Analysis of the miscibility of Cd2+ ions in CaCO3

The study of the substitution of Ca2+ ions by Cd2+ ions in precipitated calcium carbonate is reported. The synthesizing route consists in precipitating CdxCa1-xCO3 particles by rapidly mixing CdCl2 and CaCl2 precursor solutions with Na2CO3 solution. We show experimentally that Cd2+ ions are completely miscible in CdxCa1-xCO3 for x = 0 to x = 1. One interesting point was to investigate whether the vaterite polymorph could be obtained during the precipitation of the initial precursors as it is the case for x = 0. We evidence that vaterite can be obtained only when x is less than 10%. Above this threshold, the calcite structure is the only polymorph observed. Yet, SEM experiments evidence that although the calcite structure is formed at x > 10%, a spheroidal morphology is nevertheless preserved in opposition to the classical rhombohedral one usually observed for calcite. We show by quantitative X-ray analysis that the size of the coherently scattering domains strongly decreases while the strain increases when x increases yielding CdCO3 scattering over a coherent size of about 100 nm.