Oxidation of alkaline-earth-metal sulfide powders and thin films

The oxidation of CaS and SrS powders and thin films was studied in situ up to 1300 degrees C by thermoanalytical techniques (TG, DTA and high-temperature XRD) while FTIR and powder XRD were used to analyse ex situ the solid reaction intermediates and products. CaS powder starts to oxidize to CaSO4 around 500 degrees C but the oxidation is not complete because of a competing reaction, i.e. the decomposition of CaSO4 to CaO, which is significant above 1000 degrees C. SrSO4 is more stable and therefore SrS completely oxidizes to SrSO4 before the decomposition starts. FTIR and XRD failed to detect CaSO3 or SrSO3 at any stage of the heating process. Particle size appears to have a marked effect on the decomposition of powders, while moisture plays only a minor role. Thin films appear to be more stable towards oxidation but they react around 1000 degrees C with the silicon substrate.