect concentration in this experiment has changed time-and-position dependently, which then also pertains the coefficient. For the purpose of measuring gas profiles, a small precision-grinding machine was constructed, which permits the specimen to be grinded along one axis in an evacuated vessel. The radioactive gas, which is set free quantitatively during the grinding process, is measured with a proportional counter at appropriate intervals of grinding. Subsequently the change in length of the specimen along the grinding-axes is determined. The experimental result will thus be available in the form of an integrated activity versus position curve. It should be noted, that the profiles and consequently the coefficient of diffusion, can be calculated explicitly only when diffusion profiles in the specimen of interest have an unique position space dependence, e. g. in a spherical but not a cubic specimen. In the case of a cube, a qualitative evaluation is attainable if the experimental curve is compared to the one calculated from the solutions to Fick's-second-law for a specimen of the same form and the same degree of degassing, i. e. the curve that would result if the defects were perfectly stable or would anneal homogeneously. From experiment performed on cubic specimens of potassium chloride and evaluated as outlined above, it seems fair to conclude, that at least a part of the defects anneal at the surface.

Some physical properties of KCl + Pb2+ crystals

This paper presents the results of measurements of the mechanical and electrical properties of as-received pure and lead-doped crystals of potassium chloride. The critical resolved shear stress of the crystals obeys the Franks relationship\(\tau _A^* \propto \surd c_{Pb^{2 + } } \) in the whole concentration range. In addition, the correlation betweenτ0 and the Vickers hardness numberH was found and the equation is of the formτ0=k (H−H0). The solubility of Pb2+ in as-received KCl crystals was observed, from measurements of the electrical conductivity, to be low—the successive saturation of the solid solution started already in an environment of 5 mole ppm in agreement with data from the mechanical measurements. The density of cation vacancies and their mobility were represented by the following euqationsμ1T=6·25×104 exp(−·75/kT) cm2 K/volt. sec,n1=6·95×1023 exp(−2·12/kT) vacancies/cm3 The value of 0sd46±0·02 eV was found as a rough estimate of the association energy.

Creeping-film phenomenon of potassium chloride solution

A PECULIAR creeping film that is similar to the superfilm of liquid helium II, but which occurs at room temperatures and has a much lower transfer rate, takes place in saturated potassium chloride aqueous solution. In an electrolytic experiment using saturated KCl solution as the salt bridge, we noticed that without any disturbance the solution could not be kept in the beaker all the time. A thin surface film creeps up the beaker wall very slowly, about several centimetres a day, flows out over the brim and then down the outside surface of the beaker. Creeping can also be observed up the surface of the electrode and the salt bridge tube (Fig. 1). On surfaces over which the film has crept, potassium chloride crystals form as the water in which KCl is dissolved evaporates. Within a few days the solution in the beaker drains out completely and the entire surface of the beaker becomes covered with a thin layer of potassium chloride crystals, about 10−2 cm thick.

Studies of the size and the misorientation of the mosaic blocks in pure and doped single crystals of potassium chloride

Extinction coefficients for pure and doped single crystals of potassium chloride have been measured with the use of polarized X-rays and subsequent analysis in terms of Zachariasen's theory of X-ray diffraction from real crystals. The sizes of the mosaic domains and the angles of misorientation between them have been determined from the extinction coefficients corresponding to copper and cobalt radiations. It has been observed that with increase in doping the sizes of the domains diminish and the misorientation between them increases.

Polymorphic Phase Transition in a Potassium Chloride Crystal

AbstractThe phase transition data at 298 °K of potassium chloride crystals are analysed on the basis of two alternative models, (a) the Tosi‐Fumi prescription (TFP) in which overlap parameters are taken to be different in the two phases and (b) the three‐body potential model (TBPM) used by Sarkar and Sengupta [1]. The effect of finite temperature and that of zero point energy is included using the Einstein approximation. It is found that the transition data are well reproduced in both the models with TFP giving slightly better values. The temperature coefficient of transition pressure comes out with the right sign but has too high a magnitude in both the models. In elastic data, however, TFP gives poor agreement while for TBPM the agreement is much better.

Role of the distributed impurities in the etching of potassium chloride crystals

The concentration of the distributed impurity calcium in KCl crystals is varied and its influence on the etching action has been studied. As the concentration of the distributed impurity calcium is increased in KCl crystals, the etched samples show the following essential features: (a) increase in the distinctness of the etch pits; (b) a progressive increase in the degree of surface roughness; and (c) gradual variation of the etch pit morphology. These observations suggest that the distributed impurity calcium acts as an additional poison during etching.

Polymorphic phase transformation rates in shock-loaded potassium chloride

Single crystals of potassium chloride (KCl) were impacted on quartz stress gauges at velocities ranging from 0.0363 to 0.0755 cm/μsec, producing plane shocks with stress amplitudes between 17 and 31 kbar. Stresses greater than 21 kbar are sufficient to induce a polymorphic phase transformation in KCl. For impact stresses above the phase transition point, the theory of continuum mechanics is used to relate measured stress history at the impact interface to transformation rate between the phases. Experimental results show that, when shocked above the measured dynamic transition pressure of 20.8±0.5 kbar, KCl rapidly transforms to a metastable nonequilibrium mixture of phases. Phase transformation to these initial metastable states proceeds at a rate greater than 500 μsec−1 for shock propagation along the 〈100〉 and 〈111〉 directions, based on the magnitude of initial transformation and the minimum response time of the measuring system. The degree of partial transformation is observed to depend only on impact velocity and is independent of crystallographic orientation of the KCl. The observed metastable states are consistent with the postulate that the heat of formation of the nonequilibrium high-pressure phase is characterized by a specific entropy 3.9 × 10−6 cm2 μsec−2°K−1 higher than the equilibrium high-pressure phase. Subsequent phase transformation proceeds at a rate that depends on crystallographic orientation of the KCl. When the shock propagates along a 〈111〉 direction, it is observed that transformation proceeds from the metastable state to completion at a rate of 25±5 μsec−1, with final states in agreement with thermodynamic equilibrium calculations. When the shock propagates along a 〈100〉 direction, transformation proceeds at a rate of less than 5 μsec−1. For the 〈100〉 orientation, data are inadequate to allow determination of whether transformation was completed during the experiment, because of uncertainty in late-time quartz gauge response.

Anharmonic thermal X-ray scattering from potassium chloride, aluminium and silver

Experiments on diffuse X-ray scattering by potassium chloride, aluminium and silver single crystals have been performed over the temperature range 4-293K. It is observed that scattering from longitudinal modes at 293K is not as expected from known frequencies. The extra scattering can be interpreted as due to anharmonic processes. At lower temperatures, 4 and 77K, along all the symmetry axes, experimental and harmonic scatterings are practical. For all the studied crystals at any temperature, scattering from the transverse modes is predictable from harmonic theory.

Investigation of the motion kinetics of screw dislocations in sodium chloride and potassium chloride crystals

The kinetics of the motion and the surmounting of detents by dislocations in sodium chloride and potassium chloride crystals is investigated herein. Special attention is paid to a comparative study of the mean free paths of the screw and edge dislocations. It is established that the mean free paths of the screw dislocations is somewhat less than for edge dislocations under identical test conditions. The kinetic curves obtained were processed within the frame-work of theoretical representations about the motion of dislocations in crystals containing detents of two kinds.

Work function measurements on (100), (110) and (111) surfaces of aluminium

(100), (110) and (111) single crystal aluminium substrates were thoroughly outgassed by heating by electron bombardment in ultra high vacuum. Fresh single crystal surfaces were prepared by autoepitaxy on these substrates and the work functions of the surfaces were determined photoelectrically. The general effect of the deposition of an aluminium film on the substrate was to reduce the work function, but annealing the film at temperatures between 473 K and 573 K caused the work function to return to a constant value which was taken to be characteristic of the ordered surface. (100) aluminium surfaces were also prepared by epitaxy on potassium chloride crystals. The effect of argon ion bombardment on a bulk (110) aluminium surface is to increase the work function towards the polycrystalline value, but the characteristic value for the (110) face may be restored by annealing the crystal. There is a linear relationship between surface atom density and work function for the three faces investigated.

The growth of potassium chloride crystals in the presence of lead

Potassium chloride seeds (cleaved from pure, melt grown crystals) were grown from aqueous solutions with supersaturation ratios in the range 1.005 to 1.050 and with concentrations of lead chloride ⪖ 6 × 10 M. While the seeds were growing the {100} cleavage faces were studied by using reflection microscopy and two beam interference microscopy.On the initially flat cleavage faces many growth hills developed during the early stages of growth. The hills were approximately conical and their surfaces were inclined at angles ⪝ 3° to the close-packed {100} pinnes. The most steeply inclined hills grew most rapidly, both parallel and perpendicular to the crystal surface and soon overran their neighbours. Crystal growth occured solely through the continuing development of these hills.Profiles of growth hills were determined at time intervals and it was found that the development of the profiles could be described in terms of the kinematic theory of crystal growth. Furthermore, on the otherwise smooth surfaces of the hills, kinematic shock waves of sub-microscopic steps were observed. In many instances, the shock waves had the forms of spirals centered on the apices of the hills. These observations are consistent with the thesis that the hills originate at the points of emergence in the surface of groups of dislocations with large Burgers vectors.

Ionic Conductivity of Potassium Chloride Crystals

The ionic conductivity of four relatively pure crystals of potassium chloride has been measured over a wide temperature range. Precautions were taken to minimize the effects of sublimation at temperatures close to the melting point. The data have been analyzed in terms of the conventional Schottky defect model allowing for transport on both sublattices and for both nearest-neighbor and long-range (Debye-H\"uckel) defect interactions. The defect parameters obtained are used to predict cation and anion diffusion coefficients. The cation diffusion coefficients are in excellent agreement with experiment, but both the enthalpy and entropy of activation for anion diffusion are higher than the values obtained from diffusion experiments by Fuller. Possible reasons for these discrepancies are discussed.

Dielectric Low Temperature Thermometer for Use in High Magnetic Fields. II. KCl :OH, KCl :CN

Potassium chloride crystals doped with OH− or CN− ions are shown to be excellent magnetic field independent thermometers between 0.1 and 10 K. A procedure for making rugged capacitance thermometers, their sensitivity to temperature, and their independence of magnetic field at 1.2 and 4.2 K are discussed.