Observation of orientational disorder in the hexagonal stuffed tridymite Sr 0.864 Eu 0.136 Al 2 O 4 by the maximum‐entropy method

The crystal structure of a strontium europium aluminate, Sr 0.864 Eu 0.136 Al 2 O 4 , with a novel hexagonal form was investigated by a combination of Rietveld analysis and the maximum‐entropy method (MEM) with synchrotron X‐ray powder diffraction data. The electron density image calculated by the MEM/Rietveld method revealed that the apical oxygen ion in the AlO 4 tetrahedron has a broad distribution corresponding to an extraordinarily large atomic displacement parameter. This structure could be expressed by a split‐atom model, with which the Rietveld refinement gave R wp = 2.99% and R B = 4.16%. Subsequently, MEM‐based pattern fitting (MPF) decreased the R factors to R wp = 2.81% and R B = 2.34% and the electron density image clearly showed that the apical oxygen ions of the AlO 4 tetrahedra are split over three sites around a threefold axis involving an elongated distribution of the residual O ions along the c axis. These results suggest that AlO 4 tetrahedra in Sr 0.864 Eu 0.136 Al 2 O 4 are orientationally disordered.