Effect of Cd on the Nucleation and Transformation of Amorphous Calcium Carbonate

Significant work on the aqueous nucleation and growth of pure calcium carbonate has brought to light the importance of intermediate amorphous phases. In contrast, the factors that control the composition and polymorph selection of crystalline end products in mixed-cation carbonate systems are poorly understood. In this work, in situ pair distribution function (PDF) and small-angle X-ray scattering (SAXS) analyses were performed in a mixed-flow reactor to measure the structure, composition, and transformation pathways of the solid phases that form in the Ca/Cd carbonate system. A novel short-range-ordered amorphous phase that differed from amorphous calcium carbonate (ACC) and amorphous cadmium carbonate (ACdC) was identified from the PDF measurements. The transformation of this amorphous phase, referred to as Cd-ACC, to vaterite was inhibited relative to that of ACC even at a low CdCO3 content of approximately 2 mol %. The results were highly dependent on the level of supersaturation with respect to ACC, whereby, for a given Cd2+ concentration, Cd stabilized amorphous phases at high ACC supersaturation but not at low ACC supersaturation. This finding suggests that supersaturation is a pathway-determining quantity in mixed-cation carbonate systems. Overall, this work provides a quantitative understanding of the intermediate phase structure and stability in mixed-cation carbonate systems.