On the use of the term `absolute' in crystallography

Abstract

The concept of 'absolute structure' as introduced by Jones [Acta Cryst. (1984), A40, 660-662] is discussed, and the term 'absolute' is extended to cases where the structure has been related to some external macroscopic physical property. The non-centrosymmetric crystal classes are divided into seven distinct categories for which specific terms are proposed: structural chirality, when the 'absolute structure' is determined for crystals in classes 1, 2, 3, 4, 6, 222, 422, 32, 622, 23 and 432; absolute chirality, when the 'absolute structure' is linked to a chiral property such as optical rotation; absolute polarity, when the 'absolute structure' is linked to a polar property such as pyroelectricity; absolute morphology, when the 'absolute structure' is linked to the crystal habit: in this case there may be two further subdivisions - absolute chiral morphology (or absolute enantiomorphism) to describe a link to the hand of the habit, and absolute polar morphology to describe a link to the polar nature of the habit. It is further recommended that the term absolute configuration should be reserved only for molecular species and not for crystal structures.