Competitive incorporation of Ca, Sr, and Ba ions into amorphous carbonates

Amorphous alkaline earth carbonates typically occur as metastable precursors of widespread crystalline phases. Despite their transient nature, the properties of the amorphous carbonates affect the attributes of the crystalline end-products. The lack of long-range ordering in the amorphous solids allows for the incorporation of various ions into their structures, resulting in a wider range of cation compositions than in crystalline carbonate phases, furnishing these solids with peculiar physico-chemical properties. Here, we studied abiotic factors that could control the degree of incorporation of Ca²⁺, Sr²⁺, and Ba²⁺ cations into amorphous carbonates, and focused on the effects that different cation compositions have on the lifetimes and structures of amorphous solids, as well as on the structures of their crystalline end-products. The partition coefficients (D_Ca, D_Sr, and D_Ba) between the solution and the solid decreased with increasing ionic radius; however, in ternary systems the incorporation of Ba²⁺ was favored over Sr²⁺. Electron diffraction patterns and pair distribution functions calculated from them showed that incorporated Sr²⁺ and Ba²⁺ cations significantly modified the structural properties of the amorphous carbonates (relative to amorphous calcium carbonate). The changes in short-range structure resulted in exceptional kinetic stability of the ternary mixed amorphous Ca–Sr–Ba carbonates, with the lifetime of Sr_0.1Ba_.40Ca_.50C 65 times longer than that of amorphous calcium carbonate. Ba²⁺ enhanced the formation of calcite-type structures, even in the presence of Sr²⁺. These findings contribute to the understanding of both the incorporation of alkaline earth metals into amorphous carbonates and their roles in prolonging the lifetimes of amorphous solids; in addition, our results can be potentially used for technological applications.