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

Abstract

Tin (Sn ^2+ ) and strontium (Sr ^2+ ) are potential replacements to lead (Pb ^2+ ) 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 Pb ^2+ = 1.33 Å, Sn ^2+ = 1.36 Å, and Sr ^2+ = 1.32 Å. In this study, both Sn and Sr were explored in transforming calcite, a polymorph of calcium carbonate (CaCO _3 ) into a leaving group in the first step of a sequential ion-exchange reaction towards perovskite formation. Instead of forming the intermediate tin carbonate (SnCO _3 ), the reaction resulted in the formation of gypsum or calcium sulfate dihydrate (CaSO _4. 2H _2 O) and Sn in the form of oxides. These oxides, however, are useful especially when these are in the form of tin dioxide-coated CaCO _3 shell-core structures—having demonstrated flame retardant and smoke suppressant properties. On the other hand, calcite was successfully transformed into strontium carbonate (SrCO _3 ) 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.

Keywords

• perovskite synthesis
• calcite
• strontianite
• ion exchange