Mixed Alkali Halide Crystals Research Articles

Thermolouminescence studies in ternary alkali halide mixed crystals (KCl) 0.9-x (KBr) x (NaI) 0.1 doped with lithium sulphate

Single crystals of ternary mixed alkali halide crystals (KCl)0.9-x(KBr)x(NaI)0.1 of different compositions with Lithium Sulphate doping were grown by melt Technique using AR grade chemicals as starting materials.. All the crystals were grown under identical conditions. Ternary crystals cleaved from the grown bulk samples were used for the present investigation.

Color center studies in KBr–NaI mixed crystals doped with gold

Abstract Mixed Alkali halide crystals containing KBr and NaI in different compositions were grown by slow evaporation technique. Obtained transparent crystals were doped with gold to study the role of dopant on the mixed crystals. Crystals were subjected to 60Co γ-radiation. The irradiated samples were subjected to Thermoluminescence, Optical Absorption and Electron Spin Resonance studies. Effect of Au + doping on TL intensity and absorption coefficient is discussed in the present paper.

Formation of multiphased mixed crystals from miscible NaBr, KBr and KCl

AbstractMultiphased ternary and quaternary mixed crystals were grown by the melt method, for the first time, using the miscible alkali halides, viz. NaBr, KBr and KCl. X‐ray powder diffraction measurements indicate the existence of multiple phases. Density and refractive index were measured and the composition was determined. The metal atom contents determined through flame photometric measurements confirm the estimated composition of the crystals grown. The present study indicates the possibility of preparing multiphased ternary and quaternary mixed alkali halide crystals using the miscible NaBr, KBr and KCl as the starting materials. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ion size effect dependence on the glow peak temperature in europium doped mixed crystals

AbstractThe ion size effect on the thermoluminescence (TL) glow curve structure of europium doped binary mixed alkali halide crystals samples was investigated as a function of molar composition. A linear expression, that relates the characteristic glow peak temperature to the ionic size, was found and used to predict the TL temperature shifts associated to the size of Cl– and Br– anions in a binary mixed alkali halide sample at a given composition. In addition the behavior of the mixed binary crystals with components KCl and RbCl doped with divalent Europium is presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of multiphased mixed crystals of KCl, KBr and KI:1. Growth and X-ray diffraction studies

The mixed and impurity-added (doped) crystals of alkali halides are found to be harder than the end members and so they are more useful. In view of this, it becomes necessary and useful to prepare binary- and ternary-mixed crystals regardless of miscibility problem and characterize them by measuring their physical properties. In the present work, we have grown (KCl) x (KBr) y − x (KI) 1− y crystals for various values of x and y by the melt method and physically characterized. Bulk composition of the crystals was measured using the measured density and refractive index values. X-ray diffraction analysis indicates the existence of two phases in the mixed crystals. Thermal parameters were determined using the X-ray powder diffraction intensity data.

UV absorption edge shift by doping alkali fluorides in fluoroaluminate glass

The effect on transmittance and reflectance in the vacuum ultraviolet (VUV) region by doping alkali fluorides (LiF, NaF and KF) has been investigated in a ternary fluoroaluminate (18BaF2–37CaF2–45AlF3) glass. The absorption edge of the glass obeys the Urbach rule and was shifted monotonically towards higher energies by increasing the concentration of each alkali fluoride. The VUV reflection peak at 11.3eV was not sensitive to the change of the concentration of dopants. The magnitude of the edge shift was 10–20 times larger than that expected from the additive law on the magnitude of absorption coefficients of the glass and alkali fluorides. An excitonic interaction similar to that observed in mixed crystals of alkali halides is suggested from the monotonic shift toward the absorption edges of the dopants. The weak sensitivity of the reflection peak upon doping supports that the excitonic state lies predominantly around the edge energies.

Description of the migration of a cation vacancy in mixed alkali-halide crystals having a common cation

For mixed alkali-halide crystals having a common cation (of the type KCl + KBr), a new equation is proposed for calculating the hopping diffusion coefficient of a cation vacancy. It is based on the concepts of probability theory and gives good agreement with numerical calculations by the Monte Carlo method.

Spin-lattice relaxation of F centers in mixed alkali halides: KBrxCl1-x.

Measurements of the spin-lattice relaxation time, ${\mathit{T}}_{1}$, of F centers in mixed alkali halide crystals, ${\mathrm{KBr}}_{\mathit{x}}$${\mathrm{Cl}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, have been made using magneto-optical techniques: magnetic circular dichroism (MCD) and optical pumping. These techniques consist of monitoring the recovery of the MCD signal to equilibrium, after an intense pulse of circularly polarized light. They were used to obtain the dependence of ${\mathit{T}}_{1}$ on the magnetic field up to 6.5 T and for several relative halide concentrations x at pumped liquid-He temperature (2 K). A computer-assisted system designed to measure relaxation pulses of the order of 1 ms is described. A model based on phonon modulation of the hyperfine interaction was modified to introduce the relative concentration x. This proves to be the main mechanism for spin-lattice relaxation for the temperature and magnetic-field range considered here, as described for unmixed alkali halides.

Study of photo-elastic behaviour of mixed alkali halides

The present paper deals with the analysis of strain optical constants of the mixed alkali halide crystals.

Computer-simulation analysis of the ESR spectra of V-type centers in irradiated heavy-metal fluoride glasses.

Computer line-shape simulations were carried out on the electron-spin-resonance spectra of several heavy-metal fluoride glasses following exposure to ioniziong radiation at cryogenic temperatures. The primary objects of this study were certain spectra circumstantially attributable to holes trapped on a cluster of two or more inequivalent fluoride or foreign halide ions. The earlier attribution of certain spectral features to ${\mathrm{FCl}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{FBr}}^{\mathrm{\ensuremath{-}}}$ species was confirmed in Cl- and Br-doped glasses, respectively, although the simulations show that ${\mathrm{FCl}}^{\mathrm{\ensuremath{-}}}$ in the glasses has a structure quite different from the same species in mixed alkali halide crystals. Two other spectra---universally observed in irradiated glasses composed of ${\mathrm{ZrF}}_{4}$ (or ${\mathrm{HfF}}_{4}$), ${\mathrm{BaF}}_{2}$, and virtually any number of other fluoride constituents---were also examined in detail. The previous assignment of one of these spectral components to ${\mathrm{F}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ions is sustained, although the specific nature of these defects is now argued to be interstitial fluorine atoms in configurations analogous to ${\mathit{V}}_{\mathit{K}\mathit{A}}$ and ${\mathit{V}}_{\mathit{H}}$ centers (rather than ${\mathit{V}}_{\mathit{K}}$ centers) in the alkaline-earth fluorides. The second ubiquitous component, which was previously ascribed to ``interstitial ${\mathit{F}}^{0}$,'' is demonstrated to arise from an ${\mathrm{F}}_{3}^{2\mathrm{\ensuremath{-}}}$ species similar to the ${\mathit{V}}_{\mathit{t}}$ center in LiF. The ${\mathrm{F}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{F}}_{3}^{2\mathrm{\ensuremath{-}}}$ can be thought of as ``on-center'' and ``off-center'' configurations of the interstitial fluorine atom, and it appears that the former may undergo a thermal transmutation into the latter.

Dielectric Properties of Mixed Alkali and Silver Halide Crystals

AbstractThe static and optical dielectric constants are calculated for the entire range of composition in twelve systems of mixed crystals of alkali halides and silver halides with NaCl‐type structure, by adopting the method developed by Kamiyoshi and Nigara (KN). Transverse and longitudinal optical mode frequencies are also evaluated using the Lyddane‐Sachs‐Teller relation. Calculated values of static dielectric constants are found to exhibit a sharp non‐linear dependence on composition near the equi‐molar region in KClKBr, KClRbCl, KBrRbBr, KIRbI, RbClRbBr, and AgClAgBr mixed crystals and a slight non‐linear dependence on composition near the NaBr end in NaCl‐NaBr mixed crystals, which is in agreement with experimental data. The usefulness of dielectric constants and optical mode frequencies is demonstrated by investigating the composition dependence of the Szigeti effective charge parameter.

Optical Absorption of U Center in Mixed Alkali Halide Crystals

The shift with concentration variation on the optical-absorption of U band in KCl:KBr and KCl:NaCl mixed crystals was studied by use of point-ion and size-ion correction models. We found that using the Bartram et al . method there is not appreciable correction to the point-ion energies. Assuming, for the ground state, that the electrons are concentrated at the lattice site, the calculated energies are in good agreement with the experimental results for KCl, KBr and NaCl in pure state.

Cohesive Energies, Equation of State, and Thermoelastic Properties of Mixed Alkali Halide Crystals

AbstractThe cohesive energies and equations of state describing the pressure‐volume relations are studied for nine systems of mixed alkali halide crystals for the entire range of composition. The Harrison analytical potential form for the overlap repulsive energy derived from quantum mechanical considerations is used to describe the nearest neighbour as well as next neighbour interactions. The long‐range electrostatic and the short‐range van der Waals interactions are also considered in the potential model. The thermoelastic properties such as the Grüneisen parameter and its volume derivative, isothermal and isentropic Anderson parameters are also calculated and reported. The results are discussed and compared with experimental data.

Thermal and elastic behaviour of mixed alkali halide crystals within the framework of compressible ion theory

The Narayan and Ramaseshan (NR) short-range repulsive potential function, extended up to second neighbours was used to study the mixed systems of NaBr x Cl (1 − x) , KBr x Cl (1 − x) , KI x Br (1 − x) , crystallizing into the rock salt structure. The cohesive energy E, isothermal bulk modulus ( B t ), the first and second order pressure derivatives ( dB t dp and d 2B t dp 2 ) of the isothermal bulk modulus of these mixed systems were evaluated using the NR potential. The computations were also extended to calculate the Grüneisen parameter (γ) and mode Grüneisen parameter ( q) using Slater, Dugdale and McDonald, and free volume theories. The results are discussed with the interpolated experimental data.

Analysis of the Structural Phase Transition in Mixed Alkali Halide Crystals

AbstractAn analysis of structural transition in CsClKCl and CsClRbCl mixed crystals is presented by studying the lattice stability in terms of interionic potentials. The potential parameters are determined by fitting the experimental values of lattice energies in standard observed structure viz. CsCl structure for CsCl and CsBr crystals, and NaCl structure for RbCl and KCl crystals. It is found that CsClKCl mixed crystals remain stable in CsCl structure only for KCl less than 25%, and are transformed to NaCl structure when the proportion of KCl becomes equal to or more than 25%. For CsClRbCl mixed crystals, the transition from CsCl to NaCl structure is predicted when the RbCl component becomes equal to or more than 50%. These predictions are found to be in good agreement with experimental results. The lattice theory of structural stability is also applied to predict the transition temperatures for the mixed crystals viz. CsClKCl, CsClRbCl and CsClCsBr at which they transform from CsCl to NaCl structure.

Physical properties of mixed crystals of alkali halides

The physical properties of mixed crystals of alkali halides are reviewed. The properties considered are experimentally determined quantities like the lattice constants, the compressibility, the elastic constants, thermal expansion, specific heats, Debye-Waller factors, dielectric constant, refractive index, IR spectra and Raman spectra and also some calculated solid-state parameters like Debye temperatures and lattice energies. A critical analysis of composition dependence shows four groups designated as follows: Type A, properties which vary linearly with composition; Type B, properties which vary slightly non-linearly with composition; Type C, properties which depend highly non-linearly on composition with the values for mixed crystals exceeding the values for end members; and Type D, properties which are peculiar to the mixed crystals and are not shown by the parent crystals. Most of the physical properties belong to Types A or B. Variation of Type C is shown only by properties affected by differences in ion size like the Debye-Waller factor and the microhardness. Properties which are symmetry-dependent like the first-order Raman spectrum and the IR spectra are of Type D. Attention is drawn to areas where further work is needed.

Evaluation of Electronic Dielectric Constant and Fractional Ionic Character of Mixed Alkali Halide Crystals

AbstractThe electronic dielectric constant and fractional ionic character of the chemical bond are calculated for the mixed crystals NaClNaBr, KClKBr, KBrKI, CsClCsBr, CsClRbCl, and CsClKCl for the entire range of composition. The first four solid solutions are with common cations whereas the last two are formed of substances with common anions. For the electronic dielectric constant a comparison is given of the values calculated from the Clausius‐Mossotti relation and the Pantelides formulation. For evaluating the fractional ionic character application is made of Phillips' and Pauling's theories. The calculated values are found to depend sensitiviely on the composition parameter of mixed crystals.

Ionic conductivity of KBr-KI mixed crystals

Ionic conductivity, σ x , of single crystals of KBr x I 1− x solid solutions (0 ⩽ x ⩽ 1) has been measured at temperatures from 558 K to close to their melting points. There is a good correlation between the phase diagram and the variation in σ x . The composition x exhibiting the minimum in solidus temperature corresponds to the maximum in ionic conductivity. The increase in σ x as a function of x has been calculated in the intrinsic region and agrees well with the experimental results. The effect of homovalent substitution in mixed crystals of alkali halides is discussed.

Use of harrison's potential in mixed alkali halide crystals

AbstractHarrison's overlap repulsive potential is used for evaluating the cohesive energies and bulk modulus of alkali halide mixed crystals. This potential replaces the old and widely used Born‐Mayer repulsive potential in ionic crystals. The Harrison repulsive potential is represented by a quantum mechanical analytical potential form derived from the tight binding theory. The calculations are performed for the mixed crystals of NaCl—NaBr, KCl—KBr, and KBr—KI for the entire range of composition. The results are found in good agreement with experimental data and are better than those predicted from the Born‐Mayer potential.

Second-, third-, and fourth-order elastic constants of mixed alkali-halide crystals

The second-order elastic constants (SOEC), third-order elastic constants (TOEC), and fourth-order elastic constants (FOEC) of mixed alkali-halide crystals are calculated using a phenomenological potential model which takes into account the effect of Lundqvist's three-body forces along with other relevant interactions in ionic solids. Values of three SOEC, six TOEC, eleven FOEC, and pressure derivatives of SOEC have been calculated for NaCl-NaBr, KCl-KBr, and KBr-KI mixed crystals for the entire range of compositions. The results are compared with available experimental values. The applicability of knowledge of TOEC and FOEC has been discussed for predicting the conductivity and thermal expansion coefficient of mixed crystals.