The crystal structure of alpha-La2Mo2O9 and the structural origin of the oxide ion migration pathway.

We describe for the first time the full 3D atomic structure of room-temperature alpha-La2Mo2O9. The material, despite its simple chemical formula, has a remarkable 312 crystallographically unique atoms and is thus one of the most complex oxide structures reported to date. Despite this complexity, the structural results offer significant insight into the O2- migration pathway in the anion conducting high-temperature form, beta-La2Mo2O9. The material contains a mixture of 4, 5, and 6 coordinated Mo sites, suggesting that variable Mo coordination number is a key factor in providing a low-energy O2- conduction pathway. We provide quantitative analysis showing that the positions in the ordered array of 216 oxygen sites of alpha-La2Mo2O9 are directly related to the average sites occupied in beta-La2Mo2O9, providing compelling evidence that the high-temperature conducting form of the material has a time-averaged version of the low-temperature structure