Deuteron induced damage in alkali halides

Abstract

The thermoelectric and galvanomagnetic properties in the CdxHg1_xTe solid solution samples (x = 0.12; 0.15) irradiated at 300 K by electrons (3.5 MeV; by integrated fluxes up to 1.46 × 10¹⁸e/cm²) have been investigated in a wide temperature range 4.2–300 K and in magnetic fields 0–22 kOe.

It has been discovered that the electron irradiation of Cd_xHg_1–xTe crystals leads to an increase of the Hall coefficient and a decrease of the thermo e.m.f. sign inversion temperature. The concentrations and mobilities of electron and hole have been calculated on a basis of quantitative analysis of galvanomagnetic data. It has been shown that the kinetic coefficient behavior upon irradiation is due to the Te vacancy-based type radiation defect origin.     

Defects induced melting in alkali halides

In the present paper we study the pressure dependence of melting of NaCl and CsCl crystals. A formulation has been presented for the pressure dependence of melting temperature on the basis of the vacancy model using the expression for the pressure dependence of the volume of Schottky defects from the Roy-Roy equation of state. Values of pressure derivatives of melting temperature have been calculated at elevated pressures to determine the rate of change of melting temperature with increase in pressures using the data of vacancy formation energy and effective volume of Schottky defects. The vacancy model revised in the present study takes into account the variation of bulk modulus with pressure, whereas in the Ksiazek and Gorecki model, it was treated constant. Results for pressure derivative of melting temperature are calculated for the solids under study. The melting curves have also been obtained and found to compare well with results based on molecular dynamics simulation and experimental data reported in recent literature.     

Evaluation of strain polarizability parameter in alkali halides

The strain polarizability parameter λ for alkali halides under the effect of hydrostatic pressure has been evaluated. The correlations of λ with fractional ionic character, effective ionic charge and the overlap repulsive energy have been pointed out.     

Relationship between thermoluminescence and X-ray induced luminescence in alkali halides

The wavelength spectra of thermoluminescence and X-ray induced luminescence in pure and divalent cation doped alkali halides, in the temperature range LNT—RT has been studied. The more important conclusion is that the wavelength spectra in both cases are very similar. This allows a new point of view to be presented on thermoluminescence mechanisms.     

Two photoplastic effects and their temperature dependence in alkali halides

The characteristic critical temperatures for the sign of the photoplastic effect (PPE) in KCl and KBr, using also photoconductivity measurements, were determined to be 110 K and 190 K, respectively. Negative PPE is explained in terms of elastic interaction between a moving dislocation and the relaxed excited state of the F-center.     

Chemical effects of carbon ion implantation into alkali halides

Abstract

The implantation of 230 keV carbon ions into alkali halide single crystals AX (A=Na, K, Rb, Cs; X=F, CI, Br, I) at temperatures T ? 77 K gives rise to intensive, highly resolved, optical absorption bands in the visible region (2.3-2.8 eV, peak half width ～0.004 eV). They resemble very much the Swan band system of matrix isolated C₂. The influence of the matrix on the fine structure of the absorption bands is discussed. These carbon-centres are formed to a lesser extent or not at all when implanting at 200 K and 300 K, respectively. They bleach out at room temperature. Radiochemical experiments at room temperature with carbon atoms stemming from the ¹¹C(ρ, α) ¹¹C-nuclear reaction in BN/AX mixtures show the formation of carbon compounds such as CX₄, CX₃H, CX₂H₂, CXH₃ and CH₄. Thus, the primarily formed C₂-aggregate or C-X intermediates which are stable at T ? 77 K, at higher temperature are gradually transformed to stable carbon-halogen compounds. The reactions of matrix isolated carbon upon dissolving the ionic crystals are of interest for the labelling of organic compounds with ¹¹C     

Exchange effects in ODESR of F center pairs in alkali halides

The electron paramagnetic resonance of F center pairs has been measured by means of an optical detection in the following alkali halide crystals at a microwave frequency of 35 GHz: KC1, NaC1, RbC1, KBr, KI and CsBr. Besides the ESR of the ground and excited states, a third resonance line is observed and is attributed to a pair effect due to an exchange interaction between nearest neighbour F centers. A model based on this assumption is developed.     

Dislocation damping in alkali halides

In the amplitude independent region the dislocation damping is attributed to either phonon-drag (Granato-Lücke theory) or to the compensating charge-cloud surrounding electrically charged dislocations (Robinson-Birnbaum theory). The experimental results for the dependence of the damping on temperature, frequency and dislocation charge are compared with the two theories. Since it is found that in some cases it is necessary to include both forms of damping, a more complete theory is developed which includes both terms.

In the amplitude dependent region the dislocation damping was thought to be due to the dislocations breaking away from pinning points or breaking through the compensating charge-cloud. Using the piezoelectric defect results for electrically charged dislocation in KCl the force-displacement hysteresis loop for the moving dislocations is determined together with the force-displacement curves for dislocations assuming phonon and charge-cloud damping. These results are found to be inconsistent with the “break away” models for amplitude dependence but instead to be consistent with the restoring force due to an elliptical compensation charge cloud, with a size proportional to the square root of the dislocation charge.     

Exciton states in alkali halides

Recent experiments based on modulation spectroscopy have shown that it is possible to detect exciton levels in alkali halides up to n = 4. Therefore we worked out numerical calculations in order to predict the whole exciton series in KI and RbI. In our calculations the deep exciton levels are treated by considering the actual hole-electron interaction, whereas the effective mass approximation is used for the shallow exciton levels. The direct and exchange terms of the hole-electron interaction have been evaluated by performing three and four center integrals, the Wannier wave functions appearing in such integrals being approximated by suitable gaussian expansions of atomic orbitals.It is shown that by allowing the exciton state to extend up to 42 shells of neighbors it is possible to predict the exciton levels up to n = 2, the n = 3, 4 excitons being accounted for by the effective mass approximation. Similar computations performed for excitons in solid rare gases were found in excellent agreement with the experimental data and confirmed the reliability of our method.     

Muon radiolysis in alkali halides

In order to shed new light on the initial loss of muon spin polarization, or socalled missing fraction, which is commonly observed in non-metallic solids, we have studied muon-induced excitation in various alkali halides by measuring the luminescences associated with the radiative decay of the self-trapped excitons (STE). The result strongly suggests that the spin-exchange interaction between muonium and muon radiolysis products including STE's causes fast muon depolarization in those materials.     

Radiolysis of alkali halides

This paper reviews the extensive work pertaining to the creation of colour centres in alkali halides. After a brief historical survey the various mechanisms put forth to account for defect creation are examined in turn and the experimental evidence which supports or belies each of these is presented. After demonstrating from published experimental work that the defects created are of the Frenkel rather than the Schottky type and after consideration of the early stage, extrinsic coloration process, the Valey mechanism and its variants are discussed. Next, the direct displacement model proposed by Balarin is considered and, finally, the recent excitonic mechanism proposed by Pooley and Hersh is examined in detail. The conclusion reached is that, although the excitonic mechanism appears to be much the more preferable model yet proposed, a number of points need careful consideration and experimental study before it can be accepted as having general validity.     

Size effects on the spectral thermal emissivity of alkali halides

The far-i.r. vibrational modes of LiF slabs and KCl microcrystals are determined by observation of the spectral emission. For the measurement of the emissivity a special sample compartment and a high-temperature black-body radiation standard have been developed for the spectral region 30–120μm.The observed resonance absorptions of LiF slabs on metallic substrates can be assigned to definite virtual modes. The spectra of the slabs agree well with theoretical calculations if the frequency dependence of the damping in the dielectric function is taken into account.Three different types of KCl microcrystals are investigated: cubes, octahedra and irregular crystals. The emissivity spectra of the three shapes show a dominant peak in the Reststrahlenband. This peak is nearly independent of the crystal type and of the covering density. In addition a further distinct resonance occurs near the TO frequency. The intensity of this mode depends on the covering density of the particles on the sample area. An explanation of this phenomenon is proposed. Besides these two marked peaks a secondary structure in the Reststrahlenband is observed. This structure is explained to some extent by a theory recently published by Fuchs.     

Muonium centers in the alkali halides

Muonium centers (Mu) in single crystals and powdered alkali halides have been studied using the high-timing-resolution transverse field μSR technique. Mu has been observed and its hyperfine parameter (HF) determined inevery alkali halide. For the rocksalt alkali halides, the HF parameter A_μ shows a systematic dependence on the host lattice constant. A comparison of the Mu HF parameter with hydrogen ESR data suggests that the Mu center is the muonic analogue of the interstitial hydrogenH _i ⁰ -center. The rate of Mu diffusion can be deduced from the motional narrowing of the nuclear hyperfine interaction. KBr shows two different Mu states, a low-temperatureMu ^I -state and a high-temperatureMu ^II -state.     

Defect parameters in the alkali halides

Abstract

Methods are available which calculate the free energies of defects in ionic systems within the quasi-harmonic approximation. The Harwell SHEOL code is used to calculate the defect enthaplies and entropies for a number of alkali halides where reliable diffusion data are known. We discuss the trends in the calculations and the comparison with experiment.     

Cathodo-luminescence spectra of alkali halides

Cathodo-luminescence spectra of fourteen alkali halides and of some mixed crystals of NaCl and KCl have been studied. The spectra show broad structureless bands. In some ten halides it is possible to find a strong band whose peak position shifts towards longer wavelengths as the inter-atomic distance increases. For pottasium halides the relationship λ_max=1380d, where “d” is the inter-atomic distance, holds approximately but is not generally true. A mechanism of cathodo-luminescence in alkali halides has been postulated, and an interpretation of the above band and of some NaCl bands has been made on that basis.     

Raman spectra of alkali halides

In this article, we present a comparative study of the Raman spectra of alkali halides in relation to the lattice dynamics ofBorn andRaman. It is shown that the experimentally observed limit of the second-order spectra in almost all the cases can be explained well by the Lyddane-Sachs-Teller relation. It is also seen, while, an explanation of the second-order Raman spectrum of a crystal of diamond or zinc blende structure requires the frequencies from the critical points,W, Γ, X andL inBorn's analysis, the frequencies fromΓ, X andL alone are sufficient and necessary for an interpretation of the same onRaman's model. Some similarities in the determination of the long wave properties of crystals like elastic constants and limiting frequencies of the lattice vibrations in the symmetry directions in both the models are pointed out.     

Perturbed Morse oscillator calculations of vibrational effects on quadrupole coupling in alkali halides

Formulas are obtained for the variation of the quadrupole coupling constant with vibrational state of an ionic diatomic molecule, using the pertubed Morse oscillator formalism. Results are apllied to NaF, KF, RbF, CsF and KCI, and compared with measured values. Systematic discrepancies are noted, which for the alkali fluorides increase with the atomic number of the alkali metal. Possible explanations include a variation of ionicity with vibrational states and a departure from 1/r³ dependencies of the electric field gradient. Calculations of dependencies of eQq on molecular rotation are also presented.     

Atomic hydrogen and muonium in alkali halides

Atomic hydrogen can be trapped at interstitial and substitutional cation and anion sites in alkali halides. The geometrical structure of these defects was established by electron nuclear double resonance (ENDOR). From the analysis of the ENDOR spectra also detailed information was obtained on the electronic structure. In this article the major experimental and theoretical results for atomic hydrogen in several alkali halides are briefly reviewed. Special emphasis is given to the isotope effects upon replacing hydrogen by deuterium. The nature of the dynamical hyperfine and superhyperfine interactions is discussed. Its magnitude to be expected for muonium is estimated. Recent results on muonium centres are discussed on the basis of the knowledge about the hydrogen centres.