pubs.acs.org/cm Published on Web 11/06/2009 r 2009 American Chemical Society Chem. Mater. 2010, 22, 799–802 799 DOI:10.1021/cm901479g Epitaxial Nature and Transport Properties in (LaBa)Co 2 O 5þδ Thin Films † Jian Liu, ‡,§ Ming Liu, ‡,^ Gregory Collins, ‡,§ Chonglin Chen,* ,‡,§ Xuening Jiang, ) ,^ Wenquan Gong, ) Allan J. Jacobson, ) Jie He, # Jiechao Jiang, # and Efstathios I. Meletis # ‡ Department of Physics and Astronomy, University of Texas at San Antonio, Texas 78249, § Texas Center for Superconductivity and ) Department of Chemistry, University of Houston, Houston, Texas 77204, ^ Dalian University of Technology, Dalian 116024, P. R. China, and # Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, Texas Received May 29, 2009. Revised Manuscript Received October 5, 2009 Epitaxial (LaBa)Co 2 O 5þδ thin films were grown on (001) LaAlO 3 single-crystal substrates using pulsed laser deposition. Microstructure characterizations from X-ray diffraction and electron microscopy indicate that the films are highly c-axis oriented with cube-on-cube epitaxy. Transport property measurements indicate that the films have typical semiconductor behavior with a novel phase transition and hysteresis phenomena at 540 K. The chemical dynamic studies reveals that the resistance of the film changes drastically with the change of redox environment, i.e., the magnitude of resistance changes, ΔR =1  10 2 S 1  10 6 Ω, is found within a short response time (∼700 ms). These phenomena suggest that the as-grown (LaBa)Co 2 O 5þδ film have extraordinary sensitivity to reducing-oxidizing environment and the exceedingly fast surface exchange rate. Introduction Mixed ionic/electronic conducting materials are of increasing interest owing to their potential applications in various novel devices such as ceramic membranes, ultra sensitive chemical sensor, partial oxidation reactors as well as electrodes in solid oxide fuel cell (SOFC). 1-4 To improve the performance of those devices, the mixed ionic electronic conducting materials should meet the require- ments of both high oxygen diffusivity and great enhance- ment of surface exchange rate, 5,6 These two desirable attributes are typically found in the oxygen deficient doped perovskite cobaltates (Re,A)Co 2 O 5þδ , where Re is a rare earth element and A is an alkaline earth element. The A-site cation average valence favors a compensating population of oxygen vacancies at low oxygen partial pressures and therefore leads to the observed high ionic conductivity. (LnBa)Co 2 O 5.5þδ is one family of com- pounds that have received significant interest due to many intriguing mixed ionic/electronic conducting phenomena. The nature of the A-site cations, especially their size, and their distribution, ordered or disordered, may drastically influence the mixed conductivity. Taskin and co-workers have observed a remarkable enhancement of the oxygen diffusivity in A-site ordered GdBaCo 2 O 5.5þδ . 7 Our recent research showed that A-site ordered PrBaCo 2 O 5.5þδ has unusually rapid oxygen transport kinetics at low tem- peratures ranging between 300 and 500 °C. 8,9 Various interesting physical phenomena have been observed in the perovskite cobaltate family. In particular, the layered perovskite LaBaCo 2 O 5.5þδ becomes a unique case with some distinctive properties, such as the weakest tendency to A-site ordering and smallest oxygen nonstoichio- metry due to the small difference between the radii of A-site cations’ La 3þ and Ba 2þ . 10 Recently, Eeva-Leena Rautama et al. have fabricated and characterized the fully oxidized nanoscale-ordered LaBaCo 2 O 6, disordered La 0.5 Ba 0.5 CoO 3 , and the oxygen-deficient ordered LaBa- Co 2 O 5.5. They have also observed various interesting new phenomena in this system, such as the unusual magneti- zation and magnetotransport properties associated with the spin state of cobalt in low temperature. 11,12 It is well- known that the physical properties are strongly dependent † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *Corresponding author. E-mail: cl.chen@utsa.edu. (1) Steele, B. C. H. 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