Significant structures in the liquid state. I

Abstract

The mechanical properties of solids can be understood only by taking proper account of the imperfections present in the crystal lattice. Such lattice imperfections are so important in the liquid as to be an essential part of any description of this less ordered state. When a normal liquid such as argon melts, it expands 12 per cent. Such an expansion is typical. The atypical 10 per cent contraction of ice upon melting can be made to disappear if one first applies some 2,500-atm. pressure, at an appropriate temperature, to the hydrogen-bonded tetrahedral ice I and so transforms it into close-packed ice III, which is a fifth more dense. Ice III then melts with a normal 10 per cent expansion. Anhydrous salts such as sodium chloride melt with about the normal 10 per cent expansion. Metals such as sodium melt with about a 3 per cent expansion. This small expansion on melting of metals was long ago interpreted (1) as a consequence of the small size of the ion formed from atoms which no longer localize their conducting electrons. The ion, being only about one-third atomic size, requires only a third the expansion of normal liquids on melting to provide the same number of vacant sites and therefore the same increased disorder, which leads to melting.

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