Several recent investigations indicated that hetero grain boundaries between perovskite La_1-xSr_xCoO_3-d (P) and Ruddlesden-Popper phase (La_1-xSr_x)₂CoO_4+d (R) may exhibit an accelerated oxygen exchange surface reaction ("TPB effect"), see e.g. [1,2,3]. In the present work, compact La-Sr-Co-O composite films with different fractions of P and R are formed by self assembly on single crystalline YSZ substrates during pulsed laser deposition from a single PLD target [4,5]. The lateral grain size is about 50 nm, and characteristic morphologies can be assigned to the different crystalline phases. In addition to P and R two metastable phases were found: a Co-deficient perovskite phase La_0.7Sr_0.3Co_0.9O_3-d, and a higher order Ruddlesden-Popper phase.

During the self-organized phase separation, different Sr contents are established in P and R, leading for most sample compositions to a Sr accumulation in R and corresponding depletion in P [4,5]. However, during the PLD process the thermodynamic Sr distribution (as determined in [6] for powder samples annealed at 1200 °C) is not fully reached.

Since the oxygen exchange activity of P strongly depends on the Sr content, the knowledge of the actual Sr distribution is required to calculate the correct reference activity for the composite films, i.e. the hypothetical activity in absence of a "TPB effect". At 600 °C, the effective surface rate constant k^q measured by impedance spectroscopy of the self-assembled composite films with an overall La:Sr ratio of 7:3 exceed this reference activity by half to one order of magnitude [7]. This corresponds to a strength of the TPB effect normalized to the triple phase density of 2-9×10^-13 cm²/s. Furthermore, the Co-deficient perovskite phase exhibits an increase of k^q by about two orders of magnitude relative to La_0.7Sr_0.3CoO_3-d(cf. related findings in [8]). The present results are compared to literature, and possible reasons for the increased oxygen exchange activity at hetero grain boundaries are discussed.

[1] T. Kawada, M. Sase, M. Kudo, K. Yashiro, K. Sato, J. Mizusaki, N. Sakai, T. Horita, K. Yamaji, H. Yokokawa, Solid State Ionics 177 (2006) 3081

[2] E. J. Crumlin, E. Mutoro, S.-J. Ahn, G. J. la O’, D. N. Leonard, A. Borisevich, M. D. Biegalski, H. M. Christen, Y. Shao-Horn, J. Phys. Chem. Lett. 1 (2010) 3149

[3] W. Ma, J. J. Kim, N. Tsvetkov, T. Daio, Y. Kuru, Z. Cai, Y. Chen, K. Sasaki, H. L. Tuller, B. Yildiz, J. Mater. Chem. A 3 (2015) 207

[4] S. Stämmler, R. Merkle, B. Stuhlhofer, G. Logvenov, J. Maier, poster at SSI-19 in Kyoto, June 2013

[5] S. Stämmler, R. Merkle, B. Stuhlhofer, G. Logvenov, K. Hahn, P. A. van Aken, J. Maier, submitted (2016)

[6] S. Stämmler, R. Merkle, B. Stuhlhofer, G. Logvenov, J. Maier, ECS Transact. 68(1) (2015) 579

[7] S. Stämmler, R. Merkle, J. Maier, submitted (2016)

[8] A. Takeshita, S. Miyoshi, S. Yamaguchi, T. Kudo, Y. Sato, Solid State Ionics 262 (2014) 378