KBr Single Crystals Research Articles

Direct observations of pseudomorphism: compositional and textural evolution at a fluid-solid interface

Solid-fluid interactions often involve the replacement of one phase by another while retaining the morphology and structural details of the parent phase, i.e, pseudomorphism. We present in situ observations of the evolution of both the solid and fluid compositions at the interface during such a replacement reaction in the model system KBr-KCl-H2O, in which a single crystal of KBr is replaced by a single crystal of KCl. The pseudomorphism is initiated by epitaxial growth at the fluid-mineral interface, when the dissolution of the parent phase results in an interfacial fluid layer that is supersaturated with respect to a different solid composition. The subsequent evolution of the coupled dissolution and growth can be related to local equilibrium defined by a Lippmann diagram. The reaction features, including the development of porosity in the new solid phase, share many characteristics of replacement reactions in nature as well as in technical applications

Dissolution studies about the aggregation-precipitation stages of the divalent europium in KBr crystals by photoacoustic technique

Four different aggregation-precipitation (AP) stages of the divalent europium (Eu 2+ ) ions have been fixed in KBr single crystals by means of heat treating processes. To examine the dissolution of the ions configuration (AP stages), the samples are continuously heated for activating the ions diffusion. This process is monitored in situ by the pulsed photoacoustic (PA) technique. To follow the ions configuration dissolution, the PA signals at each temperature are successively acquired. AP stage of ions changes are recovered by a numerical procedure based on the correlation coefficients. This procedure applied to digitally recorded PA signals allowed the construction of an AP profile N S (T i ), where is easier to analyze the ions behaviour into the crystal host. The results agree with those obtained by the optical absorption analysis, under similar conditions. The PA signal is generated as a consequence of the non-radiative processes that have taken place after the excitation of the Europium ion. This excitation, which corresponds to an electronic transition to the low energy absorption band T 2g of the Eu 2+ ions, is achieved by means of a laser pulse at 355 nm focused on the sample.

A mechanism of mineral replacement: isotope tracing in the model system KCl-KBr-H2O

The solid solution-aqueous solution system KCl-KBr-H2O is used as a model system to determine the mechanism of the replacement process of one crystalline phase by another. A single crystal of KBr was allowed to react with a saturated KCl solution enriched in 40K and the resulting new product phases were analyzed for changes in both anion and K isotope composition, using SEM, EDX, microprobe analysis, mass spectrometry and X-ray diffraction. The results show that the new product, K(Cl,Br), contains elements derived from both the original fluid and solid phases, indicating that both anions and K isotopes are exchanged during the replacement reaction. The interface between the advancing reaction front and the original parent crystal is sharp on a micron scale, showing no reaction profile that could indicate a solid state diffusion mechanism. Macroscopically the new phase is turbid due to the development of porosity that is consistent with a net volume deficit replacement reaction and this porosity may act as an indication that a replacement process has taken place. Single crystal X-ray diffraction patterns show the preservation of the crystallographic orientation during the replacement process. The replacement mechanism is interpreted as a result of a coupled process of dissolution and recrystallization occurring at the fluid-crystal interface.

Anodic formation of microcrystals by electrolysis of KBr single crystals

Anodic deposition of ionic compound layers on insulator and metallic substrates by the combined action of ionic transport and a dc polarized plasma has been the subject of investigation in the last two decades. We now report on the growth of abundant ionic microcrystals on the anodic side of a solid electrolyte cell (W/KBr/stainless steel (ss)) formed by a KBr single crystal between a W plate anode and a stainless steel pointed cathode. KBr microcrystals are formed on the central zone of the W anodic plate. Prismatic and acicular microcrystals of some W–Br compounds are formed on the borders of the anodic plate and on the borders of the KBr anodic face. The observed crystallization can be qualitatively understood on the light of several phenomena: ionic electromigration along the axis of the KBr single crystal, existence of a dc plasma inside the small cavities between the W plate and the KBr crystal and a vapor deposition process induced by the Br 2 provided by the KBr crystal. We finally consider the possibility of growing micro and smaller size crystals of different materials on controlled surfaces of small areas.

Structural and optical characterisation of ZnO nanocrystals embedded in bulk KBr single crystal

The ZnO nanocrystals (NCs) embedded in KBr single crystal matrix has been fabricated using the Czochralski (Cz) method. X-ray diffraction has confirmed the embedding of ZnO nanocrystals inside the bulk KBr matrix. A blue shift of the absorption edge of the obtained samples has been observed. Moreover, ZnO nanocrystals embedded in bulk KBr single crystals, present photoluminescence (PL) bands, at 1.6 K, associated with free and bound excitons. These results confirm that bulk KBr single crystal is a suitable matrix-host for ZnO nanocrystals.

Studies of the thermal dissolution process of the Suzuki phase of theEu2+ ion in KBr single crystals by analysis of photoacoustic signals

An experimental investigation of the thermal behaviour of thedissolution process of the Suzuki phase (SP) by continuous heating(1 °C min−1)of KBr:Eu2+ crystals is reported in this work. The thermal profiles were determined by the correlation functionsbetween subsequent photoacoustic (PA) signals registered during the dissolution process.The behaviour of the thermal profile is directly related to the absorption coefficient of theEu2+ ion in precipitated states that are present in the crystal. The PA signal is detected as a consequenceof the non-radiative processes that take place after the excitation of the low-energy band of theEu2+ ion by means of a focused laser pulse at 355 nm. The results obtained bythis method are compared with those simultaneously obtained by thephotoluminescence (PL) technique. The samples were heated from room temperature to205 °C.The PA signal and PL spectrum were obtained every6 °C.The temperature range of the SP dissolution process was from 77 to115 °C.These results are in agreement with those obtained by the PL technique and with the datareported in the literature.

Control of laser desorption using tunable single pulses and pulse pairs

We desorb ground state Br and spin–orbit excited Br* atoms from KBr single crystals using single pulses and sequential pulse pairs of tunable nanosecond laser radiation. Irradiation of cleaved KBr crystals near the bulk absorption threshold produces hyperthermal Br emission without a significant thermal component, and with little spin–orbit excited Br* emission. The Br kinetic energy distribution may be controlled either by choice of photon energy or by excitation of transient defect centers created within the crystal. In this latter scheme, a first laser pulse generates transient centers within the bulk crystal and in the vicinity of the surface, and a second delayed laser pulse then excites the transient centers leading to atomic desorption. The Br* to Br yield ratio is significantly enhanced using two-pulse excitation as compared to resonant single-pulse desorption. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities, velocities, and spin state distributions.

Selective laser desorption of ionic surfaces: Resonant surface excitation of KBr

We demonstrate evidence of selective laser-induced desorption of ground state Br(2P3/2) and spin–orbit excited state Br(2P1/2) atoms from KBr single crystals following 6.4 eV irradiation. Laser excitation tuned selectively to a surface resonance below the first bulk absorption band excites surface states preferentially leading to surface specific reactions while inducing relatively insignificant bulk reaction. The experimental results are supported by embedded cluster ab initio calculations that indicate a reduced surface exciton energy compared to that of the bulk exciton with a slight further reduction for steps and kink sites. Low fluence irradiation of cleaved KBr crystals, near the calculated surface exciton energy of 6.2 eV, produces hyperthermal Br(2P3/2) emission without a significant thermal or Br(2P1/2) component. The hyperthermal emission is shown theoretically to be characteristic of surface induced reaction of exciton decomposition while thermal emission is attributed to bulk photoreaction.

Introductory studies on the growth and characterization of carotenoid solids: an approach to carotenoid solid engineering

AbstractSolids of seven all‐trans isomeric carotenoids, β‐carotene, β‐apo‐8′‐carotenal, astaxanthin, canthaxanthin, spheroidene, lycopene and zeaxanthin, were studied in the forms of KBr disks, aggregates, thin films and single crystals in an effort to understand and control the molecular arrangement of the solids. Optical absorption and resonance Raman scattering were adopted to characterize the arrangements. Card‐packed arrangements and head‐to‐tail arrangements were realized by changing either the end groups or the growth process. These studies, which reveal the elementary engineering of solid growth and its characterization, may open up carotenoid solid engineering to develop useful applications of the materials in the fields of electronics, photonics and mechanical engineering. Copyright © 2001 John Wiley & Sons, Ltd.

Mass-Spectrometric Study of the Temperature Variation in the Dimer-to-Monomer Ratio in the Free-Surface Vaporization Fluxes from Alkali Halide Single Crystals

A mass-spectrometric method is used to investigate the kinetics of the vaporization of LiF (970–1070 K), NaCl (722–889 K), KCl (780–900 K), KBr (724–918 K), and CsI (656–838 K) single crystals. A dimer-to-monomer ratio, JD/JM, in the fluxes vaporizing from free surfaces of these crystals are calculated, using the currents of M+, MX+, and M2X+ ions formed via electron impact ionization of the molecules MX and M2X2 (M is the alkali atom; X is the halogen). It is found that the dimer-to-monomer ratio increases with temperature at a continually increasing rate in the LiF and NaCl cases. In the KCl, KBr, and CsI cases, this rate passes through a maximum. Such a specific temperature variation in JD/JM is discussed through the analysis of proposed mechanisms of dimer formation in light of the terrace–ledge–kink and surface charge models.

Point defect production byX-rays above and below the bromineK-edge inKBr

F-center formation by monochromatic X-rays has been studied above and below the bromineK-edge in single crystals of KBr at 77 K. A concentrated beam from an undulator at the Advanced Photon Source was used to produce these point defects, which were detected by a sensitive laser-induced luminescence method. Experiments were carried out over an especially wide range of monochromatic X-ray intensity. Contrary to our previous reports, a large increase inF-center formation efficiency was not found upon crossing the bromineK-edge, which suggests that additional Auger-cascade mechanisms do not compete strongly with the usual multiple ionization electron-hole recombination processes known to generate point defects.

Remarkable properties of alkali halides and new techniques for determining Debye--Waller factors

We have used 70 Ci 183Ta Mossbauer sources to measure the Debye--Waller factors of NaCl, KCl, and KBr single crystals for several of the (h00) and (nnn) Bragg reflections. We have found that the procedure used to analyze data by earlier workers leads to incorrect parameters in the Debye--Waller factor exponent, and we use a procedure that eliminates the need to introduce an empirical parameter to account for thermal expansion effects. We observe that both the cations and anions in NaCl and KBr have identical Debye--Waller factors in the [nnn] direction. We observe Q4 terms in NaCl and KCl, with some evidence for a Q4 term in KBr. The size of this contribution varies with the direction of momentum transfer and will be reported. We observe that the Debye temperature and the coefficient of the anharmonic Q2 term also vary with direction of momentum transfer. We believe this to be the first definitive evidence for a non-spherical thermal cloud in a cubic crystal; the ions have a larger amplitude of oscillation in the [h00] direction than in the [nnn] direction. This effect is temperature dependent and greater at lower temperatures.

Sol-gel synthesis of ZnO thin films

Transparent and crackfree thin films of ZnO have been deposited on fused silica, soda glass, silicon wafers and KBr single crystals using the sol-gel technique. A sol has been prepared by reacting zinc acetate and ethylene glycol and dissolving the resultant transparent brittle solid in dry n-propanol. A proton acceptor like triethyl amine was added to assist hydrolysis of zinc acetate. The resulting solution was readily gelled on addition of water. Films were spin cast on polished substrates, gelled in humid air and pyrolysed at 450 °C to get polycrystalline ZnO thin films. The films were characterized using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and UV-Vis absorption spectroscopy.

F-center formation by Br K-hole Auger decay in KBr.

Spectral dependence of the {ital F}-center formation efficiency has been measured for KBr single crystals at liquid-helium and nitrogen temperatures in the vicinity of the {ital K} edge of bromine at 13.47 keV. The {ital F}-center formation efficiency was evaluated by the optical absorption measurement under irradiation with monochromatized synchrotron x rays. The efficiency was normalized to the deposited x-ray energy estimated by a sensitive photocalorimetric measurement. It has been found that the {ital F}-center formation efficiency shows an {approximately}24{percent} stepwise increase above the core threshold at liquid-helium temperature. This result suggests that the multiple ionization mechanism, following the core-hole Auger cascade, might participate actively in the {ital F}-center formation process. {copyright} {ital 1996 The American Physical Society.}

Fourier transform-infrared and optical studies on sol-gel synthesized SrTiO3 precursor films

SrTiO3 precursor thin films were deposited on KBr single crystals and fused silica substrates by the sol-gel technique. Fourier transform-infrared (FT-IR) spectra, ultraviolet-visible spectra and X-ray diffraction spectra were recorded after the samples were cured at various temperatures. FT-IR spectra showed a gradual decrease in -OH concentration indicating removal of hydroxyls and reduction in peaks belonging to organic groups indicating the removal of organics. After annealing at 500 °C, the peak due to SrTiO3 crystallites (∼500 cm−1) began to appear, indicating crystallization. The optical spectra showed an increase in refractive index due to densification as the curing temperature increased. With increase in curing temperature, the thickness of the film decreased, indicating densification due to pore collapse. The optical band gap also changed with annealing due to the structural transformation from amorphous to crystalline phase. X-ray diffraction spectroscopy confirmed the structural changes.

Thermoluminescence emission of X-irradiated Eu2+doped KBr single crystals

Thermoluminescence emission of X-irradiated Eu²⁺doped KBr single crystals

ESD of nonthermal halogen atoms from In-doped (001) KBr

We have measured the kinetic energy distributions of neutral halogen atoms emitted due to electron-stimulated desorption (ESD) from In-doped (001)KBr single crystals. The concentration of In + in investigated samples varied between 10 17 and 10 20 particles/cm 3. The measurements were performed at target temperature of 150°C. In all cases the energy spectra consist of two peaks. The distribution of low-energy particles can be described by the thermal (Maxwellian) energy spectrum. Particles contributing to the second peak have nonthermal kinetic energies. The emission of halogen atoms having nonthermal energies decreases with an increase in the concentration of In impurities. At the same time, the emission of thermal particles does not seem to be sensitive to variation of In + concentration. At the temperatures of interest here, In + impurities are known to be very efficient traps for migrating holes but not to influence the migration of H-centers. The obtained results support the model of nonthermal halogen desorption which requires a long range diffusion of holes from the bulk to the surface as a necessary step of the process.

Thermoluminescence emission of X-irradiated Eu2+doped KBr single crystals

Abstract In this paper we discuss the results of thermoluminescence (TL) studies carried out on freshly quenched crystals of KBr doped with ∼50ppm of Eu2+ ions which were X-irradiated at room temperature. The TL glow curve of this phosphor material consists of three glow peaks at 355, 376 and 398K whose intensities increased as a function of X-irradiation time. The TL glow peaks were analyzed by the total curve-fitting method in order to obtain the characteristic parameters; activation energy, pre-exponential factor and kinetic order. The spectral character of the emission recorded during thermoluminescence was found to be the same for all glow peaks and consists of a broad band centered at -420 nm. This band is composed of two overlapping bands peaking at 419 and 460 nm. The former band is due to the well known fluorescence of Eu2+ ions corresponding to the transition 4f6 5d (T2g) → 8 S7/2, while the latter may be associated with the recombination of F-centers with the radiation induced interstitial chlorine atoms which are thermally released from traps. It is proposed that the model of the TL process most consistent with our experimental results is one in which the Eu2+ impurity acts as an electron trap during the irradiation process and that the radiation induced center (partner of an F center) and the Vk center (partner of an Eu+ ion) are trapped together in the form of a Cl− 3 molecular ion at the Eu2+ -cation vacancy dipoles and their smaller aggregate complexes such as dimers, trimers, etc. During the TL run, the H and Vk centers escape from the trap and recombine with the complementary center giving rise to the observed TL emission spectra. The dosimetric characteristics of the KBr:Eu2+ phosphor material are also reported.

In-situ investigation of surface reactions on alkali halides by atomic force microscopy

In-situ AFM images have been produced of dissolution processes on NaCl and KBr single crystals under different organic solvents. Using a home-built sample holder stable imaging conditions have been achieved and atomic resolution could be obtained on both materials. Dissolution rates have been determined quantitatively from the AFM images and compared with empirical solvent parameters such as acceptor number AN and ET(30) value. The AFM results are in agreement with the expected behaviour of the liquids. Dissolution rates could be controlled using mixtures of polar and non polar solvents. The results are a basis for further investigations of other systems and technologically important processes such as corrosion and surface modification.

Optical absorption of theFcenter as function of the temperature

Abstract The temperature shift of the F absorption band for KBr and KCl single crystals was experimentally and theoretically determined. The theoretical calculations are based on the pseudopotential method using different wave functions and assumed a linear thermal expansion coefficient for the crystalline lattice. The change of F-center absorption energy due to collisions with the thermally vibrating lattice is calculated. This contribution is small. The measurements agree fairly well with the results obtained using the present model.