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The NaCl(100) Surface: Why Does it Not Melt?  [PDF]
T. Zykova-Timan,U. Tartaglino,D. Ceresoli,E. Tosatti
Physics , 2005,
Abstract: The high temperature surface properties of alkali halide crystals are very unusual. Through molecular dynamics simulations based on Tosi-Fumi potentials, we predict that crystalline NaCl(100) should remain stable without any precursor signals of melting up to and even above the bulk melting point $T_m$. In a metastable state, it should even be possible to overheat NaCl (100) by at least 50 K. The reasons leading to this lack of surface self-wetting are investigated. We will briefly discuss the results of calculations of the solid-vapor and liquid-vapor interface free energies, showing that the former is unusually low and the latter unusually high, and explaining why. Due to that the mutual interaction among solid-liquid and liquid-vapor interfaces, otherwise unknown, must be strongly attractive at short distance, leading to the collapse of any liquid film attempting to nucleate at the solid surface. This scenario naturally explains the large incomplete wetting angle of a drop of melt on NaCl(100).
Physics of Solid and Liquid Alkali Halide Surfaces Near the Melting Point  [PDF]
T. Zykova-Timan,D. Ceresoli,U. Tartaglino,E. Tosatti
Physics , 2005, DOI: 10.1063/1.2035096
Abstract: This paper presents a broad theoretical and simulation study of the high temperature behavior of crystalline alkali halide surfaces typified by NaCl(100), of the liquid NaCl surface near freezing, and of the very unusual partial wetting of the solid surface by the melt. Simulations are conducted using two-body rigid ion BMHFT potentials, with full treatment of long-range Coulomb forces. After a preliminary check of the description of bulk NaCl provided by these potentials, which seems generally good even at the melting point, we carry out a new investigation of solid and liquid surfaces. Solid NaCl(100) is found in this model to be very anharmonic and yet exceptionally stable when hot. It is predicted by a thermodynamic integration calculation of the surface free energy that NaCl(100) should be a well ordered, non-melting surface, metastable even well above the melting point. By contrast, the simulated liquid NaCl surface is found to exhibit large thermal fluctuations and no layering order. In spite of that, it is shown to possess a relatively large surface free energy. The latter is traced to a surface entropy deficit, reflecting some kind of surface short range order. Finally, the solid-liquid interface free energy is derived through Young's equation from direct simulation of partial wetting of NaCl(100) by a liquid droplet. It is concluded that three elements, namely the exceptional anharmonic stability of the solid (100) surface, the molecular short range order at the liquid surface, and the costly solid liquid interface, all conspire to cause the anomalously poor wetting of the (100) surface by its own melt in the BMHFT model of NaCl -- and most likely also in real alkali halide surfaces.
Alkali Halide Surfaces Near Melting: Wetting and Nanofriction Properties  [PDF]
D. Ceresoli,T. Zykova-Timan,U. Tartaglino,E. Tosatti
Physics , 2007, DOI: 10.1016/j.msea.2007.09.092
Abstract: Alkali halide (100) crystal surfaces are poorly wetted by their own melt at the triple point. We carried out simulations for NaCl(100) within the well tested BMHFT model potential. Calculations of the solid-vapor, solid-liquid and liquid-vapor free energies showed that solid NaCl(100) is a non-melting surface, and explain its bad wetting in detail. The extreme stability of NaCl(100) is ideal for a study of the nanofriction in the high temperature regime, close to and even above the bulk melting temperature (T_M). Our simulations reveal in this regime two distinct and opposite phenomena for plowing and for grazing friction. We found a frictional drop close to T_M for deep ploughing and wear, but on the contrary a frictional rise for grazing, wearless sliding. For both phenomena we obtain a fresh microscopic understanding, relating the former to ``skating'' through a local liquid cloud, the latter to softening of the free substrate surface. It is argued that both phenomena, to be pursued experimentally, should be much more general than the specific NaCl surface case. Most metals in particular possessing one or more close packed non-melting surface, such as Pb, Al or Au(111), should behave quite similarly.
Melting and Grüneisen parameters of NaCl at high pressure
中国物理 B , 2004,
Abstract: The Buckingham potential has been employed to simulate the melting and thermodynamic parameters of sodium chloride (NaCl) using the molecular dynamics (MD) method. The constant-volume heat capacity and Grüneisen parameters have been obtained in a wide range of temperatures. The calculated thermodynamic parameters are found to be in good agreement with the available experimental data. The NaCl melting simulations appear to validate the interpretation of superheating of the solid in the one-phase MD simulations. The melting curve of NaCl is compared with the experiments and other calculations at pressure 0-30GPa range.
Calculation of the melting point of alkali halides by means of computer simulations  [PDF]
J. L. Aragones,E. Sanz,C. Valeriani,C. Vega
Physics , 2012, DOI: 10.1063/1.4745205
Abstract: In this manuscript we study the liquid-solid coexistence of NaCl-type alkali halides, described by interaction potentials such as Tosi-Fumi (TF), Smith-Dang (SD) and Joung-Cheatham (JC), and compute their melting temperature (Tm) at 1 bar via three independent routes: 1) liquid/solid direct coexistence, 2) free-energy calculations and 3) Hamiltonian Gibbs-Duhem integration. The melting points obtained by the three routes are consistent with each other. The calculated Tm of the Tosi-Fumi model of NaCl is in good agreement with the experimental value as well as with other numerical calculations. However, the other two models considered for NaCl, SD and JC, overestimate the melting temperature of NaCl by more than 200 K. We have also computed the melting temperature of other alkali halides using the Tosi-Fumi interaction potential and observed that the predictions are not always as close to the experimental values as they are for NaCl. It seems that there is still room for improvement in the area of force-fields for alkaline halides, given that so far most models are still unable to describe a simple yet important property such as the melting point.


物理学报 , 1959,
Abstract: By sublimation of NaCl crystals of high purity in vacuum at a temperature near its melting point single NaCl crystals of macroscopic size were grown at zones of somewhat lower temperatures. Oriented overgrowths up to 1 mm thick were obtained over the whole cleavage faces of the crystals, but the deposition was muchmore effective on the faees normal to the temperature gradient.The influence of zonal temperature differences and the geometric form of the substrate upon the size and orientation of the crystals have been investigated. The relative importance of faces observed was found of the following sequence: 100> 111>120>122>110. of these the form 122 has not been reported for NaCl crystals growing from the solution. But there is ample evidence from the present experiment that the dominance of 111 over 120 may not be as certain as obsreved for the crystals prepared from solution.In accord with the theory generally held, initial deposition was found to take place statistically more often at the corners of cube faces than elsewhere. New patterns of interpenetration growth and surface structures have been observed and there are features indicating layer growth and screw dislocation.Preliminary experiment was also made on evaporation of a cylindrical NaCl crystal resulting in the exposition of the faces (100) and (120).
Physics and Nanofriction of Alkali Halide Solid Surfaces at the Melting Point  [PDF]
T. Zykova-Timan,D. Ceresoli,U. Tartaglino,E. Tosatti
Physics , 2006, DOI: 10.1016/j.susc.2006.02.083
Abstract: Alkali halide (100) surfaces are anomalously poorly wetted by their own melt at the triple point. We carried out simulations for NaCl(100) within a simple (BMHFT) model potential. Calculations of the solid-vapor, solid-liquid and liquid-vapor free energies showed that solid NaCl(100) is a nonmelting surface, and that the incomplete wetting can be traced to the conspiracy of three factors: surface anharmonicities stabilizing the solid surface; a large density jump causing bad liquid-solid adhesion; incipient NaCl molecular correlations destabilizing the liquid surface, reducing in particular its entropy much below that of solid NaCl(100). Presently, we are making use of the nonmelting properties of this surface to conduct case study simulations of hard tips sliding on a hot stable crystal surface. Preliminary results reveal novel phenomena whose applicability is likely of greater generality.
Incomplete melting of the Si(100) surface from molecular-dynamics simulations using the Effective-Medium Tight-Binding model  [PDF]
K. Stokbro K. W. Jacobsen,J. K. N?rskov,D. M. Deaven,C. Z. Wang,K. M. Ho
Physics , 1996,
Abstract: We present molecular-dynamics simulations of the Si(100) surface in the temperature range 1100-1750K. To describe the total energy and forces we use the Effective-Medium Tight-Binding model. The defect-free surface is found to melt at the bulk melting point, which we determine to be 1650 K, but for a surface with dimer vacancies we find a pre-melting of the first two layers 100 K below the melting point. We show that these findings can rationalize recent experimental studies of the high temperature Si(100) surface.
HM Lu,KM Fang Metallurgy School,University of Science,Technology Beijing,Beijing,China ZX Qiu,
H.M. Lu and K.M. Fang Metallurgy School
,University of Science and Technology Beijing,Beijing,China Z.X. Qiu

金属学报(英文版) , 2000,
Abstract: Multiple regression equations of liquidus temperature, electrical conductivity and bath density of the Na_3AlF_6-AlF_3-BaC1_2-NaCl system were obtained from experiments by using orthogonal regression method. The experiments were carried out in 100A cell with low melting point electrolyte, the influences of cathodic current density, electrolytic temperature, density differences of bath and liquid aluminum on current efficiency (CE) were studied; when the electrolyte cryolite ratio was 2.5, w(BaC1_2) and w(NaCl) were 48% and 10%, respectively, CE reached 90% and specific energy consumption was 10.97k Wb/kg/kg. Because of the fact that aluminum metal obtained floated on the surface of molten electrolyte, this electrolysis method was then defined as low temperature aluminum floating electrolysis. The results showed that the new low temperature aluminum electrolysis process in the Na_3AlF_6-AlF_3-BaC1_2-NaCl bath system was practical and promising.
Melting Point Shift in Supported Metal Nanoclusters  [PDF]
V. D. Borman,P. V. Borisyuk,M. A. Pushkin,I. V. Tronin,V. N. Tronin,V. I. Troyan,O. S. Vasiliev,M. V. Vitovskaya
Physics , 2010,
Abstract: The dependency of the melting point of supported metal nanoclusters as function of clusters height is theoretically investigated in the framework of the uniform approach. The vacancy mechanism describing the melting point shift in nanoclusters with decrease of their size is proposed. It is shown that the essential role in clusters melting point shift is played by van der Waals forces of cluster-substrate interaction. It is shown, that the account of layer--by--layer fusion of a cluster allows to satisfactorily describe the melting of nanoclusters of various metals, deposited onto a different substrates. The proposed model satisfactorily accounts for the experimental data.
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