Melting and crystal structure

Abstract

The role of various types of lattice flaws is discussed in their bearing on melting and crystal structure. Freezing-point determinations on lattices of a single molecule indicate the presence of such flaws in the crystals of polymethylene compounds. Freezing-point determinations on composite lattices of ketone + paraffin and ketone + ketone emphasize the importance of the rotation of the CO group for melting, and indicate that lattice holes are easily formed, without marked lowering of the freezing-point. Ordered composite lattices are formed when the number of holes does not exceed 12 % of all the methylene groups. The results emphasize the importance of co-operative effects in lattice flaws. In the equilibrium state of a crystal, such flaws might arise both from energy effects at 0° K, expecially in metals, and from energy and entropy effects at higher temperatures. A picture of melting is suggested in which a network of co-operative flaws breaks up the crystal into a mosaic, or aggregate of crystalline micelles, and in which the solid-liquid transition is comparable with a gel-sol transition.