The structure of PbCl 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 (PbCl) 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 PbCl and NaCl {100} and the structure at the surface. We find that Pb replaces a surface Na ion, while a Cl ion is located on top of the Pb. 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 PbCl is much weaker, most likely due to the fact that the Pb ion can more easily desorb. Moreover, PbCl has an even stronger effect on NaCl {111}.