Single Crystals Of Alkali Halides Research Articles

Growth and Characterization of Single Crystals of Alkali-Halides

Growth and Characterization of Single Crystals of Alkali-Halides

Hot-carrier transport processes in stimulated desorption of alkali halides

Electron- and photon-stimulated desorption of epitaxial thin films and bulk single crystals of alkali halides has been investigated. It has been found that the widely accepted model based on self-trapped exiton decay and thermally activated defect diffusion cannot account for a number of experimental observations. In particular, it cannot explain the stability of the very thin alkali-halide films against the electron beam, and it fails to interpret correctly the yield dependence of the halogen atoms emitted with nonthermal energies on the film thickness and sample temperature. It is shown that for satisfactory interpretation of those data one has to take into account early stages of crystal excitation, i.e., hot carrier formation and transport processes occurring prior to self-trapped exciton phase. Consequently, a comprehensive description of the observed desorption features is presented.

Similarities and differences between the low-temperature acoustic properties of crystalline materials and glasses

We report on measurements of the acoustic properties of single crystals of alkali halides and aluminum, and polycrystalline copper at kHz frequencies and temperatures down to 8 mK. The goal of this study was to examine in several crystals with a small number of defects the presence and influence of two-level tunneling systems which commonly govern the low-temperature behavior of amorphous solids. The samples were investigated using vibrating reed and torsional oscillator techniques. In contrast to recent reports on glasslike behavior for a large variety of pure polycrystalline metals, our results confirm the ``traditional'' point of view that the low-temperature acoustic properties of pure monatomic crystals can be well distinguished from those of amorphous solids.

Acoustic properties of crystals at very low temperatures— Comparison with glasses

In search for the microscopic nature of the so-called tunneling systems which govern the low-temperatures properties of amorphous matter it appears to be necessary to reexamine the behavior of pure crystals as a starting point. In a series of experiments we studied the acoustic properties of single crystals of alkali halides and aluminium and large-grained copper at temperatures down to 7 mK. The samples were measured in the vibrational as well as in a torsional mode. Particularly with torsional oscillators of the polycrystalline copper we attained results which allow us to state clearly that our samples do not exhibit glass-like behavior.

Electron-irradiation-induced structural and compositional changes on alkali halide surfaces

Compositional and topological changes induced by low-energy electron irradiation have been observed on thin films and single crystals of alkali halides using secondary electron microscopy and Auger electron spectroscopy. Alkali-metal-rich structures with diameters of up to several microns were found on the sample surfaces. The results give new insights ona related problem: electron- and photon-stimulated desorption.

Electron-irradiation-induced structural and compositional changes on alkali halide surfaces

Compositional and topological changes induced by low-energy electron irradiation have been observed on thin films and single crystals of alkali halides using secondary electron microscopy and Auger electron spectroscopy. Alkali-metal-rich structures with diameters of up to several microns were found on the sample surfaces. The results give new insights on a related problem: electron- and photon-stimulated desorption.

Directional emission of nonthermal halogen atoms by electron bombardment of alkali halides.

We present the first experimental results on angle-resolved kinetic-energy distributions of halogen atoms desorbed from single crystals of alkali halides because of electron bombardment. We found that the ejection of nonthermal Br atoms from the (100) surface of KBr is strongly forward peaked along the normal. We suggest that this effect is caused by a thin damaged layer on the surface due to a strong nonstoichiometry of the erosion process itself.

ChemInform Abstract: EPR Studies of Irradiated Single Crystals of Alkali Halides Doped with Cyanide Complexes of Cobalt, Rhodium, Iridium, Iron, Ruthenium and Osmium. A Review

ChemInform Abstract: EPR Studies of Irradiated Single Crystals of Alkali Halides Doped with Cyanide Complexes of Cobalt, Rhodium, Iridium, Iron, Ruthenium and Osmium. A Review

Correlation between microhardness, dislocation mobility and the isothermal stability of coloration in microcrystalline powders of KCl

Stable colour centres can be introduced in single crystals of alkali halides using electrolytic coloration. The powders obtained by crushing these single crystals lose colour very rapidly. Mobile dislocations which are present in far greater abundance in the powders are responsible for the reduced stability. Results on some KCl samples showing a correlation between microhardness, dislocation mobility and the stability of coloration are presented.

Vibrational studies of BH −4 and BD −4 isolated in alkali halides

Single crystals of alkali halides doped with BH − 4 and BD − 4 were grown from the melt. Previously unreported bands in the infrared spectra of BH − 4 and BD − 4 isolated in different alkali halides are interpreted in terms of summation bands of internal and external modes of vibration. This has allowed the torsional and translational modes of the impurity ion to be identified. The tetrahedral symmetry of the borohydride ion is retained when it is isolated within alkali halides with the NaCl structure. A reduction of symmetry towards C 3v was observed when BH − 4 (or BD − 4) was isolated within lattices with CsCl structure. Raman and far infrared spectra of alkali halide/BH − 4 systems will be reported for the first time, and high pressure infrared studies of these systems will be described. The effects of pressure in the internal mode, external modes, Fermi resonance and NaCl to CsCl structural phase changes will be discussed.

ESR studies of irradiated single crystals of alkali halides doped with ferrocyanide ion

Single crystals of RbCl, RbBr, and KCl doped with [Fe(CN)6]4− were grown from solution and ESR spectra of both x- and γ-irradiated crystals were recorded over a wide temperature range. A number of Fe(I) radicals were obtained and identified by their ESR spectra. Thermally treated crystals showed the presence of halogen-substituted radicals. Rotations were made in the crystallographic planes and the spectra analyzed using a suitable spin Hamiltonian including, where appropriate, nitrogen and halogen ligand superhyperfine interactions. Powder spectra were also recorded and used as a cross check on the single crystal analyses. The g and superhyperfine splitting tensors are shown to be consistent with having the odd electron in an antibonding orbital of a1g symmetry made up largely from the metal dz2 orbital. The spin densities in the ligand s and p valence orbitals and the p/s ratios were computed where possible. Structures have been proposed for the various radicals based on the observed ESR parameters.

Electron paramagnetic resonance studies of VO 2+ in single crystals of alkali halides

Results of electron paramagnetic resonance studies of VO 2+ in KBr and KI are reported over a wide temperature range. At room temperature there is an isotropic eight-line pattern attributed to a freely rotating VO 2+ molecular ion in the crystal. At low temperature an anisotropic spectrum attributed to the hindered rotation of VO 2+ ion is obtained. The spin-Hamiltonian parameters at room and liquid-nitrogen temperatures are determined, and study of their dependence on the lattice constant shows that as the lattice constant increases, the hyperfine parameter ( A 0) increases while g 0 and linewidth decrease.

Optical absorption of Bi+++, Hg0, and Cd0 centres

AbstractFor a systematic study of the isoelectronic series of s2‐type centres the absorption bands of some missing centres are measured. Thin layers of alkali halides doped with monodisperse Hg0 or Cd0 are formed by quenching condensation. Single crystals of alkali halides are doped with Bi+++′. In addition of the common bands of s2‐type spectra Bi+++ shows a strong D′‐band. In the isoelectronic series — like Au−, Hg0, TI+, Pb++, Bi+++ — the difference in excitation energies between centre and free ion is nearly proportional to the ionic charge.

ESR Studies of Gamma-Irradiated Crystalline 1-Bromobutane

Radiation damage in 1-bromobutane has been studied by ESR spectroscopy. The use of aligned crystals gives well-resolved spectra which can be clearly assigned to the Br2−.. center and the n-butyl radical. Other unidentifiable paramagnetic species are also produced. A quantitative interpretation of the Br2−. spectrum in the parallel orientation gives the magnetic resonance parameters A (81Br) = 472 G and g = 1.992 in satisfactory agreement with previous studies of Br2−. in single crystals of alkali halides. The spectrum of the n-butyl radical shows equal hyperfine interaction with the two β protons and as in other studies this is interpreted in terms of anisotropic internal rotation about the Cα–Cβ bond. The same two paramagnetic species can be identified in the γ-irradiated 1-bromobutane glass, but the ESR spectrum of the n-butyl radical in this case suggests that the internal rotation is frozen out and the radical adopts a nonsymmetrical configuration. Through the positive assignment of the dihalogen anion, the results provide strong confirmation of dissociative electron capture in the radiolysis of this system.

Magnetic Polarizations of Electrons at Dislocations in Alkali Halides

A new ferromagnet has been studied in the experiments reported in this paper and the hysteresis properties explored for the first time. The magnetization may arise from electrons trapped on the numerous dislocations in pure, single crystals of alkali halides. The ferromagnetic moment could be caused by some 1014 electrons/cm3 which are trapped on dislocations and whose spins are aligned. New measurements are reported in this paper giving the torque on the magnetometer as a function of applied field strength. Both the behavior of the ferromagnetic component and the induced magnetic moment are brought out clearly in the present work.

Optical Properties of Alkali-Halide Crystals

Reflectivity data from single crystals of alkali halides have been obtained in the ultraviolet region of the spectrum where excitonic and low interband transitions occur. Following recent calculations for the band structure of KC1 and KI and basing our analysis on the experimental data, it has been possible to predict the properties of the band structure in other alkali halides and describe satisfactorily the low-lying transitions. Furthermore, a close relationship between the conduction band and the energy of the $L$ bands of the $F$ center has been established.

Magnetic Polarizations of Electrons at Dislocations in Alkali Halides

A new ferromagnet has been discovered in the alkali halides. The temperature dependence of the permanent magnetic moment agrees with the Bloch spin-wave theory, $M={M}_{0}[1\ensuremath{-}{(\frac{T}{{T}_{c}})}^{\frac{3}{2}}]$, between 2 and 290\ifmmode^\circ\else\textdegree\fi{}K. A calculation of the Curie temperature ${T}_{c}$ gave (462\ifmmode\pm\else\textpm\fi{}1)\ifmmode^\circ\else\textdegree\fi{}K for two samples of NaCl and 545\ifmmode^\circ\else\textdegree\fi{}K for a sample of KCl. The magnitude of the magnetization ${M}_{0}$ at 0\ifmmode^\circ\else\textdegree\fi{}K can be accounted for by ${10}^{14}$ electrons per ${\mathrm{cm}}^{3}$. We believe all these electrons are trapped along dislocations and are responsible for the magnetic behavior described. These very pure single crystals of alkali halides contain a few parts per million of ferric ions, but they typically have some ${10}^{6}$ dislocations per ${\mathrm{cm}}^{2}$ which arise during growth from the melt. The experiments on the permanent magnetic moment show first an increase with soft x irradiation and then a decrease until the moment vanishes. The interpretation is that x irradiation excites electrons to the conduction band where they move until trapped to a depth of some 0.25 eV at dislocations. The number of electrons at first is small and the electrons may have parallel spins (Hund's rule), but as the number increases the ferromagnetic state is destroyed.

Determination of calcium, strontium and barium in single crystals of alkali halides by neutron activation

The determination by neutron activation of trace quantities of calcium, strontium and barium in single crystals of alkali halides has been studied. The matrix is eliminated before the irradiation by an ion-exchange procedure. Gamma spectroscopy after radiochemical separation is used to determine strontium and barium. Calcium is determined by measuring the β-activity of 49Sc formed by the decay of 49Ca. Long-lived radioactive tracers are used to determine the chemical yields.

Paramagnetic Resonance of Manganese in Single Crystals of Alkali Halides

Paramagnetic Resonance of Manganese in Single Crystals of Alkali Halides

Electrical Effects associated with the Mechanical Deformation of Single Crystals of Alkali Halides

IT has been reported1 that when single crystals of natural rocksalt are plastically deformed, there is a temporary increase in their ionic conductivity. The increased conductivity appears to decay exponentially with a time constant of the order of 1 min. The effect has also been reported2 for synthetic crystals of potassium chloride and bromide, both pure and coloured by the presence of F-centres. In this case the time constant of decay was of the order of 0.5 sec. Experiments have been carried out here during the past three years to investigate this effect further, using synthetic crystals of various alkali halides3. The crystals were either grown from the melt using the Kyropoulos technique or obtained from other sources.