Formation mechanisms of Fe3-xSnxO4 by a chemical vapor transport (CVT) process

Our former study reported that Fe-Sn spinel (Fe3-xSnxO4) was easily formed when SnO2 and Fe3O4 were roasted under CO-CO2 atmosphere at 900-1100 degrees C. However, the formation procedure is still unclear and there is a lack of theoretical research on the formation mechanism of the Fe-Sn spinel. In this work, the reaction mechanisms between SnO2 and Fe3O4 under CO-CO2 atmosphere were determined using XRD, VSM, SEM-EDS, XPS, etc. The results indicated that the formation of Fe3-xSnxO4 could be divided into four steps: reduction of SnO2 to solid phase SnO, volatilization of gaseous SnO, adsorption of gaseous SnO on the surface of Fe3O4, and redox reaction between SnO and Fe3O4. During the roasting process, part of Fe3+ in Fe3O4 was reduced to Fe2+ by gaseous SnO, and meanwhile Sn2+ was oxidized to Sn4+ and entered into Fe3-xSnxO4. The reaction between SnO2 and Fe3O4 could be summarized as Fe3O4 + xSnO((g)) -> Fe3-xSnxO4 (x = 0-1.0).