Pt.IV-The Atomistic Interpretation of the Melting of Binary Compounds and of the Formation of Eutectic Melts

JSGT 1951 V35 T469-T486

The Significance of the Co-ordination Requirements of the Cations in the Constitution of Glass. IV. The Atomistic Interpretation of the Melting of Binary Compounds and of the Formation of Eutectic Melts

As a preliminary to studying the problems of the melting of glasses and the viscosity of the melted material, the author discusses at length the cases of simple compounds possessing a crystalline structure, such as the metallic halides and, in particular, NaCl. The melting process begins, he assumes, when the motion of the ions through rise of temperature is sufficient to begin to detach some of the cations. The repulsion between a “loose” cation and some other cation sets going the breaking-up process of the crystal. It is assumed that in simple binary compounds the repulsion term dominates the energy and volume in the transition from the ordered to the disordered state. The repulsion is higher for noble-gas-type ions than for the more polarisable cations of the non-noble-gas type, in correspondence with the fact that compounds containing non-noble-gas-type ions have lower melting points than those with noble-gas type ions. Proceeding on the above basis, the author considered the problems of the structural changes during the melting of NaCl; the influence on the melting point of the polarisability of the cation, and of the anion-to-cation ratio; the latent heat of fusion; the thermal expansion of fused salts; and the formation of eutectic melts. In the corresponding compounds of cations having nearly the same ionic size (Mg²⁺ and Zn²⁺;Ag⁺ and Na⁺) the one containing the more polarisable cation always has the lower melting point. A parallel with this rule occurs in the temperature at which the viscosity is 10⁹ poises for the glasses derived from those of alkali–magnesia–silica when the magnesia is replaced by Zn²⁺, Co²⁺, Mn²⁺ and particularly Cu²⁺, in the order of their polarisability. In glasses the latent heat of fusion is spread over a wide temperature region; but when ions begin to pass one another a measurable endothermic effect can be observed. This endothermic effect for the alkali–silica glasses at about 500° diminishes as the smaller Na+ ions are progressively replaced by the larger and more polarisable K+ ions.

W. A. Weyl