Structure of epitaxial Ca_(2)Fe_(2)O_(5) films deposited on different perovskite-type substrates

摘要

The influence of the substrate on the structure and the microstructure of Ca_(2)Fe_(2)O_(5) (CFO) thin films has been investigated by transmission electron microscopy. Ca_(2)Fe_(2)O_(5) films have been deposited by pulsed laser deposition on (001)-oriented SrTiO_(3) (STO), LaAlO_(3) (LAO), and (LaAlO_(3))_(0.3)(SrAl_(0.5)Ta_(0.5)O_(3))_(0.7) (LSAT) substrates. Minor variations in the lattice mismatch of the CFO film with the three different substrates strongly determine the growth direction of the film. The misfit stress also promotes the development of strain accommodation mechanisms. (101) "orthogonal" twins are formed to accommodate the misfit in epitaxial films of CFO on STO(001) and LSAT(001), while CFO films on LAO(001) develop 90° rotation twins leading to film fragmentation into grains. The growth direction of the CFO film on LAO not only depends on the lattice mismatch but also on the lattice energy involved at the initial stages of the growth process. Our results also evidence the formation of an unexpected phase with composition Ca_(3)Fe_(2)O_(7) in CFO thin films on STO and LSAT. High-resolution microscopy and electron diffraction suggest that this structure belongs to the Ruddlesden-Popper family.