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.     

Calculations of Nonlinear Refractive Index of Alkali Halides

Theoretical values of the nonlinear refractive index n₂ of 16 rock salt-structure alkali halides are presented. The approach employed is based on the Phillips–Van Vechten interpretation of the bond polarizability model. Our results show that in halides with the same cation a monotonic, strong increase in n₂ with the linear refractive index can be observed.     

Dielectric properties of evaporated films

The evidence for vacancy-pair formation in alkali halide crystals is reviewed. Existing information on the dielectric properties and structure of thin films is discussed with a view to using the high defect concentrations in vacuum-deposited films to confirm vacancy-pair orientation effects. Experimental measurements of the dielectric constant and dissipation factor of evaporated films have been made at room temperature over a range of frequencies from 100 c.p.s. to 100 kc/s. Even under vacuum these films show pronounced losses at low frequencies which are attributed to excess vacancy concentrations. Such losses are contrary to the theory of ideal lattices and are not shown by single crystals.

In spite of the excess defect concentrations no evidence of vacancy-pair orientation has been found. Ageing effects have been observed in all cases, the losses decreasing with time as the defect concentration decreases. In the alkali halides the magnitude of the losses depends on the cation mobility but they decrease at a rate determined by the anion mobility. This leads to a proposed dielectric relaxation mechanism in which individual crystallites form the polarizable units, becoming polarized by migration of the cation vacancies towards one end. The losses decrease as the defects are gradually eliminated by simultaneous condensation of positive and negative vacancies at grain boundaries. This is essential to maintain electrical neutrality and the rate is determined by the diffusion of the slower anion vacancies. The measured rates are in agreement with anion activation energies obtained by tracer methods. These results cannot be explained by vacancy-pair formation even if it is assumed that vacancy pairs can form but are incapable of orientation and hence we must conclude that there is little or no vacancy-pair formation.

Measurements at atmospheric pressure show that moisture has a pronounced effect in all cases, producing dielectric losses which completely obscure the vacancy effects. The changes in dielectric properties during and after adsorption cannot be explained as conductivity effects and are in complete opposition to any modification of the Maxwell-Wagner theory. The ageing effects show that after the initial adsorption, the water molecules migrate over crystallite surfaces to positions where they are more strongly bound and contribute to the dielectric polarization by a form of hindered rotation closely analogous to mechanisms proposed for ice. At these equilibrium sites, H-bonds are frequently formed between the adsorbed molecules and the halide ions of the crystal lattice.     

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.     

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.     

Surface passivation in n-type silicon and its application in silicon drift detector

Based on the surface passivation of n-type silicon in a silicon drift detector(SDD), we propose a new passivation structure of SiO2/Al2O3/SiO2 passivation stacks. Since the SiO2 formed by the nitric-acid-oxidation-of-silicon(NAOS)method has good compactness and simple process, the first layer film is formed by the NAOS method. The Al2O3 film is also introduced into the passivation stacks owing to exceptional advantages such as good interface characteristic and simple process. In addition, for requirements of thickness and deposition temperature, the third layer of the SiO2 film is deposited by plasma enhanced chemical vapor deposition(PECVD). The deposition of the SiO2 film by PECVD is a low-temperature process and has a high deposition rate, which causes little damage to the device and makes the SiO2 film very suitable for serving as the third passivation layer. The passivation approach of stacks can saturate dangling bonds at the interface between stacks and the silicon substrate, and provide positive charge to optimize the field passivation of the n-type substrate.The passivation method ultimately achieves a good combination of chemical and field passivations. Experimental results show that with the passivation structure of SiO2/Al2O3/SiO2, the final minority carrier lifetime reaches 5223 μs at injection of 5×10¹⁵ cm^-3. When it is applied to the passivation of SDD, the leakage current is reduced to the order of nA.     

Chemically abrupt interface between Ce oxide and Fe films

A chemically abrupt Fe/Ce oxide interface can be formed by initial oxidation of an Fe film followed by deposition of Ce metal. Once a Ce oxide layer is formed on top of Fe, it acts a passivation barrier for oxygen diffusion. Further deposition of Ce metal followed by its oxidation preserve the abrupt interface between Ce oxide and Fe films. The Fe and Ce oxidation states have been monitored at each stage using X-ray photoelectron spectroscopy.     

Improvement of hot-dip zinc coating by enriching the inner layers with iron oxide

The performance of hot-dip galvanic coating formed on steel not only depends on the alloy composition of the superficial layer but also significantly, on the composition of the inner alloy layers at the coating/substrate interface. Further, the presence of barrier oxide layers, if any can also improve the performance of galvanic coating. In the present work, the effect of inner iron oxide barrier layer formed prior to hot-dip galvanization was investigated. A continuous and adherent iron oxide layer was formed on steel by anodic oxidation of the steel substrate. Although the wettability of oxide surface by liquid zinc was initially poor, the increase in dipping time and the transition of the oxide layer to unstable form due to the presence of Cl⁻ ion in the flux facilitated localized growth of Fe-Zn alloy phases. The inhibitive nature of the oxide layer was temporary, since the presence of Cl⁻ induces micro cracks on the oxide surface thereby facilitating better zinc diffusion. The modification of the substrate structure during galvanization was found to influence the galvanizing process significantly. The present study predicts scope for application of this process for protection of rusted steel specimens too.     

Quantum diffusion of muonium in insulators

In this paper we review our recent experiments conducted at TRIUMF on muonium diffusion in alkali halides. First, the technique of longitudinal-field muonium spin relaxation (T ₁) due to nuclear hyperfine interaction, an indispensabletour de force for the present work. is described. It is demonstrated in KCl that the technique provides spectacular sensitivity for muonium diffusion as well as determining the average nuclear hyperfine coupling constant. The muonium hop rate shows a minimum (T ^*≃80 K) and steep increase with decreasing temperature. The result is compared with the current theory of quantum diffusion in non-metallic crystals. A few more sets of new data may be presented for other alkali halides. In addition, we show that muonium forms a delocalized state in NaCl as evidenced by a large change of the average nuclear hyperfine parameter. Related topics of local tunneling system may be briefly reviewed.     

Specific features of the growth and stability of nanowires made of different metals

Ensembles of metal nanowires are prepared by the template method. The influence of electrodeposition conditions on the properties of the obtained samples is studied for some metals and alloys. It is shown that the observed change in the electrodeposition rate during electrolysis can be related to a change in the electrode potential as a result of an increase in the gradient potential in the near-electrode solution region and to diffusion limitations. A number of specific features of the deposition of iron, iron–cobalt alloy, and iron–nickel alloy are revealed (in particular, the dependence of the elemental composition of the obtained wires on the electrodeposition conditions). Both the topography of the nanowires and their stability are investigated by electron microscopy. It is found that the nanowire form does not always correspond to the form of growth pores. Nanowires exhibit rather high stability in alkali solutions and low stability in acid solutions.     

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。     

Vacancy pairs in sodalite

Sensitized sodalite is thought to contain paired anion and cation vacancies in the interstices of the alumino-silicate framework. Using an extension of the Born model to describe the potential in an interstice, the equilibrium configuration of a vacancy pair is calculated and the similarity between the anion site in the alkali halides and sodalite is discussed. The vacancy pair is shown to possess an electric dipole moment; hence it should exhibit dielectric loss.     

Diffusion-Limited Aggregates of Poly(Ethylene Oxide) on Alkali Halides

Poly(ethylene oxide) (PEO) was crystallized on alkali halides, such as NaCl, KCl, and KBr, from solution in isopentyl acetate. PEO dendrites first formed on alkali halide surfaces and grew to sheet the whole surface. Individual fibrils consisting of dendrites were arranged with the preferred orientation in the <110> direction on KBr and KCl, and mainly in the <100> direction on NaCl. Tetragonal lamellae grew not on the substrate directly but on the sheet platelet only, establishing a homoepitaxial relationship to the dendrites. Because the dendritic sheet that intervened between them controlled the orientation of the lamellae, tetragonal lamellae formed with a preferred orientation on alkali halide surfaces. The dendrites had morphological features of fractals, i.e., diffusion-limited aggregates (DLA). Their morphology was controlled by the diffusion process of polymer molecules on the substrate surface, as well as by epitaxial interaction. The fractal dimension (D) was obtained by analyzing the dendrites by taking the autocorrelation function of density: D ranges from 1.6 to 1.8, depending on the crystallization conditions, such as the kinds of substrate and crystallization temperature.     

Investigations into the spectral and luminescent properties of methylphenols in neutral and ionic forms in aqueous micelle solutions

The special features of the absorption and fluorescence spectra of some substituted phenols (2-, 4-methylphenol and 2-amino-4-methylphenol) in aqueous micelle solutions are investigated on addition of an alkali and acid. A fluorescence band of 4-methylphenol in the anion form is detected in the presence of a cation surfactant. The fluorescence efficiency of 4-methylphenol in the presence of the triton X100 micelle decreases in comparison with aqueous solutions. It is established that the quantum yield of fluorescence of 2-methylphenol in the presence of N-cetyltrimethyl ammonium bromide (CTAB) increases in comparison with the aqueous solution (ϕ_fl = 0.25 and 0.17, respectively). The constant of fluorescence quenching for methylphenol in aqueous micelle solutions by the alkali is two orders of magnitude higher than by the acid. The proton-acceptor properties of 2-amino-4-methylphenol are higher than of 4-methylphenol. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 54–61, November, 2005.     

Corrosion processes and their inhibition as studied by Mössbauer conversion and other electron spectroscopies

To study corrosion processes of iron and steel and measures of their inhibition, a detailed knowledge of the phase composition and of phase transformations in very thin layers close to the attacked surface of the material is necessary. The information depths of integral (ICEMS) and depth selective (DCEMS) conversion electron Mössbauer spectroscopy are well suited for such investigations, but some effort is necessary if technical samples, i.e. nonenriched in⁵⁷Fe, are to be studied. In many cases of practical importance, full information on the corroded surfaces cannot be got from Mössbauer spectra only, and a combination with Auger and photoelectron spectroscopies, in-including scanning and sputter options, is found to be most informative. This is demonstrated by three examples. The high corrosion resistance of stainless steel X1 CrNiSi 18 15 against boiling HNO₃ is found to be due to a SiO₂ layer formed on the surface and growing with duration of exposure. Nearly no iron oxides are formed, although the composition of the alloy close to the metal surface is slightly influenced. Transformer sheet steel (Fe?3%Si), commercially available after a thermal pretreatment, is covered by an insulating layer containing iron oxides and a large amount of Fe₂SiO₄. By the method combination it was found that a very thick layer below the metallic surface is also modified by the pretreatment. There, one finds pure α-iron containing clusters of SiO₂ which have been formed by internal oxidation. The passivation of iron and steel (DIN 1623) in sulphate solution and in a phosphate buffer was studied in detail. Phase composition and thickness of the passive layer, as thin as a few nm only, were analyzed in dependence on the applied anodic potential and the duration of the passivating procedure. From the experiments, a model of the passive layer was derived, which is a modification of a p-i-n junction proposed elsewhere. The real passivation is ascribed to a layer, only a few monolayers thick, which has a highly disordered structure and provides the transition from cubic to orthorhombic structure. Some thermodynamical considerations show that immediately on an iron substrate, an oxide layer should always contain a high concentration of Fe²⁺ forming an equilibrium oxide. The absence of Fe²⁺ in Mössbauer spectra is interpreted by the assumption that this layer may be extremely thin, i.e. a few monolayers only.     

First-principles study on β-GeS monolayer as high performance electrode material for alkali metal ion batteries

Based on the density functional theory calculations, we have investigated the feasibility of two-dimensional β-GeS monolayer as high-performance anodes for alkali metal ion batteries. The results show that the electrical conductivity of β-GeS monolayer can be enhanced after adsorbing the alkali metal atoms owing to the semiconductor-to-metal transition. The low diffusion barriers of alkali metal atoms on the β-GeS surface indicate a rapid charge/discharge rate without metal clustering. Moreover, the low average open-circuit voltage (0.211 V) and a high theoretical capacity (1024 mAh·g^-1) for Na suggest that the β-GeS monolayer is a promising anode material for Na-ion batteries with high performance.     

Revisited vibronic model for Li+ ion and F A (Li) centre in alkali halides

Summary The controversial far-infrared optical absorption in Li-doped alkali halides is now attributed to transitions between the low-lying energy levels of an anharmonic oscillator composed of three halogen-ion pairs centred at the cation site which vibrate in theT _1u ungerade mode. This oscillator is electronically coupled to the impurity dragging it into rotation over 〈111〉 sites. Several experimental features derive simply from our model such as the absence of any clear-cut lithium-isotope effect and the low-frequency range. The capital effect of an F centre at a 〈001〉 nearest-neighbour (nn) site is to reduce the dimensionality of vibration forcing the impurity to rotate in the perpendicular (001)-plane. This upgrades the optical frequency in proportion to (3/2)^1/2 and shrinks the off-centre displacement. We finally show that recent computer-simulation data on Li-doped alkali halides provide a solid ground for regarding the Li off-centredness as a pseudo-Jahn-Teller phenomenon. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.