Raman spectra and defect chemical characteristics of Sr(Ti,Fe)O_3−y solid solution of bulk pellets vs. thin films

Sr(Ti,Fe)O_3−y perovskite solid solutions are relevant functional materials for energy conversion and electronic devices such as solid oxide fuel and photoelectrochemical cells, electrolyzers, oxygen sensors, resistive random access memories or synaptic transistors. The Raman spectra and vibrational characteristics of the Sr(Ti,Fe)O_3−y materials class are suitable for describing their defect chemistry and the iron valence state, which governs a multitude of its mixed ionic-electronic transport and other characteristics. We synthesize a standard range of compositions containing 1-75 mol% of iron including the end members in the form of macrocrystalline bulk pellets, nanocrystalline poly- and single crystalline thin films. Through the change in both iron substitution level and microstructure, we directly see the effect of defect chemistry such as its phase, transition metal ion valence and oxygen nonstoichiometry on the Raman spectra. These are discussed in terms of in and ex situ experiments under oxidizing/reducing atmosphere. In contrast to long range structural X-ray diffraction measurements, Raman spectroscopy provides valuable insights into oxygen vacancy ordering and oxygen nonstoichiometry for the Sr(Ti,Fe)O_3−y material class.