Designing fast oxide-ion conductors based on La2Mo2O9

The ability of solid oxides to conduct oxide ions has been known for more t han a century, and fast oxide-ion conductors (or oxide electrolytes) are no w being used for applications ranging from oxide fuel cells to oxygen pumpi ng devices(1,2). To be technologically viable, these oxide electrolytes mus t exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mo bility can only be achieved with classes of materials with suitable structu ral features. So far, high mobility has been observed in only a small numbe r of structural families, such as fluorite(3-5), perovskites(6,7), intergro wth perovskite/Bi2O2 layers(8,9) and pyrochlores(10,11). Here we report a f amily of solid oxides based on the parent compound(12) La2Mo2O9 (with a dif ferent crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors(2,13), th is material undergoes a structural transition around 580 degrees C resultin g in an increase of conduction by almost two orders of magnitude. Its condu ctivity is about 6 x 10(-2) S cm(-1) at 800 degrees C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with beta-SnWO4 (ref. 14) suggests a str uctural model for the origin of the oxide-ion conduction. More generally, s ubstitution of a cation that has a lone pair of electrons by a different ca tion that does not have a lone pair-and which has a higher oxidation state- could be used as an original way to design other oxide-ion conductors.