Heteroepitaxial growth of KCl and LiF on a cleaved (001) face of KBr

An initial stage of heteroepitaxial growth of KCl and LiF on a KBr substrate was studied using RHEED, EELS and AES. Both KCl and LiF films grow heteroepitaxially on a KBr substrate with their crystallographic axes parallel to that of substrate. In the case of KCl growth occurs in a well ordered layer-by-layer fashion and it is found that thickness of an intermediate region between KCl and KBr is less than 2 nm. In contrast three-dimensional island growth occurs in the case of LiF. Differences in lattice mismatch and solid solubility seem to determine the mechanism of heteroepitaxy of KCl and LiF on a KBr substrate.     

Behavior of block boundaries during ultrasonic vibration in alkali halide crystals at temperatures 20–300°C

The behavior of block boundaries was investigated in alkali halide crystals of various orientations deformed by ultrasound at frequencies 40–73 kHz in the temperature interval 20–300°C. The specimen orientation was defined by the angle between the fourfold axis and the propagation direction of the ultrasonic wave. The initial stage of plastic deformation at room temperature is due to the generation of sources at the block boundaries in KCl and KBr, and by the heterogeneous nucleation of dislocation in NaCl and LiF. In the temperature interval 200–300°C, in NaCl and LiF dislocation multiplication begins already with the generation of sources at the block boundaries. In 30° orientation specimens the beginning of multiplication is preceded by the motion initiated by the ultrasound of sections of the block boundaries in the {1 0 0} <110> secondary glide system.Translated from Izvestiya Vysshika Uchebnykh Zavedenii, Fizika, No. 1, pp. 49–53, January, 1986.     

Observation of negative alkali ions from alkali halides during 248-nm laser irradiation

Below laser fluences where a plasma is formed (the so-called plasma or plume formation threshold) a number of fundamental phenomena can occur where particles such as atomic and molecular ions, atoms and molecular neutrals, and electrons can be emitted. An understanding of such processes is necessary to develop predictive models for material removal from laser irradiated surfaces—at the foundation of laser etching, machining, and pulsed laser deposition. We have reported on a number of the mechanisms for such emission processes. Here, due to space limitations, we present a summary of our studies on the formation of negative alkali ions from single crystal KCl during exposure to pulsed 248-nm radiation at fluences well below the threshold for plasma formation. Despite the high electron affinities of the corresponding halogen atoms, negative halogen ions were not detected. Significantly, the positive and negative alkali ion distributions overlap strongly in time and space, consistent with K formation by the sequential attachment of two electrons to K⁺. Negative alkali ions are also observed under comparable conditions from LiF, NaCl, and KBr. In each material, the strong overlap between the positive and negative alkali ion distributions, and the lack of detected negative halogen ions, suggest that negative ion formation involves a similar mechanism.     

Ab initio calculations of electronic and optical properties in O-doped LiF crystal

We present a first-principle study of electronic and optical properties in pure LiF and O-doped LiF crystals. The pure LiF crystal exhibits a wide band gap while the O-doped LiF crystal shows the less band gap due to the contribution of O 2p. Some optical constants, such as dielectric functions, reflectivity and the refractive index, have been performed. The calculated reflectivity and refractive index from the pure LiF crystal agree with the experimental and recently calculated results in the low-energy range. Meanwhile, the optical properties have also been predicted from the O-doped LiF crystal. The absorption band in 200 nm has been observed, which is relatively close to the experimental result.      

高压下LiF和NaF的结构稳定性及其电子和光学性质的第一性原理研究

基于密度泛函理论，采用全势线性缀加平面波加局域轨道方法，计算模拟了LiF高压下的相变行为，预测其在450GPa附近发生由NaCl结构(B1)到CsCl结构(B2)的结构相变.同时还计算了高压下LiF不同相的电学特性，LiF的复介电函数以及介电常数随压强变化关系.通过比较能带结构的变化行为，得出LiF在53GPa附近还存在等结构相变，即由直接带隙结构变为间接带隙结构.将LiF的计算结果与另外一个同构化合物NaF进行了比较讨论. 关键词： LiF 压致相变 从头计算     

第一性原理研究缺陷对高压下LiF的电子结构和光学性质的影响

基于密度泛函理论下的平面波超软赝势方法,本文计算了102 GPa压力下LiF理想和含空位（Li-1 、F+1）晶体的电子结构和光学性质.结果表明:102 GPa压力下理想LiF晶体有良好的透明性;高压下Li-1及F+1两种空位的存在不影响LiF的光学透明性.分析其原因：LiF是一种带隙很宽的离子晶体,电子发生跃迁需要较高的能量; LiF中F元素的电负性很大,Li的电负性却很小,二者结合的Li-F键键能很高.另外本文还计算了102 GPa压力下理想LiF晶体及Li-1 、F+1两种空位分别存在时的反射、折射率、介电函数和光损失函数等一些光学性质,并将理想和含空位的做了比较,结果表明：102 GPa压力下空位的存在对LiF在可见光范围内这些光学性质都不产生影响,但由于F+1空位的存在使LiF晶体对光的吸收发生了红移现象.     

X-ray diffraction characterization of a spatially coherent crystalline nanocomposite obtained from a melt of KCl, KBr and KI salts

Crystalline bulks, obtained by the Bridgman technique from a melt that was previously prepared by mixing equal molar fractions of the salts KBr, KCl and KI were characterized by powder and single-plate X-ray diffraction. The crystalline bulks are formed by a composite material consisting of a highly textured aggregation of crystallites of two different face-centred-cubic solid solutions with corresponding unit-cell sizes of 7.020(2) and 6.456(3) Å. The relative molar fraction concentrations of these solid solutions in the composite are about 0.348 and 0.652, respectively. From these solutions, the first is identified as the binary KBr(9.7%):KI(90.3%) mixed phase and the second is discussed to be the ternary KBr (45.9%):KCl (51.1%):KI (3.0%) mixed phase. The lattices of most of the crystallites forming the composite, no matter the phase they belong to, are spatially coherent to each other within the crystalline bulk.     

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.     

Thermal expansion of alkali halides above 300°K

The lattice parameter of NaCl, KCl, KBr, RbCl, RbBr, RbI, CsCl, CsBr and CsI from room temperature to near melting temperature, and of LiF, NaF and KI from room temperature to near 750°K, are measured using high temperature X-ray diffractometry. An orthogonal polynomial of second order with least square error approximation is fitted to the lattice parameter versus temperature data to obtain the thermal expansion coefficient. The Grüneisen's parameter is calculated using the available data, and in five salts it is found to increase gradually with temperature.     

Infrared tunable diode laser spectra of lithium fluoride at high temperatures

At temperatures up to 1300 K the high resolution spectrum has been measured for the 1-0 through 7-6 vibrational transitions of ⁷LiF and the 1-0 through 8-7 vibrational transitions of ⁶LiF. These infrared ro-vibrational measurements have been combined with microwave measurements taken from the literature to obtain a set of ten Dunham potential constants that reproduce all the measurements for both isotopic species to within their experimental uncertainty. The band center for the v = 1-0 transition is 894.34104±0.00020 cm^?1 for ⁷LiF and 946.12074±0.00024 cm^?1 for ⁶LiF.     

Determination of the elastic constants of a composite plate using wavelet transforms and neural networks

An inverse method based on a combination of the wavelet transform and artificial neural networks is presented. The method is used to recover the elastic constants of a fiber-reinforced composite plate from experimental measurements of ultrasonic Lamb waves generated and detected with lasers. In this method, the elastic constants are not recovered from the dispersion curves but rather directly from the measured waveforms. Transient waveforms obtained by numerical simulations for different elastic constants are used as input to train the neural network. The wavelet transform is used to extract the eigenvectors from the Lamb wave signals to simplify the structure of the neutral network. The eigenvectors are then introduced into a multilayer internally recurrent neural network with a back-propagation algorithm. Finally, experimental waveforms recoded on a titanium-graphite composite plate are used as input to recover the elastic constants of the material.     

OSL signal of lithium fluoride and its relationship with TL glow-curves

The widely known LiF TL detectors: LiF:Mg,Ti (MTS-N) and LiF:Mg,Cu,P (MCP-N), were investigated with respect to their OSL properties. It was found that both materials exhibit quite substantial OSL sensitivity. In particular, in the case of LiF:Mg,Cu,P this sensitivity was very high, significantly exceeding that of BeO, the standard OSL dosimetric material. LiF:Mg,Cu,P could be a very promising candidate for application in dosimetry, if not for the fading, which was found to be quite high, reaching nearly 80% loss of the signal within 60 h. The OSL signal intensity shows a correlation with the peak 2 of the TL glow curves indicating that the same trapping sites are responsible for both processes. Peak 2 of LiF:Mg,Ti shows a peculiar property, that blue light stimulation removes only about half of its initial intensity, disregarding the duration of stimulation. This suggests, that this peak may have a composite structure and originates from both light-sensitive and light-insensitive trapping centres.     

Novel ultra-fast luminescence from incipient ion tracks of insulator crystals: electron–hole plasma formation in the track core

Measurements of fast luminescence decay and time-resolved spectra revealed novel ultra-fast luminescence with the lifetime of several tens ps in heavy-ion-irradiated single crystals of LiF, NaF, NaCl, KCl, KBr, KI, RbI, CsCl, CsBr, CsI, -alumina, and MgO. The luminescence is furthermore characterized by a super-linear increase in the efficiency with increasing excitation density, non-tailed decay curve, and temperature-insensitive decay-rate and yield. The results mean that the luminescence neither originates from localized excited states such as self-trapped excitons, free excitons, excited defects, and excited impurity centers nor their interaction. A process which does not contradict the experimental results is the formation of the e–h plasmas and the luminescence from them.     

Unrestricted hartree-fock cluster analysis of F and F A centres in some alkali halides

The optical absorption energies and nearest-neighbour ground state hyperfine interaction constants are evaluated for F centres in lithium and potassium halides, and for FA(Li) centres in KC1 and KBr. For these defects, the electron-ion interaction includes self-consistent unrestricted Hartree-Fock treatment of nearest-neighbour ions, in terms of Gaussian-localized atomic orbital basis functions. The results, with the exception of FA-centre hyperfine constants, are qualitatively correct.     

Elastic constants measurement of anisotropic Olivier wood plates using air-coupled transducers generated Lamb wave and ultrasonic bulk wave

A hybrid elastic wave method is applied to determine the anisotropic constants of Olive wood specimen considered as an orthotropic solid. The method is based on the measurements of the Lamb wave velocities as well as the bulk ultrasonic wave velocities. Electrostatic, air-coupled, ultrasonic transducers are used to generate and receive Lamb waves which are sensitive to material properties. The variation of phase velocity with frequency is measured for several modes propagating parallel and normal to the fiber direction along a thin Olivier wood plates. A numerical model based mainly on an optimization method is developed; it permits to recover seven out of nine elastic constants with an uncertainty of about 15%. The remaining two elastic constants are then obtained from bulk wave measurements. The experimental Lamb phase velocities are in good agreement with the calculated dispersion curves. The evaluation of Olive wood elastic properties has been performed in the low frequency range where the Lamb length wave is large in comparison with the heterogeneity extent. Within the interval errors, the obtained elastic tensor doesn’t reveal a large deviation from a uniaxial symmetry.     

Elastic constants of the SrLaAlO4 and SrLrGagaO4 crystals measured at ultrasonic and hypersonic oustic frequencies

Elastic constants measured by Brillouin light-scattering experiments were compared with results estimated from the pulse-echo-overlap method for the ultrasonic frequency to find dispersion in their values. In this way Brillouin hypersonic experiments complete the ultrasonic results. The present investigation includes a careful analysis of the accuracy of the results. The measured values of elastic constants suggest a weak dispersion in acoustic wave velocities. However, given the magnitude of the experimental errors, it is difficult to conclude about the dispersion.     

Effect of temperature on non-destructive wave propagation in potassium halides

Temperature dependence of ultrasonic attenuation is investigated for potassium halides in the temperature range 100–400 K. These calculations are done for KCl, KBr and KI for longitudinal and shear waves along the 〈1 1 1〉 direction. The non-linearity coupling parameters and thermal relaxation time have also been obtained for these crystals. In the present investigation, it has been found that phonon–phonon interaction is the dominant cause for ultrasonic attenuation.     

LaNbO4晶体的弹性系数和声速的计算

利用群论方法,对LaNbO₄晶体,建立了计算弹性系数的一般公式和由弹性系数反演力常数的解析式。在4个近邻力常数的近似下,计算了LaNbO₄晶体各种弹性波的传播速度。 关键词：     

Ce activated potassium bromide phosphor for lyoluminescence dosimetry of ionizing radiation

The lyoluminescence (LL) properties of gamma irradiated KBr:Ce phosphor are reported in this paper. The samples were prepared by wet chemical route. The prepared material was characterized by lyoluminescence technique. LL in KBr:Ce have been recorded for different gamma doses. The nature of variation of peak LL intensity is found to be sublinear with gamma irradiation dose, and the peak LL intensity is found to be dependent on concentrations of added Ce in the samples. Negligible fading in the prepared KBr:Ce (0.5 mol%) sample is observed.