Periodic Cation Segregation in Cs_0.44[Nb_2.54W_2.46O₁₄] Quantified by High-Resolution Scanning Transmission Electron Microscopy

Abstract The atomic structure of Cs_0.44[Nb_2.54W_2.46O₁₄] closely resembles the structure of the most active catalyst for the synthesis of acrylic acid, the M1 phase of ${\rm Mo}_{{{\rm 10}}} {\rm V}_{{\rm 2}}^{{{\rm 4{\plus}}}} {\rm Nb}₂ {\rm TeO}_{{{\rm 42}{\minus}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 Cs_0.44[Nb_2.54W_2.46O₁₄] and the related catalytic compounds also extends to the level of the cation segregation.