Strain relaxation in epitaxial SrTi0.5Fe0.5 O3−δ thin films on LaAlO3: An X-ray reciprocal space mapping study

SrTi0.5Fe0.5O3−δ (STF) has emerged as a promising cathode material for solid oxide fuel cells due to its excellent high ionic and electronic conductivity. We investigated the strain-state in epitaxial STF thin films on LaAlO3 (001) using synchrotron x-ray reciprocal space mapping measurement. The STF layer consists of two groups of compressively strained domains mixed horizontally in the film plane with a strain difference of about 1.4 %. The stress-free relaxed unit cell of more compressively strained domains, where a large degree of Sr segregation is expected, is relatively larger indicating that Sr segregation causes unit cell expansion. The lattice strain becomes gradually relaxed as the film thickness increases and the in-plane domain size decreases. We conjecture that the segregated Sr from compressively strained domains diffuse to the surface to form a layer of SrOx islands.