Oxides in the Ln2O3-MO3 (M = Mo and W) system are of significant technological interest for their laser applications , ionic conduction , catalytic  and ferroelectric  properties. The La2O3-WO3 phase diagram has been studied by a number of groups [5-7], but little detailed crystallographic information was reported due to the lack of good single crystals. Some of the reported compositions have not been appropriately characterized. Recently, the structures of La2WO6, La18W10O57 and La10W2O21 were solved using X-ray powder diffraction (XRPD) [8-10].
For the La6W2O15 compound phase transitions at 630 and 930 °C have been reported [1-3]. The structure of the high temperature phase α-La6W2O15 was determined ab-initio by XRPD . The lower-temperature forms β and γ, however, couldn’t be determined due to the large number of reflections in the X-ray powder diffraction pattern and the relatively low symmetry of the system. The existing literature on γ-La6W2O15 only relates two sets of unit cell parameters [5-6], that almost match the XRPD pattern of γ-La6W2O15, but some weak peaks remain without indexation and can’t be explained by the presence of any impurity.
Here we present the structure solution using transmission electron microscopy of the complex structure of γ-La6W2O15. From zone axis precession electron diffraction the unit cell was determined to be monoclinic with cell parameters a=1.57 nm, b=1.21 nm, c=1.57 nm, β=110°. As an example, the  zone axis is presented on figure 1. Due to the low symmetry of the crystal system and the large unit cell, a huge number of reflections needed to be acquired, so that electron diffraction tomography was used to record the intensities. The cation positions were obtained but the distribution of the cations on the sites was not evident. Z-contrast imaging showed that disorder on some cationic sites has to be considered (fig.2).
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To cite this abstract:Stéphanie Kodjikian, Christophe Lepoittevin, Holger Klein, Thomas Schönenberger, Oleg Lebedev, Olivier Leynaud, Marie-Hélène Chambrier, François Goutenoire; Structure solution of the complex γ-La6W2O15. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/structure-solution-of-the-complex-%ce%b3-la6w2o15/. Accessed: April 21, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/structure-solution-of-the-complex-%ce%b3-la6w2o15/