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