Concepts and Nomenclature in Chemical Crystallography

Abstract

The concepts relating to the structures and properties of crystals involve numerous terms that might be especially bewildering to the nonspecialist. Many of these terms have been borrowed and/or adapted from mineralogy, a much older branch of crystallography and involving a different community of researchers, while others have been coined at will to describe either new concepts or concepts for which suitable terms did not exist before. As a result, crystallographic nomenclature is far from systematic and the lexicon of modern crystallography has evolved as a haphazard collection of terms. This contribution presents a survey of the concepts that arise during the crystallographic studies of crystals, with the rather ambitious goal of achieving disambiguation regarding nomenclature. The authors specifically discuss terms that they have identified as being highly relevant to modern chemical crystallography.

Is a Bent Crystal Still a Single Crystal?

“…[20] Theindividual crystalline domains are often referred to as grains,a nd the junctions between these crystals are known as grain boundaries.P olycrystalline matter is notably different from asingle crystal with respect to properties related to optical, thermal, diffraction, and electrical properties,among others. Theword "crystal" is anoun and defines an object that complies with the IUCr definition of crystallinity.T he word "crystalline" is an adjective and describes the property of an object, and specifically an object that has properties of acrystal, that is,anobject that is either one crystal or is composed of several or many crystals,that is, it is not amorphous.T herefore,asingle crystal is crystalline.…”

Ist ein gebogener Kristall immer noch ein Einkristall?

“…[20] Die einzelnen kristallinen Bereiche werden oft als Kçrner bezeichnet. Das Wort "Kristall" ist ein Substantiv und definiert ein Objekt, das der IUCr-Definition der Kristallinitäte ntspricht.…”

“…[20] Andererseits werden die Rçntgenstrahlen von einkristallinen Materialien aufgrund der Kristallgitterausrichtungn ur in bestimmte Richtungen gestreut.…”

The role of co-crystals in pharmaceutical design

“…Much of the debate has concerned nomenclature and definitions. Barbour et al have very recently provided a lucid and concise summary of the issues (12) and for the most part their definitions and suggestions will be adopted here. Very broadly, a co-crystal is a multi-component molecular crystal, i.e.…”

“…However, there 3 is a practical and 'inventive' difference between co-crystals that include solid co-formers and those which include molecules that might have been included accidentally because they are a potential solvent for the active pharmaceutical ingredient (API). As a result, some workers feel that multicomponent molecular crystals in which one of the components is a solvent molecule are not "proper" co-crystals and it has been proposed that therefore none of the components of a co-crystal should have played the role of solvent during the crystallization process (12). Although in practical terms the deliberate design of co-crystals rarely focuses on solvates, the distinction is a subjective one and should not form part of a formal definition.…”