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

Abstract

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 (CaSO4.2H2O) 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.