title: Periodic cation segregation in Cs0.44[Nb2.54W2.46O14] quantified by high-resolution scanning transmission electron microscopy.
abstract: The atomic structure of Cs0.44[Nb2.54W2.46O14] closely resembles the structure of the most active catalyst for the synthesis of acrylic acid, the M1 phase of Mo10V2(4+)Nb2TeO42-x. Consistently with observations made for the latter compound, the high-angle electron scattering signal recorded by scanning transmission electron microscopy shows a significant intensity variation, which repeats periodically with the projected crystallographic unit cell. The occupation factors for the individual mixed Nb/W atomic columns are extracted from the observed intensity variations. For this purpose, experimental images and simulated images are compared on an identical intensity scale, which enables a quantification of the cation distribution. According to our analysis specific sites possess low tungsten concentrations of 25%, whereas other sites have tungsten concentrations above 70%. These findings allow us to refine the existing structure model of the target compound, which has until now described a uniform distribution of the niobium and tungsten atoms in the unit cell, showing that the similarity between Cs0.44[Nb2.54W2.46O14] and the related catalytic compounds also extends to the level of the cation segregation.
authors.affiliation: 1Central Facility for Electron Microscopy,RWTH Aachen University,52074 Aachen,Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Heidelmann
authors.affiliation: 1Central Facility for Electron Microscopy,RWTH Aachen University,52074 Aachen,Germany.
authors.forename: Juri
authors.initials: J
authors.lastname: Barthel
authors.affiliation: 3BASF SE,Department of Polymer Physics,67065 Ludwigshafen,Germany.
authors.forename: Gerhard
authors.initials: G
authors.lastname: Cox
authors.affiliation: 1Central Facility for Electron Microscopy,RWTH Aachen University,52074 Aachen,Germany.
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Weirich