Structure of Diamond

Abstract

DIAMOND is generally thought to be one of the most perfect crystals occurring naturally: its atomic structure was first investigated in the very early days of X-ray crystal analysis. It turned out that the atoms of the crystal were arranged on two interpenetrating face-centred cubic lattices, the corner of one cube lying one quarter of the way along the cube diagonal of the other. Whether the resulting crystal possessed full octahedral symmetry or was of the lower tetrahedral type, a question on which crystallographers were divided, could not be settled by the additional X-ray evidence. Some ten years ago, new interest in the structure of diamond was aroused by the work of Robertson, Fox and Martin¹, who showed that the infra-red absorption and ultraviolet transparency of diamonds placed them in two classes. Of the two types, the first and commoner was opaque to both infra-red and ultra-violet radiation, while the second, Type II, is rare and is transparent to these radiations. The more recent X-ray investigations of Lonsdale and Smith² have shown that although the two types are structurally identical on an atomic scale, there are abnormalities in the diffraction patterns of Type I diamonds which suggest some kind of 'mosaic' structural difference in the two types. Dr. Lonsdale's observed effects are in fact very similar to the kind of abnormality observed in the X-ray spectra of age-hardened alloys and alloys with high magnetic coercivity; they suggest a laminated structure.