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Alkali Halides (alkali + halide)
Selected AbstractsMolecular dynamics of phase transitions in clusters of alkali halidesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2001Pedro C. R. Rodrigues Abstract Molecular dynamics simulations of unconstrained alkali halide clusters with 8, 64, 216, 512, 1000, 1728, 2744, 4096, 5832, and 8000 ions have been carried out using the Born,Mayer,Huggins potential. All the clusters exhibit first-order melting and freezing transitions. The melting temperature increases with the number of ions and approaches the melting temperature of the bulk. Clusters with a number of ions less than approximately 1000 present hysteresis cycles and practically do not have phase coexistence. Clusters with a number of ions over 1000 present phase coexistence during a significant part of the transition region and hysteresis is progressively eliminated as the clusters size increases. It is suggested that hysteresis is an intrinsic characteristic of small clusters. In the transition regions the calculations have been performed by fixing the total energy of the clusters. It is shown that such a technique provides a better way of analyzing the transition mechanism than the usual procedure of fixing the temperature by ad hoc rescaling the velocities or by using canonical molecular dynamics or Monte Carlo. A detailed analysis of the melting transition is presented. The effects of interfaces and impurities are discussed. A method based on the velocity autocorrelation functions is proposed, in order to determine the molar fraction of the ions present in the solid and liquid phases as well as to produce colored snapshots of the phases in coexistence. The overall agreement of the estimated melting points and enthalpies of melting with the experiment is fairly good. The estimated melting point and enthalpy of melting for KCl in particular are in excellent agreement with the experimental values. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 169,180, 2001 [source] The role of breathers in the anomalous decay of luminescencePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2006Eva Mihóková Abstract Luminescence of alkali halides doped by heavy ns2 ions exhibits an anomaly in the slow component emission decay. The anomaly is explained by the formation of a discrete breather in the immediate neighborhood of the impurity. We study properties of these breathers, their phase space structure, robustness, and propensity for formation. Under a wide range of parameters and interionic potentials they form 2-dimensional Kolmogorov-Arnold-Moser tori (less than generic) in phase space. We show strobed views of these tori, useful in quantization. All features support the thesis of breather formation as the explanation for the luminescence decay anomaly that first motivated our breather proposal. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Model of temperature dependent crystal relaxationPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2006Eva Mihóková Abstract Anomalous decay of the slow emission component in certain doped alkali halides was explained by the creation of a discrete breather in the immediate neighborhood of the impurity. It was experimentally observed that with rising temperature the decay anomaly becomes less pronounced, finally disappearing in the range 100 K to150 K (depending on the substance). In view of the breather explanation, this temperature dependence should be a consequence of the effect of temperature on breathers. We present an improved model of the dissipative effects of temperature on lattice breathers so as to compare it to the observed experimental temperature behavior. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Novel Superalkali Superhalogen Compounds (Li3)+(SH), (SH=LiF2, BeF3, and BF4) with Aromaticity: New Electrides and AlkalidesCHEMPHYSCHEM, Issue 5 2006Fang-Fang Wang Dr. Abstract Optimized structures, with all real frequencies, of superalkali superhalides (Li3)+(SH), (SH=LiF2, BeF3, and BF4), are obtained, for the first time, at the B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ computational levels. These superalkali superhalides possess three characteristics that are significantly different from normal alkali halides. 1) They have a variety of structures, which come from five bonding mode types: edge,face, edge,edge, face,face, face,edge, and staggered face,edge. We find that the bonding mode type closely correlates with the Li3SH bond energy. 2) The valence electrons on the Li3 ring are pushed out by the (SH), anion, and become excess electrons, conferring alkalide or electride characteristics on these Li3SH species, depending on the bonding mode type. 3) The highest occupied molecular orbital of each Li3SH species is a doubly occupied delocalized , bonding orbital on the Li3 ring, which indicates its aromaticity. It is noticeable that the maximum negative nucleus-independent chemical shift value (about ,10 ppm) moves out from the center of the Li3 ring, owing to repulsion by the SH, anion. We find that these superalkali superhalides are not only complicated "supermolecules", but are also a new type of alkalide or electride, with aromaticity. [source] | ||||||||||