Phase development in the formation of CaLa2S4 powder and pellet

Phase transformations were determined by IR, XRD, DTA/TGA, SEM, EDX and TEM in the formation of CaLa2S4, as powders or pellets. The CaLa2S4 powders were formed by decomposition of coprecipitated carbonates in vacuum and exposure to hydrogen sulfide at 650–950°C. Oxygen was not completely eliminated formed precursors by sulfurization at 950°C for 48 h. The intermediate compounds formed by heating the CaLa2S4 powder in air were La2O2S2, La2O2S, La2O2SO4 and CaSO4 in sequence, while La2O2S2 was formed by grinding the CaLa2S4 powder in ethanol solution. Pellets were formed by sintering the green compact in a pure H2S atmosphere followed by hot isostatic pressing (HIP). After annealing in pure H2S, the pellet was translucent yellow in appearance.