The control of defect structure and surface composition is of great importance to achieve performance improvements of a wide variety of materials for electrochemical applications. Effects of temperature, PO₂ and bias have been widely studied. However, the potential of strain-induced oxygen non-stoichiometry requires further study to obtain a deeper understanding of the effects on controlling oxygen stoichiometry and modifying surface characteristics. In this work strained epitaxial thin films were investigated as a method to design the defect structure and surface characteristics of the La_0.5Sr_0.5Mn_0.5Co_0.5O_3-δ (LSMC) perovskite. This perovskite was chosen due to its ability to accommodate a wide variety of oxygen stoichiometries¹. Thin films of LSMC were grown using pulsed laser deposition on substrates with lattice mismatch of +0.46%, between SrTiO₃ and the bulk reduced La_0.5Sr_0.5Mn_0.5Co_0.5O_2.38(2) resulting in tensile strain, and -1.50%, producing compressive strain between LaAlO₃ and the bulk oxidised La_0.5Sr_0.5Mn_0.5Co_0.5O_2.98(2). Crystal lattice volumes of the thin films were determined by X-ray diffraction and confirmed that fully strained (00l) epitaxial films were achieved. Low energy ion scattering (LEIS) was used to probe both surface and sub-surface composition of the thin films. LEIS analysis confirmed that whilst both tensile and compressively strained films were A-site terminated, with strong strontium dominance, there was cobalt enrichment of the sub-surface layer which was more prominent in films under tensile strain. Density functional theory calculations show that the vibrational contribution to the vacancy free energy, i.e. thermal effects, strongly depends on the strain.  In particular, the computational investigation demonstrated that the equilibrium oxygen vacancy concentration increases (decreases) under tensile (compressive) strain compared to the bulk LSMC case.

1         A. Aguadero, H. Falcon, J. M. Campos-Martin, S. M. Al-Zahrani, J. L. G. Fierro and J. A. Alonso, Angew. Chem. Int. Ed. Engl., 2011, 50, 6557–61.