Mechanisms of spin-lattice relaxation of vk and related centers

An experimental and theoretical investigation of the spin-lattice relaxation (SLR) mechanisms of V_K-centers in alkali and alkaline earth halides has been carried out. It has been shown that at low temperatures the main role in SLR is played by rapidly relaxing centers (RRC). In some cases, however, an intrinsic one-phonon SLR process due to modulation of both the isotropic and anisotropic hyperfine (HF) interaction seems to be important. At higher temperatures SLR can be explained by an anharmonic Raman process with the participation of the resonant molecular vibrational (RMV) mode of the V_K-center. In this SLR process, similarly to the low-temperature intrinsic process, the spin-phonon coupling is due to the modulation of the HF interaction. Deviations from the Debye phonon spectrum and the local anharmonicity of V_K-centers turn out to be important. The SLR of several other hole and interstitial type centers in alkali halides has also been investigated and their SLR mechanisms are discussed.     

Farbzentren in Ammoniumhalogeniden

Color centers produced by X-rays in ammonium halides at various temperatures between 20°K and room temperature have been investigated by means of paramagnetic resonance and by optical methods. Two kinds of paramagnetic defects were found to be predominant, the self-trapped hole (V _K-center) and another electron deficiency center involving a NH₃ ⁺-radical. The electronic structure of theV _K-center is the same as in the alkali halides, except that the orientation of the molecular axis is along [100] instead of [110]. The kinetics of the thermally activated motion of theV _K-centers and of their recombination with electrons has been studied. The electronic structure of the second center was derived from the hyperfine spectrum of the paramagnetic resonance. The rotation of the NH₃ ⁺ ion and its connection with the order-disorder transition in NH₄Cl has been studied.     

Optical transitions of Ag− centers in alkali halides

Ag^? centers in alkali halides give rise to a strong absorption band in the 300 nm region (formerly called “B band”). Its resolved triplet structure in CsCl suggests that it corresponds to the C band of the isoelectronic In⁺ center. Two very weak bands are found in several alkali halides in the 400 nm region. These new bands are assigned to the A and B transitions of the In⁺-type centers. This is supported by the doublet structure in the A band, and by the temperature dependence of the oscillator strength of the B band. In KCl∶Ag^? the ratios of the oscillator strengths are found to bef _c /f _A =610 andf _c /f _B =3,400 at low temperatures. The energy parameters of Ag^? centers are computed and compared with those of others ²-type centers. The electron-lattice coupling parameters are estimated from the Jahn-Teller splitting of the C band in CsCl and of the A band in KC1. The temperature dependence of the lifetime of the visible fluorescence suggests that a metastable state is involved in the emission process after a C band excitation.     

Thermostimulated recombination processes in LiBaF3 crystals

The creation of radiation defects in LiBaF₃ crystals at 10 K and the processes of their thermostimulated recombination are investigated. The methods of optical absorption, thermal bleaching of color centers, thermostimulated luminescence and fractional glow technique are used. The radiation defects anneal in a multi-stage process accompanied with thermo-luminescence at 20, 46, 105, 130, 170, 210 and 270 K. Differences in the optical absorption spectra measured before and after the TSL peaks are obtained and recombination parameters are determined. The TSL peak at 20 K arises from the delocalization of H-centers. The presence of two TSL peaks related to V_K-centers at 105 and 130 K indicates that 60° and 90° migration hops occur.     

Diffusion measurement of implanted Sb into Si,using SiO2 encapsulation

Large, asymmetric atomic relaxations have been shown to play a crucial role in the structure and properties of several point defects in oxide materials. Examples include trapped hole centers in alkaline-earth oxides and E1′ and E4′ oxygen-vacancy centers and peroxy-radical defects in silicon dioxide. Schirmer's “bound small polaron” model, applied in particular to the alkaline-earth oxide defects, and model treatments of the E1′ center in SiO2 by Yip, Griscom and Fowler clearly illustrate the important spectroscopic consequences of such atomic relaxations. In fact, such effects had been incorporated in Lüty's classic model of the Type II FA center in alkali halides. Edwards and Fowler have recently applied MNDO and MINDO/3 quantum-chemistry approaches to the E1′, E4′, and peroxy radical defects in SiO2. These calculations generally corroborate suggested models and bear as well on possible creation mechanisms. Large relaxation effects are likely to be important in many other defects in oxide materials.     

Analysis of melting based on the diffusional force theory for alkali halides

The theory of melting based on the concept of diffusional force given by Bosi is used for studying the melting of alkali halides. Values of thermal expansivity and the Anderson-Gruneisen parameter are used to predict the interionic separations in 16 alkali halides at melting temperatures with the help of the Anderson formula. A model for melting is developed by estimating the diffusional force from the knowledge of interionic potentials based on ultrasonic data for bulk modulus and its temperature and pressure derivatives. The model thus developed is found to yield satisfactory results in agreement with the experimental data on melting.     

Phonon induced tunneling of ions in solids

A quantum mechanical analysis is given of the change of position or orientation of an atom, ion or molecule in a crystal as it occurs e.g. in the processes of diffusion or hindered rotation. One-phonon processes, Raman processes and indirect processes are discussed. The limits of applicability of the classical rate theory are given. Specifically, the analysis is applied to the reorientation of the O₂- center in alkali halides under the influence of external mechanical stresses at low temperatures, which has been investigated experimentally byKänzig. The measured dependence of the reorientation time upon temperature and applied stress can be explained by one-phonon processes. Random internal strains in the crystal are shown to play an important part. As further application of the theory the proton motions in ice and in iron are elucidated. Finally, an estimate is given of the effect of direct processes involving imperfections on the thermal conductivity of alkali halides.     

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.     

碱金属卤化物键长、德拜温度和热膨胀系数的温度效应

以碱金属卤化物为研究对象,根据局域键平均近似方法,从热膨胀系数的定义出发,建立热膨胀系数和键长的温度效应函数表达式。定量获取碱金属卤化物的键长、刚性因子、德拜温度和热膨胀系数。结果表明：①碱金属卤化物的热膨胀系数与结合能成反比;②同类碱金属卤化物中,从氟碱化物到碘碱化物德拜温度逐渐减小而热膨胀系数逐渐增大;③碱金属卤化物的键长在其德拜温度的三分之一处,由非线性转变为线性热伸长。     

Absorption Spectrum Study on Colloidal Centers in KCl:NCO:K

The absorption spectrum study on metallic colloidal centers in KCl:NCO:K crystals have been performed as a function of annealing temperature in the temperature range of 500–750K. Conversion process between F-centers and colloidal centers was observed and it is found that the NCO ions play an important role in the formation and stability of aggregative centers in alkali halides.     

Morphology of alkali halide thin films studied by AFM

Thin layers of alkali halides were investigated by atomic force microscope (AFM). The studied systems were: LiBr/KBr(0 0 1) with −16.7% misfit, LiF/Si(0 0 1) with +4.4% misfit, LiBr/LiF(0 0 1) with +36.8% misfit and NaCl/Si(0 0 1) with +46.5% misfit. The results show that the surface morphology strongly depends on the temperature of layer formation. The alkali halides deposited on the foreign substrate at elevated temperatures or at room temperature and subsequently annealed form preferentially 3D islands leaving uncovered substrate areas between them. It is suggested that Ostwald ripening takes place at elevated temperatures.     

The three reorientation processes of O-2 centres in unstressed alkali halide crystals

Measurements of the reorientation of O^-₂ centres in RbI and RbBr at low temperatures together with earlier measurements in different alkali halides suggest the following sequence of relaxation mechanisms in an unstressed srystal: Below about 3°K a one phonon tunneling process, then an activated tunneling process through the first librational state, and above 10°K a classical jumping process over the potential well.     

Ranges and range theories

Irradiation of ionic crystals causes the displacement of lattice ions and the formation of primary defects in the form of vacancies and interstitials. At high temperatures when these defects are mobile secondary defect reactions will produce various types of defect clusters. In some compounds clustering can lead to the formation of small particles of the metal constituent, referred to as colloids. A well-known example of this effect occurs in the alkali halides, where the colloids form as the result of large-scale aggregation of the primary F centres, so that the metallic region in this case derives from primary defects on the anion sublattice. The latent image of the photographic process in silver halides is also an example of the formation of a small metal colloid, and other crystals such as hydrides and azides can also be partially decomposed into metallic particles by irradiation with ionizing radiation. Recently metal colloids have been found as a result of displacement damage in the oxides Li₂ and Al₂O₃. This article reviews some of the background properties of colloids in ionic crystals and describes some examples of colloid formation by irradiation. Colloid growth in NaCl is described in more detail, since recent experimental and theoretical work provides a more complete picture than in other compounds. The Jain-Lidiard theory explains many features of the behaviour observed during high dose irradiation at high temperatures, and some comments are made about ways in which the theory could be developed further.     

Anion vacancy centers in alkaline earth oxides

For many years, study of the defect state of oxide crystals was overshadowed by activity in the alkali halides. However, during the last decade, work on oxides has emerged as an important field of study in its. own right. This burgeoning of interest came about for many reasons. First the properties of defects in the alkali halides became well understood, and it was natural for workers in this field, to search for new pastures. Secondly, reliable supplies of good? quality single crystals became more readily available. Furthermore, a greater understanding of band structure, lattice dynamics, and mass transport became apparent, and concerted efforts were made by various researchers to introduce defects into oxides in a controlled manner.     

Detection of defects binding free polarons in colored alkali halide crystals

The assumption has been made that defects binding free polarons in colored alkali halide crystals are F'-center, i.e., defects that slow down the motion of dislocations (photoplastic effect). This assumption has been confirmed by the experiments performed in this study. Thus, the anion vacancy in alkali halide crystals at a low temperature can capture three electrons: two electrons at a deep level (F'-center) and one electron in a bound polaron state. This electron is retained due to the energy gain in the interaction of a local deformation of the polaron and a local deformation surrounding the F'-center, despite the presence of the Coulomb repulsion.     

Sputtering of molecules during low-energy electron bombardment of alkali halides

The emission of molecules during bombardment of several alkali halides with a 540 eV electron beam has been investigated. Using a time of flight method the energy distributions of the halogen molecules have been measured at various temperatures of the target. The relative halogen molecule to atom ratio has also been examined as a function of the target temperature. It has been found that the molecules were formed from the atoms at the alkali halide surface. This process and the subsequent desorption of the molecules account for the experimental results.     

Absorption bands of lead in mixed KCl + KBr crystals

The absorption spectra of mixed KCl + KBr crystals with lead added were measured. The main attention was devoted to A bands and their fine structure. Measurements were made at different temperatures. The results are discussed from the point of view of the existence of lead complexes in crystals of alkali halides.     

The resonant secondary emission of the F center in alkali halides

The secondary radiation after resonant excitation of F center and its linear polarization P correlated to the polarization of resonant light have been measured for five alkali halides at low temperatures. In KC1, the spectrum of P over the whole Stokes region is divided into three successive regions, the depolarization range at the one-phonon Raman scattering, the near plateau range, and the depolarization range down to vanishing. The former two have common relevance to resonant energy and symmetries of coupled phonons. These relevances are interpreted adopting a configuration coordinate model for 2s- and 2p-like excited states.     

Theoretical Studies of g Factors and Defect Structures for Cubic, Tetragonal, and Orthorhombic Fe+ Centers in Alkali Halides MX (M=Li,Na;X=F,Cl)

The EPR g factors for cubic, tetragonal and orthorhombic Fe centers in alkali halides MX (M = Li, Na;X = F, Cl) are calculated from second-order perturbation formulas of g factors based on cluster approach for 3d7 ions in three symmetries. From calculations, the g factors of these Fe centers in MX crystals are reasonably explained and the defect structural data for the tetragonal and orthorhombic Fe centers are estimated. The results are discussed.