Preparation of Thin Films of Fast Oxide-Ion Conductor La 2 Mo 2 O 9 by rf-sputtering: I. Films prepared by in situ sputtering X. Y. Chen 1,2 , P. Laffez 1* , G. Banerjee 3 , T. Pezril 1 , M. Rossel 4 , G. Van Tendeloo 4 , P. Lacorre 5 , J. M. Liu 6 , and Z. G. Liu 6 1 Laboratoire de Physique de l'Etat condensé - UMR CNRS 6087, Université du Maine, Avenue O. Messiaen, 72085 Le Mans cedex, France 2. Department of Materials Science and Engneering, Nanjing University, Nanjing 210093, China 3. Eperimental Condensed Matter Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Calcutta 700064, India 4. Laboratorie des Fluorures, Faculté des Science UMR CNRS 6010, Université du Maine,72085 Le Mans cedex 9, France 5.Electron Microscopy for Materials Research, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium 6. National Laboratory of Solid Sate Microstructure Physics, Nanjing University, Nanjing 210093, China Abstract Preparation of La 2 Mo 2 O 9 thin film on Al 2 O 3 ceramic substrate was studied by rf-sputtering technique. It was found that the deposition temperature and oxygen partial pressure were most important factors to obtain the film of the pure La 2 Mo 2 O 9 phase. Varying the two parameters, the Mo-rich, the stoichiometric and the Mo-deficient films could be obtained. With the increase of the La:Mo, the film became denser. A crust layer was observed to exist on top of the Mo-rich and the * Corresponding author, patrick.laffez@univ-lemans.fr 1