Highly crystalline strontium ferrites SrFeO3-δ: an easy and effective wet-chemistry synthesis

The synthesis of strontium ferrite SrFeO3−δ has been explored through wet-chemistry methods in order to optimize a quick, easy and reproducible method to obtain the perovskite in pure crystalline form with a high yield. Among the three investigated synthetic paths, (i) coprecipitation of hydroxides, (ii) coprecipitation of oxalates and (iii) polyol-assisted coprecipitation, only the second one was effective in obtaining the desired perovskite modification as a single phase. The products were analyzed by means of powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), to determine the crystalline structure and the chemical composition of the sample surface, respectively, and to optimise the synthetic process. Pure samples were further characterised by means of inductively coupled plasma (ICP-AES) analysis, nitrogen adsorption, elemental analysis, temperature programmed reduction (TPR) and Mössbauer spectroscopy.

Due to their important and diverse properties, mixed transition metal oxides having perovskite crystal structures have long been object of academic studies and technological applications. In this framework, strontium ferrite SrFeO3-x holds particular interest due to its capability to act as both an electronic and ionic conductor at high temperatures (oxygen ions migrate through vacancies in its crystal structure) and thus employable both in fuel cells (as an electrode) and in oxygen concentrating devices. Moreover this compound is also endowed with remarkable magnetic and catalytic properties. In this work, the perovskite SrFeO3-x has been obtained through an easy, reproducible and cost-effective wet-chemistry coprecipitation route. Firstly, three synthetic paths have been explored: i) coprecipitation of hydroxides from an aqueous solution, ii) coprecipitation of oxalates from an aqueous solution and iii) polyol-assisted coprecipitation. Products of all three synthetic paths were analyzed by means of powder X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) to determine the crystalline structure and the chemical composition of the sample surface and to optimize the synthetic process. These analyses revealed that only the coprecipitation of oxalates was effective in obtaining the desired perovskite modification as a single phase. Pure samples were further characterized by means of Inductive Coupled Plasma (ICP-MS) analysis, nitrogen adsorption, elemental analysis, temperature programmed reduction (TPR) and Mössbauer spectroscopy. In particular, Mössbauer spectroscopy was used to analyze the oxidation states and magnetic properties of the iron atoms contained in the sample, thus also allowing to estimate the correct oxygen content.