The fact that deformation and failure are facilitated by the reversible physico-chemical influence of the medium has been established by now for all types of solids: for metals (and also certain covalent crystals) in contact with liquid metals, for ionic crystals and inorganic glasses in the presence of molten salts, water, alcohol, or other polar media, and for molecular crystals of organic compounds in contact with nonpolar and low-polarity organic liquids. In general these phenomena facilitate the breaking and realignment of the interatomic bonds in the presence of certain foreign atoms or molecules (which have sufficient mobility to ensure their penetration into the bond-breaking zone) and can be described as a lowering of the free surface energy of the given solid under the influence of the surrounding medium. The main condition under which the medium exerts a strong influence on the mechanical properties of the body (in cases of reversible adsorption interaction not connected with dissolution, corrosion, or other chemical processes) is that the solid and the medium be of related nature, to make the surface energy low on the boundary between the solid and liquid phases. At the same time, the form and degree of manifestation of these effects depend in a complicated manner on the real structure of the body (defects) and on the deformation conditions (stresses, temperature, strain rate, time of contact, etc.) An optimal combination of these factors makes it possible to use the influence of the medium to facilitate dispersion and treatment, particularly of solid materials that are difficult to work. To the contrary, by eliminating individual factors that lower the strength by adsorption it becomes possible to protect against the influence of the medium.
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The fractional frequency shift in the nmr signal from protons in a spherically-shaped pure water sample has been measured directly as -10.36(30) X OC-l over the range 5°C to 45 "C. A revised value of 25.69 (14) x is suggested for the proton diamagnetic shielding correction in water at 25 "C, leading to pb(2.5 OC)/pB = 0.001 520 993 136(17).
Current interatomic potentials for compound semiconductors, such as GaAs, fail to correctly predict the ab initio calculated and experimentally observed surface reconstructions. These potentials do not address the electron occupancies of dangling bonds associated with surface atoms and their well established role in the formation of low-energy surfaces. The electron counting rule helps account for the electron distribution among covalent and dangling bonds, which, when applied to GaAs surfaces, requires the arsenic dangling bonds to be fully occupied and the gallium dangling bonds to be empty. A simple method for linking this electron counting constraint with interatomic potentials is proposed and used to investigate energetics of the atomic scale structures of the GaAs(001) surface using molecular statics methods.
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