Microhardness studies in ammonium halide crystals

Microhardness studies of NH₄Cl (pure and doped), NH₄Br and alkali halide crystals are presented. The hardness of ammonium halides is found to be less as compared to alkali halide crystals. Doping NH₄Cl crystals with copper (Cu²⁺) is found to increase the hardness enormously and the results obtained with various concentrations of copper are presented. The results have been analysed and the various factors contributing to the increase in hardness at lower loads have been discussed.     

Tensile plastic strain localization in single crystals of austenite steel electrolytically saturated with hydrogen

The effect of interstitial hydrogen atoms on the mechanical properties and plastic strain localization patterns in tensile tested Fe-18Cr-12Ni-2Mo single crystals of austenite steel with low stacking-fault energy has been studied using a double-exposure speckle photography technique. The main parameters of plastic-flow localization at various stages of the deformation hardening of crystals have been determined in single crystals of steel electrolytically saturated with hydrogen in a three-electrode electrochemical cell at a controlled constant cathode potential.     

Micro-macro property correlations in alkali halide crystals

Correlations are developed between the microscopic parameters, Debye-Waller factor and lattice constant and the macroscopic properties, melting temperature and hardness of alkali halide crystals with NaCl structure. Proper equations are proposed and physical justification is provided for the observed correlations. Further, these properties are correlated with the force constant calculated from the compressibility and these correlations are, again, shown to have physical justification. Finally, a kaleidoscopic relationship is shown to exist between all these properties.     

Effect of growth temperature gradients on residual stresses in halides

Thermal gradients experienced by alkali halide crystals during growth by the Czochralski process were measured. The optical homogeneity of the crystals was measured interferometrically. Refractive index variations are shown to result from the temperature gradients during growth which produce thermoelastic strains that are quenched and result in a stressed crystal. Minimization of thermal gradients is shown to produce crystals that have better optical uniformity.     

Calculation of Interaction Parameters from Immiscible Phase Diagram of Alkali Metal or Alkali Earth Metal-Halide System by Means of Subregular Solution Model

1. IntroductionBeside the information of phase transition andphase structure in Hanson'S model of phase diagram,that of metastable phase and thermodynamics of systems should be involved in the document of theirphase diagrams, and the phase boundaries should benumerical. In the previous paper[1] 1 by using the subregular solution model the calculation of activity coefficieflt from a binary alloy phase diagram involvingtwo coexisting phases has been introduced. The purpose of this paper is devo…     

Electron transfer and intershell spectroscopic parameters of Pb2+: Changes when lead halide films interact with alkali halide substrates

UV spectral data for Pb²⁺ highly diluted in alkali halide crystals and in lead halide thin films are shown to be consistent in the context of current inorganic spectroscopy. For Pb²⁺ in (i) alkali halides and (ii) lead halides the optical electronegativities are 1.2 and 1.4 respectively, while for the ¹S₀→³P₁ frequency (v_s?p) relationship (v_s?p = v_f?v_fmh where h is the nephelauxetic parameter of the complexing halide ion), the values of v_f are 60 700 cm^-1 and 64 400 cm^-1 and the values of m are 0.20 and 0.22 respectively. The results indicate that the modified Pb²⁺ ion model used for Pb²⁺ ions in alkali halide crystals can be extended to Pb²⁺ halide salts. These rationalizations indicate the usefulness of UV spectroscopic measurements for studying the penetration of lead halide thin films into crystalline alkali halides. Spectral changes occuring when sandwiches of lead halide films between NaCl and KBr substrates are maintained at 300°C indicate substantial evaporation of the lead halide film and preference of Pb²⁺ to diffuse into KBr rather than into NaCl.     

Non-linearity in NaCl single-crystal films

Epitaxial Ag/NaCl/Ag capacitors have been fabricated on hot NaCl substrates and their dielectric properties have been measured. Only one loss peak in the relaxation spectrum of the epitaxial films has been observed, which has been attributed to the blocking of cation vacancies at the electrode-dielectric interfaces. The activation energy associated with the loss peak has been calculated, and is of the order of 0.83 ± 0.1 eV. The charge carrier concentration has been estimated to be about 10²⁴ m^?3. The increase in capacitance observed at low frequencies suggests that the formation of space charge is responsible for the polarization in alkali halides.A strong non-linearity is associated with the polarization and has been observed in the form of variations of dielectric loss and capacitance in alkali halide films. Harmonic generation and cut-off in the current through the alkali halide epitaxial capacitors have been observed. All these observations imply that the space charge polarization mechanism is predominant in alkali halide films. A built-in potential of several hundred millivolts across the freshly prepared epitaxial capacitor has been observed, and this causes an asymmetry in the electrical behaviour. No evidence of vacancy pair orientation has been found and the results have been attributed to the migration of cation vacancies through the whole thickness of the epitaxial film with subsequent blocking at the electrodes to form a space charge.     

Surface controlled crystallization of poly(tetramethylene terephthalate)

Epitaxial crystallization of poly(tetramethylene terephthalate) from dimethyl phthalate solution on alkali halide crystals has been studied using electron microscopy and electron diffraction techniques. Rod-like crystals oriented in the 〈110〉 directions of the substrate were found to form at short crystallization times. At longer times, however, lamella-like crystals, which were nucleated at the sides of the rod-like crystals, grew in directions dictated by the substrate. Besides the normal bidirectional orientations induced by the substrate, the polymer crystals showed twinning which was found to be due to the triclinic lattice structure of the polymer.     

Features of plastic strain localization at the yield plateau in Hadfield steel single crystals

Spatiotemporal distributions of local components of the plastic distortion tensor in Hadfield steel single crystals oriented for single twinning have been studied under active tensile straining conditions using the double-exposure speckle photography technique. Features of the macroscopically inhomogeneous strain localization at the yield plateau are considered. Relations between local components of the plastic distortion tensor in the zone of strain localization are analyzed.     

Simulation of structural phase transitions in ionic crystals under conditions of high pressures and temperatures

Structural phase transitions of the B1–B2 type in alkali halide crystals are investigated at a temperature other than 0 K. The pressure of B1–B2 phase transition is calculated as a function of temperature. The calculated pressure of B1–B2 transition in these crystals is weakly and almost linearly dependent on temperature. For a number of ionic crystals, the p ₀(T) curve has a maximum at some value of temperature.     

Dielectric—metal structural phase transition in alkali halide crystals under conditions of extremely high pressures

The dielectric—metal structural phase transition in alkali halide metals under conditions of extremely high pressures is studied using the method of electron density functional (MDF). The values of metallization pressure are calculated for both infinite crystals and nano-size samples. The dimensional dependence of metallization pressure is revealed, namely, the dependence of the value of phase transition pressure on the initial size of crystal.     

The optical properties of alkali nitrate single crystals

Absorption of non-polarized light by a uniaxial crystal has been studied. The degree of absorption polarization has been calculated as a function of the ratio of optical densities in the region of low and high absorbances. This function is proposed for analysis of the qualitative and quantitative characteristics of uniaxial crystal absorption spectra. Non-polarized light spectra of alkali nitrate single crystals, both pure and doped with thallium, have been studied. It is shown that the absorption band at 300 nm is due to two transitions, whose intensities depend on temperature in various ways. There is a weak band in a short wavelength range of the absorption spectrum of potassium nitrate crystal, whose intensity increases with thallium doping. The band parameters of alkali nitrate single crystals have been calculated. Low-energy transitions in the nitrate ion have been located.     

La（Ce）Br3闪烁晶体的研究历史及存在的问题

最近20年来,对于无机闪烁晶体,尤其是卤化物闪烁晶体的研究而言是一段收获颇丰的历史,这期间,先后研究发现了大量光输出高、能量分辨率好、衰减时间短的新型卤化物闪烁晶体。着重介绍La(Ce)Br3晶体的研究现状以及对晶体结构、发光机制、闪烁性能和最新研究进展进行系统整理和总结,同时指出在La(Ce)Br3晶体研究中急需解决的难题、发展方向和潜在的应用前景。     

Morphological development on alkali halide surfaces during evaporation

The sequential changes of the surface morphology of evaporated alkali halide crystals were investigated. The origin of surface features and their dependence on temperature are described for cleaved (100) surfaces evaporated in vacuum. Qualitative and quantitative determinations of the velocity of atomic steps during evaporation were made through the use of the gold double decoration technique. The results are related to theoretical expectations and compared to previous studies.     

Stages of surface erosion under ion irradiation

Initial stages of surface erosion have been studied for NaCl and LiF single crystals bombarded by Ar⁺ ions with 20 keV. For irradiation with doses D=10¹⁰-10¹¹ ions/cm², exoelectron emission has been used, whereas for higher doses, we have used electron microscopy. Two stages of initial surface erosion have been identified; for small doses, there is slow development of atomic-scale roughening, which reaches its peak when areas damaged by closest incident ions start to overlap, and then, beginning with D=10¹⁶ ions/cm², there is rapid etching, deep into the crystal, followed by the emergence of secondary microscopic structure, i.e. caverns, concentric closed steps and terraces. Ion-induced surface structure of alkali halide crystals has been shown to depend strongly on the presence of foreign particles on the surface, as well as on segregation. Topography of thin carbon films and some metals have been discussed.     

Quantum molecular tunneling of NH4 + ions in mixed ammonium-alkali halide systems

The rotational excitations of NH₄ ⁺ ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.     

On inhomogeneous straining in compressed sylvinite

Spatiotemporal distributions of local components of the distortion tensor of quasi-plastic materials—saliferous rocks (sylvinite)—have been studied under active compressive straining conditions using double-exposure speckle photography techniques. The strain localization patterns are presented and the features of macroscopic strain inhomogeneity are considered for inelastic behavior of the material. Results obtained for the slow wave processes in deformed saliferous rocks are compared to analogous data available for ionic crystals.     

High‐Temperature Ionic Epitaxy of Halide Perovskite Thin Film and the Hidden Carrier Dynamics

High‐temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high‐performance electro‐optic devices in III–V semiconductor field. A typical example is the successful growth of p‐type GaN by VPE for blue light‐emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high‐temperature processing condition and controllable deposition rate. For the first time, single‐crystalline high‐temperature VPE halide perovskite thin film has been demonstrated—a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer‐scale epitaxial and grain boundary‐free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo‐Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic–inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high‐quality large‐scale single‐crystalline halide perovskite films requiring precise control of defects and morphology.     

Assessment of small fatigue crack growth driving forces in single crystals with and without slip bands

Strain localization under low amplitude cyclic loading is a manifestation of plastic irreversible deformation associated with early crack growth. However, traditional constitutive models cannot usually reproduce strain localization in smooth single crystals, which can affect crack growth predictions for crystallographic fatigue cracks. This work analyzes the influence of bands of localized plastic shear strain on the cyclic crack tip displacement and on a fatigue indicator parameter by making special provision of a crack along the interface of a deformation band. Furthermore, the quality of local and volume-averaged fatigue indicator parameters are assessed using finite element models of a Cu single crystal cycled to induce plastic deformation under multiple loading conditions.     

Observation of large Rashba spin–orbit coupling at room temperature in compositionally engineered perovskite single crystals and application in high performance photodetectors

Indirect absorption extended below the direct transition edge and increase in carrier lifetime derived from Rashba spin–orbit coupling may advance the optoelectronic applications of metal halide perovskites. Spin-orbit coupling in halide perovskites is due to the presence of heavy elements in their structure. However, when these materials lack an inversion symmetry, for example by the application of strain, spin–orbit coupling becomes odd in the electron’s momentum giving rise to a splitting in the electronic energy bands. Here we report on the observation of a large Rashba splitting of 117 meV at room temperature, as predicted by relativistic first-principles calculations, in halide perovskite single crystals through a facile compositional engineering approach. Partial substitution of organic cations by rubidium in single crystals induces significant indirect absorption and dual peak photoluminescence as a result of a large Rashba splitting. We measured circularly polarized photoluminescence and magneto-photoluminescence in perovskite films printed by single crystals as well as magneto-electroluminescence and magneto-photocurrent in spin-LEDs based on perovskite single crystals. They indicated significant spin-momentum locking due to the large Rashba effect. A hybrid perovskite single crystal photodetector achieved record figures of merit, including detectivity of more than 1.3 × 10¹⁸ Jones which represents a three orders of magnitude improvement compared to the to date record. These findings show that facile compositional engineering of perovskite single crystals holds great promise for further advancing the optoelectronic properties of existing materials.