Francois Goutenoire

Designing fast oxide-ion conductors based on La2Mo2O9

The ability of solid oxides to conduct oxide ions has been known for more than a century, and fast oxide-ion conductors (or oxide electrolytes) are now being used for applications ranging from oxide fuel cells to oxygen pumping devices. To be technologically viable, these oxide electrolytes must exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mobility can only be achieved with classes of materials with suitable structural features. So far, high mobility has been observed in only a small number of structural families, such as fluorite, perovskites, intergrowth perovskite/Bi2O2 layers and pyrochlores. Here we report a family of solid oxides based on the parent compound La2Mo 2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors, this material undergoes a structural transition around 580 °C resulting in an increase of conduction by almost two orders of magnitude. Its conductivity is about 6 × 10 -2 S cm-1 at 800 °C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with β-SnWO 4 (ref. 14) suggests a structural model for the origin of the oxide-ion conduction. More generally, substitution of a cation that has a lone pair of electrons by a different cation 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.

Reducibility of fast oxide-ion conductors La2−xRxMo2−yWyO9(R = Nd, Gd)

The substitutional range and cell parameter evolution of fast oxide-ion conductors La2−xRxMo2−yWyO9 (R = Nd, Gd) are investigated. In the whole series, the cubic β-La2Mo2O9 structural type is stabilized at room temperature. The effects on reducibility of both single and double substitutions are presented. Lanthanum substitution by rare earth appeared to be responsible for an increase in the reducibility and a strong but reversible amorphization under dilute hydrogen. On the contrary, the favourable role of tungsten on the compound stability under reducing conditions is evidenced: it depletes oxygen loss while making the La2Mo2O9 structural type less affected by it.

Structure determination of β-Pb2ZnF6 by coupling multinuclear solid state NMR, powder XRD and ab initio calculations

The results from one-dimensional multinuclear (19F, 207Pb and 67Zn) magic-angle spinning nuclear magnetic resonance experiments combined with the use of the ISODISPLACE program allow for the space group determination of beta-Pb2ZnF6 (no. 138 P4(2)/ncm). The structure was refined from X-ray powder diffraction data (a = 5.633 (1) A and c = 16.247 (1) A, Z = 4). beta-Pb2ZnF6 has one six-fold coordinated Zn, one eleven-fold coordinated Pb and five F non-equivalent crystallographic sites and is built from alternated layers parallel to the (a, b) plane; tilted ZnF4(2-) layers of corner sharing ZnF6(4-) octahedra and FPb+ layers of edge sharing FPb4(7+) tetrahedra. The structure of beta-Pb2ZnF6 was then optimized using the ab initio code WIEN2k and the calculated 67Zn EFG is in agreement with the NMR results. 19F-19F proximities and 19F-207Pb connectivities were evidenced using through-space and through-bond NMR correlation experiments, respectively, and support the proposed structure. 19F-207Pb J-coupling was also used to select fluorine resonances depending on the number of neighbouring lead ions, leading to an unambiguous assignment of the different 19F resonances.