Electron localization and the non-metal-metal transition in alkali-alkali-halide solutions

Summary We evaluate and extend an earlier proposal for a microscopic theory of the non-metal-to-metal (NM-M) transition which occurs on dissolving an alkali metal in it molten halide. The transition is viewed as involving a balance between the free-energy gain from the binding of valence electrons into localization centres and the excess free energy of the ionic assembly screened by the electrons. Using parameters estimated for solutions of potassium in potassium chloride and assuming that the elementary process of electronic trapping is the formation ofF-centre-like clusters, Thomas-Fermi screening by metallic electrons is shown to lead to a very sharp NM-M transition at a concentration in the range of 25–30% added metal. Thermally activated hopping of the localized electrons and the evolution of the localization centres with composition are next crudely taken into account by allowing for an additional contribution to the inverse screening length, which is estimated from the electronic localization length. This is shown to lead to a progressive break-up of the localization clusters, accelerating into a NM-M transition in the same concentration range. This simplified theoretical scenario is consistent with the available experimental evidence.     

Changes in work function due to charge transfer in chemisorbed layers

In a simple tight-binding model well suited to transition and noble metals, we calculate the work function change due to adsorption of alkali metals. A decrease is found to be caused by ionic bonds formed on the surface through charge transfer between the two metals. The local density of states and the number of electrons in the substrate and the chcmisorbed layer are evaluated for various alkali metals and coverages. The method is applicable in both the high and low coverage limits in contrast to existing theories.     

Pressure-induced phase transitions in some AnS (An = Th,U, Np,Pu) Compounds

Pressure-induced structural phase transitions at high-pressure in monosulfides of thorium, uranium, neptunium and plutonium (AnS) have been studied theoretically by an inter-ionic potential theory with modified ionic charge introduced to include the Coulomb screening effect due to localized ‘f’ electrons. These AnS compounds undergo a phase transition from sodium chloride (NaCl) to cesium chloride (CsCl) structure at a very high-pressure. The present theoretical investigation carried out up to 120?GPa reveals that these compounds undergo NaCl–CsCl phase transitions at 100, 81, 75 and 105?GPa for ThS, US, NpS and PuS, respectively. The first-order pressure derivatives calculated from the present theory agree well with observed data.     

Bipolarons in a KCl melt

A theory of two excess electrons in alkali halide melts is developed using variational estimates of path integrals. As a result of the strong screening, the average field generated by the ions has little influence on the electrons and the problem reduces to a study of a bipolaron type of free energy functional. The behavior of this functional is determined as a function of the thermodynamic and structural characteristics of the melt. Variational bipolaron calculations are made using the approximation of uncorrelated electrons and using Kohn-Sham theory to allow for electron-electron correlations. The results of the calculations using Kohn-Sham theory agree with the data obtained by quantum molecular dynamics and show that a correct choice of trial wave function which allows explicitly for the correlation of two electrons is required to obtain a correct estimate of bipolaron stability.     

Vapor-liquid (insulator-metal) phase transition in alkali metal vapors

A simple physical model is proposed that describes a vapor-liquid phase transition in alkali metal vapors. The model is based on an assumption made on the character of binding between atoms in the gas phase near the critical point. This is the collective quantum cohesive energy, well-known in the theory of liquid alkali metals, which arises due to the appearance of conduction electrons and is extended to the gas region near the critical point. The parameters of the critical points of the transition and of the binodal are determined on the basis of the model calculation of the binding energy for all alkali metals. Combined, these parameters well agree with experimental results and the predictions made by other authors. The minimum metallic conductivity is evaluated. Its behavior allows one to conclude that vapor-liquid and insulator-metal transitions in alkali metal vapors coincide. This fact sheds light on the Zel’dovich-Landau problem as applied to alkali metal vapors.     

The anion and cation effects in the magnetic anisotropy of rare-earth compounds: Charge screening by conduction electrons

The formation of magnetic anisotropy (MA) in rare-earth compounds with transition metals has been analyzed. The screening of the charges creating the crystal field by conduction electrons has been shown to play an important role. The calculations took into account the Friedel charge-density oscillations. The model used for RCo₅ is the point-charge crystal field including nonuniform screening by conduction electrons with an anisotropic Fermi surface. The mechanisms of strong MA due to light-element impurities (hydrogen and nitrogen) are considered. The effective charge of an impurity can heavily depend on its ionic radius and the characteristics of the Fermi surface (in particular, on the Fermi momentum k _F ) of the screening electrons. The screening of the cation and anion charge in hydrides and nitrides based on the R₂Fe₁₇ and RFe₁₁Ti intermetallic compounds is discussed.     

A new theory of compressible ions — structures of the alkali halides

Ions in ionic crystals are considered to exist in compressible space-filling polyhedral cells analogous to the Wigner-Seitz cell in metals. Repulsion arises from the compression energy of the ions written as a surface integral over the ionic cells. Two adjustable parameters are introduced per ion with the provision that the same parameters can be used in any crystal of any structure in which the ion occurs. The 18 parameters for the 5 alkali and 4 halogen ions have been determined from PV data on the 20 alkali halides. The important successes of the theory are: (i) All the twenty alkali halides are correctly predicted to occur in their observed structures (ii) The thermal transition in CsCl is explained (iii) The pressure transitions in the alkali halides are predicted well (iv) The calculated values of the variation of transition pressures with temperature agree well with experiment. These results are much better than those obtained by earlier theories.     

Thermal and non-radiative transitions of electrons at imperfections in ionic crystals

The theory of non-radiative and thermal transitions of electrons at imperfections in ionic crystals is elaborated. The results obtained are used to confirm Mott's and Gurney's theories on the detachment process in two steps of electrons from F centres during the absorption of light in the F band and to explain theoretically the temperature dependence of quantum efficiency. Good agreement with experiment was obtained. The possibility of the luminescence of alkali halide crystals containing F centres at sufficiently low temperatures was also determined.     

Magnetic 4d systems studied by implanted ions

By application of the perturbed -ray distribution method following heavy-ion reactions and recoil implantation techniques, we have found an experimental way of producing and investigating magnetic 4d states in metals. Strong 4d magnetism has been found for 4d ions in alkali metal hosts and in Pd hosts. In alkali metals, 4d ions reflect the phenomena of well-defined ionic ground states, orbital magnetism, mixed valence, and crystal field splittings smaller than theLS coupling. Magnetic 4d states in alkali metals cannot be described by one-electron approaches based on Anderson-type models, but requires an analysis in terms of many electron ionic configurations exhibiting basic features common to the physics of stable and unstable f stales in metals. In contrast, the local moment formation of 4d and 3d ions in Pd is governed by inter-atomic interactions of the magnetic d states with host d-band electrons, giving rise to spin magnetic behavior of the 4d impurity and to strong spin polarizations of the 4d electrons of the Pd host. Thus, the magnetism and electronic structure of 4d ions in metals exhibit qualitative differences in alkali metal hosts compared to Pd. The existence of magnetic 4d systems strongly depends on the 4d ion species and the host matrix, and on spin fluctuation rates or the corresponding Kondo temperatures. The results can be directly compared to theoretical work and also to the magnetic behavior of 3d ions in sp metal hosts and in hosts with d-band electrons.     

Ionic and temperature aspects of the photoresponse of metallic clusters

A calculation of the electronic response of alkali metal clusters is carried out in the Adiabatic Time-Dependent Local Density Approximation. The role played by the ionic structure is investigated in the framework of second-order pseudopotential perturbation theory. The calculations are carried out at different temperatures, and the effect of temperature in the decay of the collective excitations is analyzed. It is found that the coupling of the electrons to the thermal vibrations of the ions accounts for the width of the plasmon resonances. Presented at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.     

Critical behavior of dilute electrolyte solutions

A theory of the critical behavior of a dilute ionic solution is constructed. An expression for the susceptibility in a wide temperature range is obtained. It is shown that ionic solutions belong to the universality class of the Ising model. The Ginzburg parameter of the ionic solutions decreases with the increase of the solvent concentration. In the general case, the critical exponent of susceptibility nonmonotonically depends on the temperature in the crossover region from the Ising-like to the mean-field behavior. In the vicinity of the transition point, the Debye-Hückel screening radius is proportional to the correlation length. As T→T _c, the screening radius tends to infinity and the screening disappears. The voltage between the two phases of the ionic solution is proportional to the order parameter and changes as |T/T _c?1|^β in the vicinity of the phase transition point.     

Applying electrohydrodynamic atomization to enhance mass transfer of metal salts from an aqueous phase towards ionic liquids

In this work a two coaxial nozzles configuration is used to investigate whether electro hydrodynamic atomization of an ionic liquid in combination with an aqueous metal salt solution could enhance the removal of metal salts. The technique was evaluated for the removal of manganese (II), cobalt (II) calcium and sodium chloride. Good metal salt extraction was observed for the water-presaturated ionic liquid, tetraoctylammonium oleate, at an applied electrical potential of 5 kV, which was slightly lower compared to mechanical mixing, but a higher separation factor was obtained between the transition metals and the alkali and earth alkali metals.     

The effective ion-ion interaction of heavy alkali metals

The effective ion-ion interaction for the heavy alkali metals is calculated by a full non-local model potential, including exchange and correlation in the screening according to Vashishta and Singwi and in the RPA. The inclusion of the s-d hybridization effects merely following Harrison's theory for transition metals turns out to be inadequate for these metals.     

Electronic structure and positron annihilation in alkali metals: Isolation of ionic core contribution and valence high-momentum components

Momentum densities of annihilation pairs from valence as well as from ionic core electrons in alkali metals are calculated ab initio and compared with the experimental results. It is shown that the valence high-momentum components constitute a great deal (23–34% in Na-Cs and probably even more in Li) of the Gaussian part of the angular correlation curves. The average core enhancement factor γ_c ranges from 1.5 (Li) to 7.1 (Cs) and may be well expressed by a logarithmic function of ionic core polarizability. The presented values of γ_c are much higher than the core enhancement factors in the high-momentum (?15 mrad) region which, according to the recent theory of Bonderup, Andersen and Lowy, should not be very different from unity.     

Pressure induced phase transition in rare earth mono-antimonides

Pressure induced structural phase transition of mono-antimonides of lanthanum, cerium, praseodymium and neodymium (LnSb, Ln=La, Ce, Pr and Nd) has been studied theoretically using an inter-ionic potential with modified ionic charge which parametrically includes the effect of Coulomb screening by the delocalized f electrons of rare earth (RE) ion. The anomalous structural properties of these compounds have been interpreted in terms of the hybridization of f electrons with the conduction band and strong mixing of f states of Ln ion with the p orbital of neighbouring antimonide ion. All the four compounds are found to undergo from their initial NaCl (B₁) phase to body centered tetragonal (BCT) phase at high pressure and agree well with the experimental results. The body centered tetragonal phase is viewed as distorted CsCl structure and is highly anisotropic with c/a=0.82. The transition pressure of LnSb compounds is observed to increase with decreasing lattice constant in NaCl phase. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-Sb) distances at high pressure. The calculated values of elastic constants are also reported.     

过渡金属原子在离子晶体上的化学吸附

本文研究了一种描写过渡金属原子在离子晶体上的化学吸附模型。其中离子晶体用半无限A—B交替原子链代表,而吸附原子d轨道电子间的库仑排斥作用则用Anderson-Newns方法表述。对d轨道电子与晶体表面相互作用的几种不同耦合常数,用自洽格林函数方法计算了化学吸附能和吸附原子的电荷转移量。讨论了各种自洽解的存在条件和性质,得到了一些有趣的定性结论。 关键词：     

Hydroxide degradation pathways for guanidimidazolium cation: A density functional theory study

The chemical stability of anion exchange membrane especially in strong alkaline medium under high temperature is one of the crucial challenges for the commercialization of alkaline anion exchange membrane fuel cells. Herein, the degradation mechanism of guanidimidazolium ionic group used in anion exchange membrane was calculated by density functional theory method. The whole transition states of every elemental reaction were systematically searched, and the changes of crucial bonds length were further analyzed for understanding of the reaction mechanism. It was found that the reaction pathway of OH^? attacking guanidinium group of guanidimidazolium was the dominant way for the degradation of guanidimidazolium. Through comparing the energy barriers of guanidimidazolium, imidazolium, and guanidinium, it can be concluded that the electron‐inducing effect had an intimate connection with the alkali stability of ionic groups. The chemical stability of the conjugated structure groups decreased when linking with electron‐drawing groups, while it increased when linking with electron‐donating groups. This work provides theoretical basis for the design of decent alkali‐resistance and high‐performance anion exchange membrane.     

Lattice dynamics of ionic and covalent crystals

Lattice dynamics has some claim to being the oldest branch of solid state physics. Planck's Theory of Radiation and the Theory of Specific Heat was published by Einstein in 1907, On Vibrations in Space Lattices by Born and von Karman in 1912, and On the Theory of Specific Heat by Debye in the same year. Other early papers on lattice dynamics included those by Debye and by Waller on the effect of temperature op the scattering of x-rays by a crystal. In the 1920's Peierls made fundamental contributions to the theory of thermal conductivity and of electrical conductivity involving lattice dynamics, and in the 1930's Blackman brought to an end the acceptance of the complete validity of the Debye theory of specific heat and also contributed the first papers on anharmonic effects in the absorption of infrared radiation by ionic crystals. The work we have mentioned used the formal theory of lattice dynamics to account for the thermal and other properties of crystals. It was not until 1940 that Kellermann made the first moderately successful attempt to predict the dynamical properties of a crystal—sodium chloride—from something approaching first principles, and it was in the late 1950's that the development of the technique of neutron inelastic scattering made it possible for the first time t o acquire detailed knowledge of the spectrum of lattice vibrations of a crystal. The development of this experimental technique, pioneered by Brockhouse, was possibly the main impetus for the greatly increased activity in the subject at the present time. The formal theory of lattice dynamics in the harmonic approximation is now on a firm foundation, largely provided by the work of Born and his collaborators, and was recently reviewed by Maradudin, Montroll and Weiss.¹ Much recent work has been concerned with anharmonic effects and their influence on the thermal, dielectric, and radiation-scattering properties of crystals, and on the theory of the dynamics of imperfect crystals. The subject has become comparatively wide-ranging. In this review we shall concentrate on work where the ultimate objective is to understand the dynamical properties of a crystal in terms of the interaction between electrons and nuclei or, more realistically, between ‘ion cores’ interacting directly and through the valence electrons. This objective has been most nearly achieved for the alkali metals because of their relatively simple electronic structure, but we largely exclude metals from consideration since the subject, Lattice Dynamics of Metals, has recently been reviewed by Joshi and Rajagopal.² Kellermann's theory³ of the dynamics of sodium chloride involved interactions between point charges representing the ions, Born having already shown⁴ that the cohesive energy of alkali halides could be understood on this basis. The theory of the cohesive energy of ionic crystals has recently been reviewed by Tosi.⁵ We begin at this point in the development of the theory for nonmetallic crystals since many of the concepts in more recent work can be expressed in terms of those which apply to the simple point ion model of an ionic crystal. The dielectric and dynamical properties of ionic crystals are so intimately connected that we also briefly discuss the former.     

Optical properties of the surface transition-metal structures in fluoride ionic crystals and peculiarities of their formation during thermal diffusion

The possibility of forming surface films with an elevated concentration of an impurity metal during high-temperature diffusion has been analyzed for a wide series of ionic crystals: LiF with Co, Ni, Mg, Ca, Ba, and Sr impurities; NaF with Co, Mn, Mg, Ca, and Sr; MgF₂ with Co and Ni; and CaF₂ with Co. It is established that films are formed only on alkali halide crystals with impurities of transition metals and are not formed on alkaline earth fluorides with transition metals, as well as on alkali halide crystals activated with other divalent cationic impurities. The dynamics of the increase and decrease in the intensity of centers related to impurity-vacancy dipoles during thermal diffusion is shown. The mechanisms of film formation are explained in terms of the features of growth and structure of ionic crystals with cationic impurities and on the basis of isomorphism rules.