Crystal structure of La2Mo2O9, a new fast oxide-ion conductor

The crystal structure of the new fast oxide-ion conductor La2Mo2O9 (ionic conductivity of 0.06 S cm(-1) at 800 degrees C) has been studied. This compound presents a reversible phase transformation around 580 degrees C from a low-temperature form alpha-La2Mo2O9 to a high-temperature form beta-La2Mo2O9. The high-temperature form beta-La2Mo2O9 has a cubic structure (at 617 degrees C, space group P2(1)3; alpha = 7.2014(5) Angstrom; Z = 2; R-Bragg = 5.8%, R-p = 10.9%, R-wp = 6.5%, chi(2) = 7.7) which derives from that of beta-SnWO4. Partial site occupation by oxygen atoms, strongly anisotropic thermal factors, and short-range order with a distance characteristic of O-O pairs have been evidenced. An original concept is proposed for the origin of oxide-ion conduction in this compound, which could be applied to the design of new oxide-ion conductors. The low-temperature form alpha-La2Mo2O9 exhibits a slight monoclinic distortion and a large superstructure relative to beta-La2Mo2O9 (2 x 3 x 4), most probably due to the localization of oxygen atoms. The large cell (similar to 8800 Angstrom(3)) did not allow us to determine the crystal structure of alpha-La2Mo2O9.