Intergrowth structures in inorganic solids: A new class of materials

Abstract

An increasing number of inorganic solids forming long-period structures due to recurrent intergrowth of two chemically distinct but structurally related units are getting to be known in recent years. These novel structures have given rise to new chemistry at solid-solid interfaces. Besides intergrowth structures with long-range order, many solids with random intergrowth (similar to stacking faults in polytypes) are known. Ordered integrowth gives rise to homologous series of structures in many systems. Barium ferrites, the Aurivillius family of oxides and other perovskite-related oxides, siliconiobates, and tungsten oxide bronzes are some of the systems exhibiting ordered intergrowth structures. Both ordered and disordered intergrowths are fruitfully investigated by high resolution electron microscopy. The main emphasis in this article is on intergrowth structures where the component units are compositionally different. These systems are obviously most fascinating since compositional change occurs across each interface (intergrowth plane), unlike in polytypic materials where the composition remains constant. Even in ordered intergrowth structures, there is always some disorder. If order in an intergrowth structure does not prevail over large distances, but occurs only over shorter stretches (say, a few repeats of the sequence), it becomes difficult to describe the solid except in terms of the gross composition and where possible, the unit cell dimensions. Structures with occasional intergrowths are found in a variety of materials such asβ-alumina, Magneli phases, silicates, ferrites and several other oxide systems. In addition to examining the structural features of various intergrowths, the origin of the intergrowth phenomenon is discussed.