KBr Single Crystals Research Articles

E.P.R. of MnO2-4in KBr crystals under uniaxial stress

E.P.R. of MnO2- 4 centres in KBr single crystals has been investigated at 9·5 and 35 GHz in the temperature range 1·6-77 K. At 4·2 K, the spectrum exhibits a more nearly cubic anisotropy. A temperature variation study indicates that the spectra disappear reversibly above about 8 K. Uniaxial stress measurements at X-band have shown the importance of random strain.

Nucleation of AuAg binary alloys from the vapour phase on KBr single crystals

The nucleation, growth and composition of particulate AuAg alloy deposits on a KBr(100) crystal cleavage surface was investigated by coevaporation of the metals in ultrahigh vacuum. Series of specimens were prepared by increasing the deposition time at constant fluxes of the vapour beams. Analysis of the nucleation rates by transmission electron microscopy revealed that they are mainly determined by the nucleation of gold. Correspondingly, the preferred condensation of gold was found in the early stages of particle growth, as measured by quantitative X-ray fluorescence analysis. During further deposition the composition approached that of the vapour beam, while the substrate coverage increased. The results were interpreted by means of an extended kinetic nucleation model in which it is assumed that the nucleation and growth of alloy particles is mainly governed by the differences in the adatom adsorption energies E a, in the activation energies E d for surface diffusion and in the pre-exponential constants of the components. Evaluation of the experimental data resulted in E a, Au − E a, Ag = 0.03 eV and E d, Au − E d, Ag = -0.02 eV, in close agreement with data for AuAg on NaCl reported in earlier work. As small variations of these values were found to result in large deviations in the condensation behaviour, particularly in the composition in the nucleation phase, the error limits were only about ±0.02 eV. The ratio of the pre-exponential constants was found to be about unity.

BIREFRINGENGE INDUCED BY SPATIAL DISPERSION IN ALKALI HALIDES

Optical anisotropy induced by spatial dispersion of KI, Cs1 and KBr single crystals near their absorption edge has been measured by means of optical transmision experiments. The birefringence has strong dispersion near the absorption edge of these materials. By fitting the results with a microscopical model one obtains the warping parameter Y2 - )13. The warping parameter is much more smaller in KBr than for iodides.

Refractive index changes due to F → M conversion in KBr single crystals and their applications in holography

Refractive index changes due to the F → M colour centre conversion in a KBr crystal are given as a function of angular frequency and temperature. The applicability of the results obtained for holographic recording is discussed.

Hot luminescence and ultrafast configurational relaxation of F centers

Hot luminescence of F centers in RbCl, KBr and KCl single crystals at liquid helium temperatures is discovered and interpreted as the emission from the classical turning points of a configurational coordinate. The energy relaxation time is estimated by two different methods providing interesting information about the excited state relaxation dynamics.

Landau-Placzek ratio of KBr single crystals—I

The Mandel'shtam-Brillouin and Gross scattering was measured for a series of purified KBr crystals. The ratio I G 2I MB , which for a pure, perfect crystal would be the Landau-Placzek ratio, was found to vary with the type of reactive gas treatment used in the final purification. With these crystals both the longitudinal and mixed mode bands were observed with θ and φ equal to 90°. The ratio I L I M was in agreement with calculated intensity ratios and the Δμ for these bands gave C 11 + C 12 = 41.6 GPa and C 44 = 5.18 GPa.

Optical absorption and fluorescence investigation of the precipitated phases of Eu 2+ in KBr single crystals

Europium precipitation in monocrystalline KBR has been studied using optical absorption and photoluminescence techniques. The annealing of quenched samples at temperatures below 100°C produces the growth of an emission band peaking at 433 nm which appears to be related with the presence of Suzuki-type precipitates in the crystals. The aging at higher temperatures, however, produces the appearence in the emission spectrum of two other bands peaking at 427 and 459 nm. The data presented in this paper strongly suggest that the former band is associated with the stable dihalide phase EuBr 2 in the host KBr, while the latter is due to a metastable precipitate whose structure appears to be quite similar to that of europium bromide. Some of the characteristics of the second phase precipitates were obtained by measuring the crystal field splitting (10 Dq) of the 4f 65d configuration of the Eu 2+ ions in each one of them.

Characterization of nearly perfect alkali halide crystals by high resolution diffuse x-ray scattering measurements

Diffuse X-ray scattering (DXS) measurements have been made on nearly perfect KC; and KBr single crystals using a triple-crystal X-ray diffractometer in the (+, -, +) configuration. A highly collimated MoK α1 X-ray beam was used as the exploring beam. The diffraction curves of these crystals had half width in the range: 10–30 s or arc. Reciprocal space has been explored around 002 relp in [001], [001̄], [011], [01̄1̄], [010] and [01̄0] directions. In both, KCl and Kbr, the observed distribution of DXS shows that phonons do not contribute appreciably to this scattering at room temperature. The observed DXS is mainly from point defects and their aggregates. In the as-grown crystals, vacancy-type defects have been observed. The DXS distribution is very sensitive to the thermal history of the specimen. Some crystals were cooled after growth at rates slower than normal. This produced a significant effect on the distribution of the DXS intensity. The slopes of the lines in the absolute DXS intensity versus 1/ K ∗2 curves increased as the rate of cooling was decreased. K ∗ is the scattering vector. The nature of anisotropy with respect to the sense of K ∗ also changed, showing the formation of defect clusters of interstitial type.

Optical investigation of divalent europium in the alkali chlorides and bromides

Detailed optical absorption data are reported for the divalent europium ion in NaCl, KCl, RbCl, KBr, RbBr, and KI single crystals at 300 and 77 K. Values of the oscillator strength are calculated for all the observed bands in the optical spectra at both temperatures. It is established that the optical spectra are well accounted for by a strong crystal field split 5d electron having a Coulomb interaction with the 4f6 core about one-half the free ion value. It is also shown that the measured 10 Dq cubic field splitting of the 4f6 5d configuration into its t2g and eg components is in accord with the expected order of the fields exerted by the different crystal media. In addition, calibration constants are reported for determining the Eu2+ concentration in KBr, RbBr, and KI single crystals by optical techniques, without destroying the sample.

K +3P-core excitons in potassium halides studied at low temperatures with high resolution

Reflection spectra of KF, KCl, KBr and KI single crystals have been measured at photon energies of approximately 20 eV with a bandwidth of 10 meV using synchrotron radiation in order to study fine structure and temperature dependence of the excitonic transitions associated with the K +3 p core level. The crystals were cleaved under ultrahigh vacuum and cooled down to 20 K. Information on energy positions, halfwidths and line shapes for the K +3 p-core excitons and their temperature dependence has been obtained. A new exciton predicted by the ligand field model was observed. For KI we evaluate an electron-hole exchange energy of only 30 (± 7) meV.

ITC measurements in potassium bromide single crystals doped with selenium

The dielectric relaxation of impurity-vacancy (I-V) dipoles in single crystals of KBr doped with Se2- has been studied by measuring the ionic thermocurrent (ITC). It was observed that a single relaxation process is involved in the KBr:Se2- system. A pi jump seems to be the mechanism responsible for the orientation of the I-V dipoles. The corresponding dielectric relaxation parameters were evaluated as tau 0=(2+or-2)*10-14s; Ea=(0.72+or-0.01)eV.

Luminescence and Color Center Foramation by the Optical Conversion of STE in KBr and NaCl

The optical conversion of self trapped excitons ( STE ) from 3 Σ + u state to 1 Σ + u state has been confirmed by observing the σ emission spectra in KBr and NaCl single crystals under the double excitation by a pulsed electron beam and a Q-switched ruby laser light. The conversion efficiencies are 0.3±0.1 and 0.6±0.1 for KBr (5.8 K) and NaCl (6.5 K), respectively, and decrease at higher temperatures. From their temperature dependence, thermal activation energies for the quenching of the conversion are estimated to be 5±1 meV (KBr) and 11±2 meV (NaCl). The conversion efficiency from the STE to the F centers is 0.7±0.2 in KBr. Possible relaxation process of the STE is discussed.

Thermoluminescence in KI, KBr, NaCl and NaF crystals irradiated at room temperature

The thermoluminescence and the thermal stability of the F centres in pure KI, KBr, NaCl and NaF single crystals x and gamma irradiated at room temperature have been studied. It is concluded that the observed thermoluminescence is due to the recombination with F centres of halogen interstitial atoms which are thermally released from traps. The kinetics, activation energy and pre-exponential factors of these thermally activated processes are given. Some measurements of the M centre thermal stability are presented.

Temperature dependence of positron annihilation in KBr single crystals

Positron lifetimes and peak counts of 2-photon angular correlations in KBr single crystals have been measured as a function of temperature in the range between 20°C and 400°C. Results are discussed with current existing models.

Solution Hardening and Softening in KCl–KBr Single Crystals at Low Temperatures

The temperature and concentration dependence of the critical resolved shear stress (CRSS) for KCl and KCl–KBr solid solutions was measured from 4.2 K to 293 K. At lower temperatures the solid solutions with the concentration below 10 mol% KBr and above 90 mol% KBr showed solution softening. The other concentrations of solid solutions were hardened at all temperatures tested. The concentration dependence of the CRSS was given by a c(1-c) relation except for the case of lower concentrations at lower temperatures. The activation free energy of deformation varied linearly with temperature and independently of the concentration. It was concluded from the analysis of the activation volume that the dislocation motion in the higher concentration region was impeded only by the solute ions, and that in the lower concentration region by the Peierls barriers in addition to the solute ions.

Irradiation damage of alkali halide crystals during positron bombardment

Abstract The bleaching effect of positron irradiation of KCl and KBr single crystals previously coloured with electrons or protons was investigated. Positrons injection in the coloured alkali halide samples reduces the F-centres concentration considerably. For KCl crystals thicker than the positrons range the appearance of additional bands in the absorption spectra is noticeable. The experimental data show that the bleaching phenomenon should be observed merely throughout the positron exposure both for irradiated and non-irradiated regions of the sample. Irradiation effects, due to positron source, on the peak counting rate of (γ-γ) angular correlation in KCl crystals under applied magnetic field were also investigated. The growth of peak counting rate shows the increase of positronium-like states formation near defects of cation sublattice.

On the Hologram Recording Based on the F→Z1 Colour Centre Conversion in KCl:Ca and KBr:Ca Crystals

Use of the F → Z1 colour centre conversion for hologram recording in KCl and KBr single crystals doped with Ca has been studied. A new technique for measurement of the H-D curve (optical density versus exposure energy) has been described. Further, measured diffraction efficiency values for holograms by taking advantage of the H-D curves are presented. The readout of the hologram was observed to be destructive, and experimental data describing this property are also given.

Quadratic electrostrictive effects in LiF, NaF, NaBr, KCl, KBr, KI, RbCl, RbBr and RbI

Quadratic electrostrictive effects of both longitudinal and transverse type have been measured on LiF, NaF, NaCl, NaBr, KCl, KBr, KI, RbCl, RbBr and RbI single crystals with an improved frequency-modulation technique with a 100 MHz oscillator. The three independent tensor components d1111, d1122, d1212, defined as dijkl = ½ ∂2∊ij/∂Ek∂El have been determined for all these crystals. ∊ij are the components of the strain tensor and Ek the components of the electric field vector. All longitudinal components d1111 and all 'shear' components d1212 are negative; all transverse components d1122 are positive. The electric field causes a volume contraction in these crystals. The theoretical values for d1111 as derived by Grindlay & Wong [Canad. J. Phys. (1969), 47, 1563-1571] agree well with the experimental data. Larger discrepancies are found between the predicted and observed behaviour of d1122 and d1212. Within the isotypic crystal group of alkali halides of rocksalt type the components d1111 show a positive correlation with the dielectric constants. The appreciable electrostrictive contribution to the total electro-optical effect is discussed.

Transient and steady electron currents limited by ionic space charge in electrolytically coloured potassium halides

Transient electrical currents have been measured when potassium halides are electrolytically coloured under constant temperature and applied voltage. An analysis of the characteristics of these currents leads to the conclusion that they are electron currents limited by ionic space charge. A very detailed study of the transient current as function of temperature ( T ⪅ 825°K) and applied voltage ( V ⪅ 10 3 V) has been accomplished in KBr, KI and KC1 single crystals. A quadratic law of the type J s αV 2, has been observed to be obeyed in the stationary level of the current, J s . From the dependence of the steady value ( J s ) on temperature we have obtained activation energies ( W) for the process responsible for the propagation of the colour ( F-centre) cloud from the cathode to the anode. The experimental values are W = 1.6, 1.6 and 1.8 eV for KBr, KI and KC1, respectively. According to these results and others obtained by other authors, it is proposed that the F-centre diffusion takes place by movement of the F-electron from the centre to the nearest anion vacancy under the action of the electric field and without passing through the conduction band. The propagation process appears to be controlled by the anion vacancy density and mobility. From the above values of W, we have obtained activation energies for the diffusion of anion vacancies of 0.8, 0.8 and 0.7 eV for KBr, KI and KC1, respectively.

Growth striations in KBr(PbI2) single crystals grown from aqueous solution

The growth striations revealed by etching (100) cleavages of cubo-octahedral transparent KBr(PbI2) crystals grown from solution by the evaporation method are described and discussed. The formation of these striations is attributed to fluctuations in temperature during crystal growth. It is shown that at first the crystals grow with an octahedral habit by the deposition of layers on (111) faces and that at a later stage cubic faces also develop. The implications are discussed.