The Crystal Structure of α-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 α-La 2 Mo 2 O 9 . 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 O 2- migration pathway in the anion conducting high-temperature form, β-La 2 Mo 2 O 9 . 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 O 2- conduction pathway. We provide quantitative analysis showing that the positions in the ordered array of 216 oxygen sites of α-La 2 Mo 2 O 9 are directly related to the average sites occupied in β-La 2 Mo 2 O 9, providing compelling evidence that the high-temperature conducting form of the material has a time-averaged version of the low-temperature structure.