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Crystal Structure of an Indigo@Silicalite Hybrid Related to the Ancient Maya Blue Pigment
The
structure of the indigo@silicalite pigment, an analog of ancient Maya
Blue, has been determined by combining X-ray Laue microdiffraction
and powder diffraction techniques. After the adsorption of indigo
into the calcined (monoclinic) silicalite sample, the powder diffraction
pattern contained peaks from both orthorhombic (major phase) and monoclinic
(minor phase) silicalite. Assuming that the orthorhombic phase was
induced by the adsorption of indigo, Laue microdiffraction was used
to map the unit cell changes (and thereby the indigo distribution)
within a single crystal. It was found to be highly heterogeneous with
empty monoclinic and indigo-induced orthorhombic domains. The Laue
diffraction data indicated that the space group of the orthorhombic
domains was <i>Pnma</i> rather than <i>P2</i><sub>1</sub>2<sub>1</sub>2<sub>1</sub>. With this information, the indigo@silicalite
structure could be solved and refined from the powder diffraction
data. The starting positions for two independent indigo molecules,
described as rigid bodies, were obtained by simulated annealing, with
a first molecule positioned in the straight channel and the second
one in the sinusoidal channel. The positions and occupancies of these
molecules and the positions of the framework atoms were then refined
using the Rietveld method. Approximately four indigo molecules per
unit cell were found, two per independent site, and possible local
arrangements are suggested. The size of the indigo molecule prevents
the structure from being fully ordered