Strontianite-aragonite solid solutions Sr xCa 1− xCO 3: Effect of composition on the orthorhombic-brhombohedral phase transition and the conversion to oxide solid solutions Sr xCa 1− xO

Abstract

The conversion of orthorhombic Sr x Ca 1− x CO 3 solid solutions to Sr x Ca 1− x O has been studied by thermal analysis and X-ray diffraction. The decomposition occurs in two stages. In the first stage, occurring at 950–1150 K in N 2, the cations demix to produce CaO and leave a Sr-enriched carbonate phase. In the second stage (1150–1350 K) the residual carbonate decomposes and the cations remix to form the oxide solid solution. By heating in CO 2, thereby displacing decomposition to above 1250 K, the orthorhombicrhombohedral phase transition characteristic of these carbonates has been studied over the whole range x = 0 → 1. The transition temperature ( T tr) increases progressively with x. For the Srenriched carbonate obtained after the first stage of decomposition in N 2, T tr is independent of the initial composition and close to that for SrCO 3. ΔH for the transition, however, is less than that expected for well-crystallized SrCO 3, suggesting that the demixed carbonate is partly amorphous.