KBr Single Crystals Research Articles

Effect of channeling on positron backscattering from crystals

The dependence of the positron backscattering counting rate on the orientation angle of KCl, KBr, and KI single crystals relative to the incident beam axis is investigated. In the case where the initial positron beam was coincident with the 〈100〉 crystallographic axis, a dip in yield for backscattered positrons was observed. In comparison with positron measurements the backscattered electrons show a peak in yield. The minimum for positrons and the maximum for electrons disappeared in crystals heated up to about 800 °K. [Russian text ignored]

Spiral etch pits in KBr single crystals

Freshly cleaved (100) cleavages of KBr single crystals grown in the laboratory have been etched in an etchant containing 10 ml absolute ethyl alcohol, 0·5 ml n-butyl alcohol, 0·1 ml formaldehyde and 0·1 ml distilled water. Both clockwise and anticlockwise spiral etch pits are produced. By etching (100) matched cleavage pairs in the same conditions, one-to-one correspondence in number, position and shape but not the sense of some of the spirals has been established. By re-etching a cleavage face having spiral etch pits in dislocation etchant, an etch pit is formed exactly at the centre of each spiral, indicating thereby that the spiral etch pits nucleate at the sites of dislocations terminating on the crystal face. The implications are discussed.

Temperature dependence of photostimulated electron emission from ionic crystals

The temperature dependence of photoelectron emission from previously X-rayed or UV-excited NaCl, KCl, KBr, and KI single crystals has been studied in the region 100 to 420 °K at various wavelengths in the visible range. The observed thermal changes of the emission intensity and spectral distribution are attributed to chemisorption phenomena affecting the surface potential barrier of the samples. [Russian Text Ignored].

Colour centre modulation spectroscopy in KBr single crystals

Colour centre modulation spectra are reported for coloured KBr single crystals illuminated with a chopped laser beam. This new technique allows the R 1, R 2 and R 1 +, R 2 + bands to be resolved at room temperature so their behaviour can be followed during bleaching.

Discharge propagation through single crystals of KBr

Cooper and Elliott have recently shown that the electric breakdown of KBr single crystals is initiated in the cathode region. An explanation was offered which depended on the accumulation of positive space charge arising from electron impact ionization. Experiments are described in the paper which show that an alternative explanation of cathode field enhancement by polarization due to the migration of positive ions is improbable.

Initial and discharge time-dependent conductivity in alkali halide single crystals

Initial transient conductivity σ tr and discharge conductivity σ d measured in KBr and NaCl single crystals from 50 to 200°C is explained in terms of charge transport and charge blocking without any need to postulate dipole relaxation. Experiment determines a low-temperature athermal linear transient, which fits a simple exhaustion process of charge flow into traps, and a higher-temperature thermal non-linear transient which correlates with the cation-vacancy flow of normal conduction. The low-temperature traps are identified with sites in the dislocation substructure, whilst transient conductivity at higher temperatures results from a pattern of inhomogeneous charge transport determined by the physical substructure. Direct evidence for inhomogeneous space charge is provided by the anomalous reversal of charge flow in certain discharge transients.

Mobility and diffusion constant of 110Ag in KBr single crystals

Activation enthalpy for mobility of 110Ag in KBr single crystals between 260° and 400°C is 0.67 eV. equal to that for cation vacancy mobility. The corresponding activation enthalpy for diffusion is anomalously smaller. 0.54 eV.

The penetration of electrons in alkali halide single crystals and the effect of nonuniform irradiation on colour centre growth curves

Growth curves of F centres in KCl, KBr, KI and NaCl single crystals up to saturation are measured at room temperature by irradiation with 5-45 keV electrons. The optical density at saturation was found to be proportional to En, with E the energy of incident electrons and n a parameter equal to between 1.35 and 1.69 according to the material. The F-centre formation efficiency at the beginning of the late stage of coloration was found to be independent of electron energy. A hypothesis about the behaviour of the absorption coefficient alpha is made, by means of which it is shown that the electron range T obeys the same relationship T varies as En. For alpha as a function of time, two approximating functions are given.

Microhardness of crystals of KBr-KI solid solutions with Sr2+ and OH? impurities

The microhardness, conductivity, and dielectric-loss angle have been studied in KBr and KI single crystals and in their solid solutions with Sr2+ and OH− impurities. It is shown that the Sr2+ impurity dissolves only slightly in KI crystals and in KBr-KI solid solutions having a large KI content. Addition of the anionic OH− impurity to the molten salts reduces the micro-hardness of KBr but it does not affect that of KI crystals or KBr-KI solid solutions having a large KI content. The composition dependence of the microhardness for KBr-KI solid solutions is constructed on the basis of the microhardness values for the corresponding crystals purified of cationic impurities of different valence.

Photostimulated Electron Emission Spectra of Vacuum‐Ultraviolet‐Irradiated Ionic Crystals

AbstractBy the investigation of the spectral characteristics of photostimulated electron emission it has been shown that F‐centres are created in NaCl, KCl, KBr, and KI single crystals by ultraviolet irradiation in the 5 to 11 eV region. The F‐centres are formed as a result of the photoproduction of excitons as well as electron–hole pairs, the exciton mechanism being especially effective. Under stimulation with visible light, electron emission appears from UV‐created F‐centres due to a two‐step photothermal process in crystals with sufficiently low electron affinity χ and by the usual photoemission mechanism in crystals with relatively high χ.

Low temperature annealing of neutron and gamma induced defects in KBr

Monoenergetic neutrons from 7Li (p,n) 7Be and/or Co 60 γ-rays have been used to produce metastable defects in the form of stored charge in single crystals of KBr at 10°K. Three discrete thermally stimulated conductivity peaks are observed below 40°K. Correlation with optical absorption experiments suggests that the processes responsible for these peaks are controlled by the motion of the Br - interstitial.

Thermal Diffuse Scattering of X-Rays by Transverse Lattice Waves of Potassium Bromide Crystal

Intensity measurements of X-ray diffuse scattering by single crystals of KBr were made on and around the symmetry axes [010] passing through the reciprocal lattice point 400 and [110] passing through the point 004 in the reciprocal space at 293°K. The measured intensities were compared with the calculated intensities in which the first-order thermal diffuse scatterings by the transverse lattice waves were computed on the bases of a simple-shell model and a rigid-ion model. The measurements were found to be closer to the calculated values based on the shell model than to those based on the rigid-ion model. The intensity correction of diffuse scattering till up to 20% for the divergence of X-rays was found to be necessary for the calculation corresponding to the measurements on the symmetry axis [010] passing through the reciprocal lattice point 400.

Dielectric relaxation of KBr doped with KOH and KOD

The complex dielectric constant of KBr single crystals doped with KOH and with KOD has been measured in the temperature range from 0.32°K to 300°K using a small a. c. signal. A relaxation timeτ0 was determined by means of Cole-Cole plots. ForT<4°K the temperature dependence ofτ0 can be approximated byAT−n, wheren is between 1.2 and 1.3 for the dilute samples, and between 0.8 and 1.0 for two samples with large hydroxyl concentrations.n has the same value for OH− and OD− dipoles. The constantA roughly doubles upon substitution ofH byD. The relaxation behavior was found to be independent of concentration in the range between 3×1018 cm−3 dipoles and 1019 dipoles cm−3.

Directional dependence of energy loss of fast protons in a single crystal of KBr

Directional dependence of energy loss of fast protons in a single crystal of KBr

Photostimulated Thermoluminescence in KBr Single Crystals

KBr single crystals are investigated by means of photostimulated thermoluminescence from 50 to 250°K. The spectral range of photostimulation is 240-700mµ. No evidence of V band stimulation has been found; only electron traps are involved in the stimulation process. Filling of the traps occurs either through capture of photoelectrons from the conduction band, or by tunneling of electrons from excited F centers to neighbouring traps.

Nuclear Magnetic Resonance and Quadrupole Effects in KBr and NaBr Single Crystals

Nuclear Magnetic Resonance and Quadrupole Effects in KBr and NaBr Single Crystals

Fermi Doublet ν1, 2ν2 in the Cyanate Ion

Cyanate ion enriched in 13C, 15N, or 18O has been introduced into single crystals of KBr and KCl grown from the melt. The infrared absorption of such matrix-isolated molecular ions has been observed between 1150 and 1350 cm−1 from room temperature down to liquid-N2 temperature. On the basis both of isotopic frequency shifts and changes in the relative intensities, an earlier assignment of ν1 and 2ν2 is reversed; the new assignment parallels the situation recently brought to light for carbon dioxide.

POLARIZATION CAPACITANCE AND DISTORTION IN KBr AT LOW FREQUENCIES

A bridge to measure high impedances in the frequency range 0.01 to 100 Hz is described. The conductance and capacitance of calcium doped KBr single crystals were measured between 50 and 330 °C. The polarization capacitance was not inversely proportional to the square of the thickness, a condition expected from the linearized theory. Severe harmonic distortion was observed at low frequencies, and was attributed to the onset of space-charge control following completion of a vacancy monolayer at the surface, forming a blocking boundary. At the higher frequencies this monolayer is not completed, the crystal boundaries are not blocking, and distortion is not observed. An expression is derived for the limiting frequency at which distortion sets in, and is shown to be in good agreement with experiment.

Rare Gas Mobility in Pure and Doped Potassium Bromide

The mobility of the inert gases argon and xenon in pure and doped single crystals of KBr was studied. The gases were introduced by ion bombardment. The recently established system of stages was used to interpret the results. Normal volume diffusion was found for xenon at low gas concentration, the activation enthalpy being 1.4 ± 0.2 eV. This release was independent of doping, thus favoring a transport process for xenon independent of single vacancies. As most probable alternative, a mobility via small vacancy clusters, e. g. vacancy pairs was suggested. Trapping of xenon was found at higher xenon concentrations. This retarded release was shown to depend on doping and was more pronounced in crystals with an excess of cation vacancies. The diffusion of argon occurred with a similar activation enthalpy, but was most probably due to an interstitial motion.

Effect of State on the Shape of the Dislocation Rosette near an Indentation in Some Alkali Halide Crystals

AbstractThe structure of a dislocation rosette near an indentation in KBr and KCL crystals is studied. The motion of dislocations out of the rays of the dislocation rosette due to external impulse stress is shown to depend on the amplitude and duration of the pulse. The effect of stress on the shape of the dislocation rosette near the indentation is shown for KBR, KCI, and NaCI single crystals. During indentation of the stressed crystal the uniaxial surface contraction elongates the rosette rays that adjoin the direction of contraction, whereas the uniaxial dilation elongates the rays that adjoin the direction normal to the direction of extension. The effect is enhanced by increasing the preliminary stresses on the surface to be indented. The observed phenomenon is explained qualitatively by considering the structure of the rosette near the indentation. This effect may be used to determine the magnitude and sign of the local residual elastic stresses at the surface.