The structure of PbCl₂ on the {100} surface of NaCl and its consequences for crystal growth

The role that additives play in the growth of sodium chloride is a topic which has been widely researched but not always fully understood at an atomic level. Lead chloride (PbCl₂) is one such additive which has been reported to have growth inhibition effects on NaCl {100} and {111}; however, no definitive evidence has been reported which details the mechanism of this interaction. In this investigation, we used the technique of surface x-ray diffraction to determine the interaction between PbCl₂ and NaCl {100} and the structure at the surface. We find that Pb²⁺ replaces a surface Na⁺ ion, while a Cl^- ion is located on top of the Pb²⁺. This leads to a charge mismatch in the bulk crystal, which, as energetically unfavourable, leads to a growth blocking effect. While this is a similar mechanism as in the anticaking agent ferrocyanide, the effect of PbCl₂ is much weaker, most likely due to the fact that the Pb²⁺ ion can more easily desorb. Moreover, PbCl₂ has an even stronger effect on NaCl {111}.