Functional Crystals: Search Criteria and Design Principles

Conceptually, there are three levels of design strategy for functional crystals. The first level is the point group symmetry. In fact, point-group symmetry dictates the presence or absence of a particular physical response (and its anisotropy) of a crystal; hence, it can be used to search and/or screen for new functional crystals. The symmetry requirements of a number of technologically important physical properties of functional crystals are described. However, point-group symmetry is a necessary requirement but not a sufficient condition for a functional crystal. For a crystal to exhibit a particular property, it must be augmented by the second level of design strategy--the structure pattern or the space-group symmetry. Finally, in order to enhance or optimize the response, the third level of design strategy, molecular engineering, which involves the fine tuning of the electronic or magnetic structures of the building blocks (atoms, molecules, clusters, etc.) of the crystal, must be considered. This three-level strategy for designing functional crystals is discussed with the aid of examples from a new class of hybrid crystalline materials based on inorganic polymers with organic spacers/controllers systems recently developed by us.