Halide-ion conductivities of the complexes of cryptand[2.2.2] with alkali-metal halides and alkaline earth metal halides

Halide ion conductivities of the complexes of cryptand[2.2.2] with alkali-metal halides or alkaline-earth metal halides were measured at 20–160°C. The halide ion conductivities of the complexes were found to be much higher than those of the crown ether complexes with alkali-metal halides or alkaline-earth metal halides. This is caused by the much weaker Coulomb attraction between metal-cation and halide ion in cryptand complexes than in crown ether complexes.     

卤化银微晶中光电子衰减谱的特性

光电子在卤化银材料潜影形成过程中发挥着重要的作用 ,光电子衰减行为在很大程度上决定于卤化银的晶体结构。采用光学与微波双共振技术 ,测量了自由光电子和浅俘获光电子的衰减谱 ,得到了卤化银中电子陷阱的密度和深度分布。以自由光电子寿命为纽带 ,通过分析掺杂卤化银晶体中电子陷阱的分布情况 ,可以确定浅电子陷阱掺杂剂的最佳掺杂浓度。     

X-ray and UV photoemission studies of the thallous iodide phase transformation

The orthorombic to simple cubic thallous halide phase transformation induced by temperature increase is studied by means of X-ray and UV photoelectron spectroscopy. The structural transformation is well evidenced by an abrupt decrease of the binding energy separation between the iodine and the thallium core levels at the phase transition temperature. Valence band spectra recorded for both crystal forms by using the HeI radiation evidence modification in the band structure.     

The second-order elastic constants of an AgBr-(56.6%) AgCl mixed crystal from 20°C to 400°C

Abstract

Initial measurements of the second-order elastic constants of an AgBr-(56.6%) AgCl mixed crystal have been made from 20—400°C. Over this temperature range, C'₁₁ decreased by 44%, C' by 71%, C₄₄ by 22%, C₁₁ by 53%, C₁₂ by 40%, and B _s by 46%. The decreases in the elastic constants are linear until approximately 280°C, whereupon all but C' begin to decrease more rapidly than linearly with variations from linearity of 4—20% at 400°C and C' remains linear at all temperatures. This anomalous behavior is similar to other silver halide physical properties and may be attributed to the unusually high defect concentration at high temperatures. Similar elastic constant changes are seen in superionic conductors near the superionic transition, indicating that the silver halides may be starting the transition to a superionic state when the halide sublattice melts and the transition is frustrated.     

Comparison of the interaction of Cl2 and Br2 with Cu(100)

The adsorption, reaction and etching of Cu(100) by Cl₂ was studied using temperature programmed desorption (TPD) and low energy electron diffraction (LEED), and the results were compared with recent results for Br₂. Although the general etching mechanism was the same for both gases (adsorption rate limited Cu halide formation followed by halide sublimation), significant differences between the behavior of Cl₂ and Br₂ were observed. The desorption of CuCl was characterized by a single zero order sublimation peak, independent of CuCl coverage, while limiting the CuBr coverage resulted in a desorption peak at temperatures lower than a prediction based on vapor pressure data of all known phases of CuBr. In addition, Cl₂ was found to be at least an order of magnitude less reactive than Br₂ towards halide formation. For both Cl₂ and Br₂, the halide formation rate reversibly decreased with increasing reaction temperature. However, for Br₂, but not Cl₂, annealing a chemisorbed halogen layer prior to further reaction irreversibly increased the halide formation rate. Structural differences between CuCl and CuBr on Cu(100) were also observed. For CuCl, LEED data suggested that highly faceted crystallites form at 325 K and remain stable until desorption, while LEED data for CuBr reveal a compressed epitaxial (111) layer that disorders below 400 K and then desorbs. The implications of these differences on etching and oxidation processes are discussed.     

混合卤化物中的极化子与双极化子

应用紧束缚双带模型对混合卤化物的电荷掺杂性质进行了计算,发现单电荷掺杂在混合卤化物中形成极化子,双电荷掺杂则形成稳定的双极化子;单体的长度和界面耦合都对极化子和双极化子的产生和稳定性有影响.混合卤化物的单体对掺杂电荷具有选择性;电子-电子相互作用并不能使形成双极化子的两个同号电荷相互排斥而形成两个单极化子.在具有电荷自发转移的材料中,混合卤化物单体对掺杂电荷没有像其他混合卤化物一样的选择性,掺杂电荷的定域位置取决于两种单体对该电荷的束缚能力. 关键词： 混合卤化物 极化子 电荷约束     

Halides‐Assisted Low‐Temperature Synthesis of Hexagonal Boron Nitride Nanosheets

Hexagonal boron nitride nanosheets (hBNNS) have various promising applications in dielectric and thermal conductive materials. However, in the bottom‐up method, well‐crystallized hBNNS could only be attained at very high temperatures. In this paper, a facile halide‐assisted hydrothermal assembly (HAHTA) method for the low‐temperature synthesis of well‐crystallized hBNNS is prepared. In HAHTA, the lateral growth of hBNNS is enhanced by the halide addition in the precursor, and the dimension reaches several micrometers. In addition, the obtained hBNNS enhance the oxidization resistance of vanadium dioxide (VO₂) though the formation of a VO₂/hBN composite, which is meaningful in facilitating the potential applications of VO₂ as smart phase transition materials.     

Paraelastic behavior of potassium cyanide

The low temperature Curie-type paraelastic behavior of low concentration anionic impurities dissolved in simple cubic alkali halide crystals has already been analysed for a number of cases. The CN⁻ radical in particular can substitute the halide ion in any concentration (up to 100% forming pure cyanide crystals), producing room temperature alkali halide-like crystals that above about 85% concentration show low temperature ferroelastic phases of crystallization. This paper presents the experimental evidence of room temperature paraelastic alignment exploring some analogies between elastic and magnetic (or electric) properties in the high concentration impurity range and above the phase transition (Curie) temperature.     

Effect of replacing calcium oxide with calcium halide on crystallization and some physical properties of calcium vanadium phosphate glass ceramics

The effect of halide ions on density, electrical, magnetic and crystallization kinetics for (20X-50P₂O₅-30V₂O₅) mole% has been investigated, where X=CaO, CaF₂, CaCl₂ and CaBr₂. Halide ions reduce the glass transition temperature, crystallization temperature and activation energy of crystallization. Density, electrical conductivity and magnetic susceptibility increase while molar volume, glass thermal stability and interatomic distance between transition metal ions decrease as the halide ions replace the oxygen ions in these glasses.     

Collisions of electrons with molecules in excited vibrational-rotational states

Results are presented of calculations of cross sections for scattering of electrons by diatomic molecules in specific excited vibrational-rotational states. The calculations were made using an approximation based on a quantum theory of scattering in a system of several bodies which can be applied to calculations of direct reactions and reactions involving the formation of an intermediate transition complex. Results of calculations of cross sections for collisions of electrons with hydrogen, nitrogen, lithium, sodium, and hydrogen halide molecules are compared with existing experimental data and the results of calculations made by other authors.     

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.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Stabilization of LiBH4 superionic phase by halide doping and H disordering

We performed first-principles density-functional theory calculations to investigate the structural properties and the effect of halide ion doping to the superionic-conducting, high-temperature phase and the effect of halide doping on the phase of LiBH₄. It is computationally demonstrated that the superionic phase is stabilized owing to the halide doping with the large ions which fill the interlayer space of the superionic phase. The H-disordered phase is observed in the structure and is found to contribute to the stabilization of the superionic phase.     

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.     

Auger-free luminescence due to interatomic p–d transition and self-trapped exciton luminescence in Rb2ZnCl4 crystals

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb₂ZnCl₄. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p–d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton.     

Regularities in the luminescence spectra of solutions with molecular anion impurities

This article deals with the absorption, excitation and luminescence spectra of frozen aqueous solutions of the halide salts and acids with molecular anion impurities CN^-, SCN^-, SeCN^- and CNO^-. It is shown that the spectra at 4.2°K display vibronic structure which is explained by the interaction of electronic transition with intramolecular oscillations of the impurity. Conclusions are also deduced from the results of investigation of temperature dependence of the quantum emission efficiency and the Raman spectra of such systems.     

Cesium halide photodissociation cross sections

The cross sections for the photodissociation of CsCl, CsBr and CsI have been measured using a cesium halide seeded shock tube. These measurements were made by using the ratio of two absorption signals: one in the incident and one in the reflected shock region. The cesium halide was the absorption species in the incident shock region and a halogen negative ion species absorbed in the reflected shock region. Previously measured and calculated halogen negative ion photodetachment cross sections were used in calculating the cesium halide photodissociation cross section. Measurements were made between 4000 and 2000 Å at 1900°K.     

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.     

The pumping efficiency of copper halide lasers

A simple relation between the copper halide laser output and the densities of ground state (²S) and metastable state (²D) copper atoms was found. This relation was used to analyze the experimental results of Nerheim. In all the analyzed systems it was found that about four percent of the ground state copper atoms take part in the lasing process. This may have implications for the efficiency of copper halide lasers.     

Structural phase transitions of the B1–B2 type in small-sized ionic crystals

The features of structural phase transitions that occur under high pressure are studied. The density functional theory is used to calculate the B1–B2 phase transition pressure as a function of the crystal size for small-sized alkali halide crystals. A size effect (an increase in the phase transition pressure as the crystal grain radius decreases) is revealed for the B1–B2 transitions in all halogen compounds except for lithium fluoride, for which the dependence is inverse.