KBr Crystals Research Articles

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.

SERS from Ag Colloids in Alkali Halide Crystals

AbstractSurface‐enhanced Raman scattering is investigated on NaCl, KCl, KBr, and KI crystals doped with Ag. By stepwise annealing the controlled formation and growth of Ag colloids out of Ag− centres can be performed. Raman scattering and optical absorption at 20 °C under comparable conditions allow the conclusion that “active sites” at the colloid surface are responsible for the increase and decrease of the SERSspectrum. Comparative SERS at LNT show the preferred excitation near the upper limit of the crystal phonon spectrum.

Refractive index change of f-aggregate centres as a function of temperature

The refractive index change of a KBr crystal containing F, M and R centres is given as a function of temperature. There are angular frequency values for which the change is constant.

Sophisticated equipment developed for growth and evaluation of perfection of nearly perfect crystals

This paper reviews four major equipment developed at the National Physical Laboratory for growth and perfection evaluation of single crystals, namely (i) a crystal puller for growth of nearly perfect crystals by the Czochralski method; (ii) a microfocus x-ray generator; (iii) an x-ray diffraction topography camera; and (iv) a triple crystal x-ray diffractometer. The crystal puller can provide smooth, uniform and variable pulling rates. The maximum length of pull is nearly 60cm. Efforts have been made to isolate vibrations. Nearly perfect single crystals of KCl, KBr and NaCl with maximum diameter of ∼ 60 mm have been grown. The crystals give diffraction curves with half width in the range of 10–30 sec of arc. In the projection topographs, dislocations can be resolved and characterized. The microfocus x-ray generator is a demountable continuously evacuated system with specially designed electron gun and anode assembly. The vacuum is continuously monitored for ease of maintenance. In the point focus mode the spot size is 40 µm on the anode. X-ray topography system is a versatile equipment used for projection and section topography. It can provide 360° rotations to the specimen disc around an axis perpendicular to it. Rotations of a few sec of arc can be given to the specimen around a vertical axis. Typical diffraction curves of a dislocation-free crystal and a crystal with boundaries are shown. Well-resolved images of dislocations are shown in a topograph as an illustration. In the triple crystal x-ray diffractometer a highly collimated and monochromated Kα1 exploring x-ray beam is obtained by combining microfocus source, a special collimator and crystal monochromators of Bonse-Hart type. With this beam very narrow diffraction curves with half width of about 5 sec of arc can be recorded. Typical results of measurement of diffuse x-ray scattering (dxs) on nearly perfect silicon single crystals are discussed. It has been observed that the contribution of phonons to thedxs is negligible. Thedxs is mainly due to point defect aggregates.

Low‐Temperature Pair Associates of H Centres in KBr

AbstractIn the course of investigating the annealing of KBr crystals X‐rayed at 4.2 K the creation of new pair associates of interstitial halogen atoms (di‐H centres) with an absorption band at 3.85 eV is detected. The 3.85 centres arise only as a result of thermodestruction of the well‐known di‐H centres with an absorption band at 4.4 eV. The properties of these new centres are studied and the possible ways of halogen association in KBr are discussed. As a model for the 3.85 centres a model of (Br)2 dimers is proposed, in which the optical transition differs from that in Br ions both by size and direction. The range of KBr absorption spectra between Br and Br absorptions (3.6 to 4.4 eV) is attributed to the absorption of (Br)2 dimers. The V4A centres in KBr with Li or Na impurities have obviously also a (Br−2)2 structure.

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.

Acoustic and photon emissions during mechanical deformation of coloured alkali halide crystals

Acoustic and photon emissions take place in the elastic and plastic as well as the fracture region of X-irradiated KBr, KCl and NaCl crystals. The rate of photon emission is linear with the strain rate: however, the RMS value of the acoustic emission is proportional to the square root of the strain rate. The acoustic emission is maximum for X-irradiated NaCl crystals; however, the photon emission is maximum for X-irradiated KBr crystals. From the similarity between the acoustic emission and the photon emission, it seems that mobile dislocations are responsible for the acoustic emission in coloured alkali halide crystals. It is concluded that similarly to the acoustic emission, photon emission measurements may also be useful in non-destructive testing of materials. For materials which do not emit photons, to emit less photons during their deformation, a coating of a thin film of an intense mechanoluminescent phosphor on the materials may be helpful.

Spectroscopic properties of Cr(CN)63− doped in a KBr crystal

Abstract The ESR, IR, RS electron absorption and emission spectra of Cr(CN) 6 3− trapped in a KBr crystal are studied. The results reveal two different site symmetries of the Cr(III) ion in the host. The effect of the site symmetry on the radiative and non-radiative transition is discussed.

Structure of flash-evaporated Pb 1−xHg xS alloy films

Electron diffraction and differential thermal analysis investigations were carried out on films prepared by the flash evaporation of PbS-HgS mixtures on freshly cleaved single crystals of KBr. The results suggest that under appropriate growth conditions PbS alloys with α-HgS to form Pb 1− x Hg x S over the composition range 0.07 ⩽ χ ⩽0.93. Lead-rich films (0.07 ⩽ χ ⩽0.50) deposited at a substrate temperature T of 100°C had f.c.c. structures and grew epitaxially with 〈100〉 grain orientation. Depositions at lower substrate temperatures resulted in polycrystalline films with distorted f.c.c. lattices; the lattice parameters increased with increasing mercury concentration. The mercury-rich films (0.60 ⩽ χ ⩽0.93) grown at T = 25°C were amorphous. An increase in T resulted in a change of the amorphous structure through polycrystalline simple cubic at T = 50°C to distorted f.c.c. at T = 100°C.

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.

Radiolysis of nitrate and nitrite ions in a KBr matrix

Abstract Gamma-ray induced decomposition of the nitrate ion in solid solution of KBr crystal has been studied by infrared spectroscopy. The disappearance of the nitrate follows the rate law derived from the mechanism (1) NO3−→NO2− + O, (2) NO2− + O → NO3−, and (3) NO3− + 0 → NO2− + O2. At room temperature ki =l.3 ± 0.2 hr−1 and k2/k3=70 ± l at a dose of 5.9 × 1019 eV hr−1 g−1. The appearance of the nitrite ion follows the same rate law only at the initial stage of the reaction when its secondary destruction by the radiation is negligible. The nitrite ion itself decays by an apparent second order process but only about a third or less of its decomposition produces the nitrate ion.

On Piezoluminescence in Irradiated Alkali Halides

The emission of light under application of pressure was recorded from irradiated crystals of NaCl, KCl, KBr and polycrystalline chips of LiF (TLD-100) as a function of dose of gamma rays imparted to the specimens. The paper gives details of experimental procedures and apparatus. The dose range covered was from 0.3 Gy to 1 kGy.

On Piezoluminescence in Irradiated Alkali Halides

Abstract The emission of light under application of pressure was recorded from irradiated crystals of NaCl, KCl, KBr and polycrystalline chips of LiF (TLD-100) as a function of dose of gamma rays imparted to the specimens. The paper gives details of experimental procedures and apparatus. The dose range covered was from 0.3 Gy to 1 kGy.

Double excitation transitions inMn2+-doped alkali halides

It is shown in this work that the room-temperature excitation spectra of as-grown crystals of LiF, NaF, NaCl, KCl, and KBr doped with ${\mathrm{Mn}}^{2+}$ reveal the existence of double excitation peaks. These peaks are related to the formation of precipitated phases containing ${\mathrm{Mn}}^{2+}$ in the alkali halide lattice. In the case of LiF:${\mathrm{Mn}}^{2+}$ the position of the [$^{4}A_{1}(G)$, $^{4}E(G)$] and $^{4}T_{1}(G)$ peaks suggests an anomalously small ${\mathrm{Mn}}^{2+}$- ${\mathrm{F}}^{\ensuremath{-}}$ distance for the precipitated phase.

Dislocation Mobility in Hydrostatically Compressed KBr Crystals

Dislocation Mobility in Hydrostatically Compressed KBr Crystals

Ionic and electronic processes during frenkel defect annealing in KBr crystals above LHeT

AbstractThe relaxation of radiation‐induced optical absorption spectra and a set of thermally stimulated curves (current, depolarization current, diffusion current, emission‐desorption of gas, thermal bleaching) of KBr crystals X‐irradiated at 4.6 K are recorded synchronously or step by step. A multistage annealing of defects is detected: (i) from 4.6 to 32 K about ten substages controlled by the I centre delocalization are accompanied by electron and energy release, (ii) from 32 to 60 K about six substages controlled by the H, centre delocalization are accompanied by corresponding electron and energy release and gas desorption peaks. A recombination of the migrating I or H centres within such complementary defect systems (pairs, triplets et al.) takes place: (i) {I…α}, {I… F… Vk}, {I… F'… Vk… Vk}, (ii) {H… F} and {H… F'… Vk}. Following phenomena in KBr at 4.6 K are observed and discussed: (i) the “F‐photostimulated” electron tunneling transfer within the excited pair {F* γ. HA} → (α… IA}, (ii) the effect of dc field (3kV/cm) caused Frenkel defect formation efficiency increase (by ≧ 10%), (iii) the photoelectrical dissociation of the α centres. Comments about the anion Frenkel primary defect generation mechanisms are made.

E⊗ε-type Jahn-Teller effect of ReO42-in KBr crystals: an EPR study

EPR experiments have been carried out on irradiated KBr crystals doped with ReO4- ions at 4.2K. They indicate the formation of ReO42- (5d1, 2E ground state) subjected to an E(X) epsilon -type Jahn-Teller distortion. Uniaxial stress experiment show that the ReO42- ion reorientates locally and the spin Hamiltonian parameters are unaltered by the application of the stress. A quadratic stress coupling term cannot be neglected. A model based on the approach of M.C.M. O'Brien is proposed to explain the experimental results.

Thermoluminescence emission spectra of KCl-KBr mixed systems

Thermoluminescence (TL) emission spectra of gamma-irradiated KCl, KBr and KCl-KBr mixed crystals have been obtained at various glow peak temperatures. The variation of emission-band wavelength with composition at different glow peaks has been studied. It shows a minimum for the intermediate composition. Also, the addition of Br- ions to the KCl lattice has more influence on the TL emission of a KCl-KBr mixed system than the addition of Cl- ions to KBr.

Thermoluminescence studies in KCl-KBr mixed crystals

Thermoluminescence glow curves of gamma-irradiated KCl, KBr, and KCl-KBr mixed crystals have been studied in the temperature range 25-300 degrees C. The variation of peak temperature and the relative intensity of the peaks with respect to the composition have been studied. A plot of the peak height against composition shows a minimum at KCl0.70Br0.30. The glow curves were analysed using derivative plots and the variation energy with composition has also been studied. The addition of Br- ions to the KCl lattice has been found to be more effective in enhancing the thermoluminescence of KCl-KBr solid solution.

Thermoluminescence and optical absorption studies of KBr crystals doped with ca2+

AbstractThermoluminescence studies of KBr crystals doped with 0.5 mole % of Ca2+ both in the ‘as‐grown’ state and after quenching them from various elevated temperatures indicate that the position of the glow peak which has been assigned to F‐centers is found to depend upon the state of dispersion of impurity. Bleaching of the X‐irradiated samples result in an additional peak and this has been attributed to Z1‐centers. Optical absorption measurements also confirm the formation of Z1‐centers in Ca2+ doped KBr crystals.