Correlation of Oxygen Nonstoichiometry on Chemical Expansion in LCCO Thin Films

Mixed electronic/ionic conductors (MIECs) are of great technological interest with applications, for example, in solid oxide fuel cells (SOFCs). Reduction of operation temperatures is needed both for extended life and reduced costs, but this increases the demand for more electrocatalytically active electrodes. The MIEC layer compound lanthanum cerium cuprate (La_1–xCe_xCuO₄₊_δ, LCCO) shows promise in this regard given relatively high electronic conductivities at reduced temperatures [1], as well as the ability to accommodate both oxygen excess and deficiency, the latter making it an attractive candidate for both the SOFC cathode and anode [2], and thus a promising material for reversible SOFCs. In this work, oxygen nonstoichiometry in LCCO thin films is characterized in the temperature range of 500 to 700 °C. The determination of the oxygen nonstoichiometry in thin films is a challenging task, especially at elevated temperatures. Piezoelectric nanobalances, based on catangasite (CTGS, Ca₃TaGa₃Si₂O₁₄) single crystals, were applied to monitor changes in δ in these thin oxide films [3] reaching values of up to δ = –0.118 at e.g. 600 °C and log (<i>p</i>_O2) = –13.<br/>Changing oxygen content in LCCO films, for positive or negative values of δ, results in chemical expansion that can lead to stresses resulting in cracks and malfunction. In the case of thin films this chemical expansion occurs mainly perpendicular to the substrate. In lateral direction of the substrate, mechanical stress is generated leading to substrate bending. The surface near film expansion is characterized by X-ray diffraction under temperature and <i>p</i>_O2 variation. We characterize the overall thin-film expansion of LCCO films deposited on single crystalline yttria stabilized zirconia (YSZ) substrates acting as oxygen pumping cells by high-temperature laser-Doppler vibrometry (LDV) [4]. An applied voltage results in an oxygen activity which corresponds to an effective oxygen partial pressure and can be calculated via the Nernst relation. When a sinusoidal excitation voltage is applied to the cell, the reversible <i>breathing</i> of the LCCO film can be detected via LDV. If the excitation frequency is low enough, a quasi-stationary state is reached. At 500 °C and an excitation frequency of 100 µHz, positive and negative voltages of +/– 0.5 V correspond to oxygen activities of log (<i>a</i>_O2) = –10 and +10, respectively. In both cases thin-film expansion of only a few nm, results in total displacements (i.e. thin-film expansion plus substrate bending) of 64 nm and 239 nm respectively.<br/>[1] C.S. Kim, H.L. Tuller: Sol. State Ion., 320, 233-238 (2018) DOI: 10.1016/j.ssi.2018.03.015<br/>[2] Q. Liu et al.: Int. J. Hydrogen Energy, 46, 9818-9825 (2021) DOI: 10.1016/j.ijhydene.2020.06.063<br/>[3] S. Schröder et al.: Appl. Phys. Lett., 112, 213502 (2018) DOI: 10.1063/1.5025389<br/>[4] H. Wulfmeier et al.: Z. Phys. Chem., 236, 1013–1053 (2022) DOI: 10.1515/zpch-2021-3125