Substitution of Ca2+ by Sn2+ and Sr2+ cations in P. placenta shells and single-crystal calcite through ion exchange reactions

Tin (Sn²⁺) and strontium (Sr²⁺) are potential replacements to lead (Pb²⁺) in perovskite synthesis since Sn is on the same IVA group in the periodic table as Pb while Sr is a promising alternative according to Goldschmidt’s rules and quantum mechanical analysis. The crystal radii of their ions are also nearly identical with Pb²⁺ = 1.33 Å, Sn²⁺ = 1.36 Å, and Sr²⁺ = 1.32 Å. In this study, both Sn and Sr were explored in transforming calcite, a polymorph of calcium carbonate (CaCO₃) into a leaving group in the first step of a sequential ion-exchange reaction towards perovskite formation. Instead of forming the intermediate tin carbonate (SnCO₃), the reaction resulted in the formation of gypsum or calcium sulfate dihydrate (CaSO_4.2H₂O) and Sn in the form of oxides. These oxides, however, are useful especially when these are in the form of tin dioxide-coated CaCO₃ shell-core structures—having demonstrated flame retardant and smoke suppressant properties. On the other hand, calcite was successfully transformed into strontium carbonate (SrCO₃) or strontianite through the cation exchange reaction. X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) were used to observe the resulting materials and understand the transformation of both Placuna placenta (or Capiz) shells and single-crystal calcite from the ion-exchange reactions.