On electro-crystalline properties as conditioned by atomic lattices

Abstract

The method of analysis of crystal lattices by ultra-optical spectra, introduced and applied by W. L. and W. H. Bragg, has led to the view that the ultimate crystalline element is (or may be) the atom and not the chemical molecule: so that each atom, presumably with its ionic electric charge, often stands in identically the same relation to a number of surrounding atoms of the other kind with opposite charge, and therefore cannot be associated physically with any one of them. The alternative view, previously perhaps the more natural on the physical as distinct from the tactical side, was to regard the chemical molecule as the crystalline unit: its associated positive and negative atomic ions might then form an electric diad with definite polar moment orientated in definite direction,—and with this the new schemes come into contrast. The following general remarks may be useful towards comparison of these two fundamentally different conceptions. Other reasons will incidentally emerge, besides the results of analysis by X-rays, favouring the view that facts of nature point rather towards the atomic mode of crystal structure. These considerations deal with fundamental ideas that must be reckoned with, on an ionic view, apart from the detail essential to the science of crystallography. It will however be convenient to fix attention by simple cases; for example, the typical space-lattice for an alkaline halide such as rocksalt NaCl, in which the crystalline unit has been ascertained to be for each ion a “face-centred” cube, the corners for one component being the centres of the edges for the other. In the two structural plane diagrams, which represent the atoms projected perpendicular to a diagonal, the letters a may represent the negative atomic ions Cl, and the letters b the positive ones Na. In the second diagram a diagonal boundary face constituted of Cl atoms is succeeded by parallel planes, alternately of Na and Cl, and finally one of Na, these planes of like atoms being obliquely placed: this diagram invites description at sight as that of a very strongly polarised aggregate, each bipolar element being made up of a Cl combined with an adjacent Na below it. On the other hand in the first diagram the boundary face is composed of both kinds of ion in equal numbers: and if the aggregate is regarded as sorted into bipolar elements, each consisting of an atom in one face and the adjacent atom in the next parallel plane, the polarities of the consecutive elements in all directions are equal and opposite, so that the mass appears as unpolarised when the effective element of volume is large enough to include a considerable number of such bipolar elements.