Nanoporous Intergrowths: How Crystal Growth Dictates Phase Composition and Hierarchical Structure in the CHA/AEI System

Some of the most important nanoporous materials that are used for industrial applications are formed as intergrowths between structurally related phases. Further, the specific properties and functions are often strongly related to the nature of these intergrowths. By their nature such structures are notoriously difficult to characterize in detail and thereby formulate a structure/property relationship. We approach the problem of the industrially relevant CHA/AEI intergrowth system by getting insight into not only the structure of the materials but also the crystal-growth mechanism and show that the former is crucially dependent upon the latter. Through a detailed X-ray diffraction analysis with optimization of the CHA/AEI layer stacking sequence, it is shown that up to three distinct components are present. These consist of the two end member structures intimately cocrystallizing with an intergrowth structure. The intergrowth composition is further corroborated by nuclear magnetic resonance and unit cell measurements. The mechanism by which these complex intergrowth structures form is revealed by atomic force microscopy that shows there are at least two competing mechanisms of growth at the surface: layer-by-layer and spiral. This has profound consequences on the resulting intergrowth materials, as intergrowth formation is not permitted in spiral growth. The competition from the lower energy spiral growth at screw dislocations does not allow intergrowth formation and consequently results in blocks of pure-phase AEI or CHA. Owing to this competitive growth nature, the different possibilities furnish the material with its higher level hierarchical structure.