Treading in the Limited Stability Regime of Lanthanum Strontium Ferrite — Reduction, Phase Change and Exsolution

We present an in situ investigation of the iron exsolution from lanthanum strontium ferrite perovskites. Using in situ X-ray diffraction experiments at the synchrotron, the exact onset of exsolution was determined by a change in the lattice parameter before any iron reflexes become visible. For an initially orthorhombic thin film, on the other hand, a phase transition to a fluorite/rock-salt structure is observed to occur during the exsolution. Also, a difference in the iron oxidation states between bulk and surface is found since photoelectron spectroscopy and X-ray absorption spectroscopy both indicate the existence of the Fe(III)/Fe(II) couple in oxidation/reduction cycles, whereas magnetic measurements would suggest Fe(IV)/Fe(III).