Oxygen-ion arrangements and concerted motion in β- La2 Mo2 O9

We have performed ab initio molecular dynamics simulations coupled to a simulated annealing procedure and the nudged elastic band method to study the arrangements and diffusion of O ions in β- La2 Mo2 O9. It is found that the occupancies of three crystallographic distinct O sites O(1), O(2), and O(3) are 100%, 91.7%, and 25%, respectively, consistent with experiments. Each Mo cation is surrounded by four or five O ions, forming Mo O4 tetrahedra or Mo O5 hexahedra. The analysis of Mo-O(1) bond length demonstrates the split of O(1) sites into two groups. The O ions move in a cooperative fashion. Two diffusion channels are found to be responsible for the high conductivity, one involves the migration of O(2) with low barrier energy and short migration length assisted by O(3), and another is the migration of O(3) with large barrier energy and long migration length assisted by O(1) and O(2). The thermally activated and concerted motions of three different kinds of O ions instead of the single-particle jump indicate that the diffusion or relaxation behaviors in β- La2 Mo2 O9 could be understood using the combined model of Arrhenius law and Vogel-Tammna-Fulcher equation.