Most substances can crystallise into two or more different crystal lattices,
called polymorphs. Despite this, there are no systems in which we can
quantitatively predict the probability of one competing polymorph forming,
instead of the other. We address this problem using large scale (hundreds of
events) studies of the competing nucleation of the alpha and gamma polymorphs
of glycine. In situ Raman spectroscopy is used to identify the polymorph of
each crystal. We find that the nucleation kinetics of the two polymorphs is
very different. Nucleation of the alpha polymorph starts off slowly but
accelerates, while nucleation of the gamma polymorph starts off fast but then
slows. We exploit this difference to increase the purity with which we obtain
the gamma polymorph by a factor of ten. The statistics of the nucleation of
crystals is analogous to that of human mortality, and using a result from
medical statistics we show that conventional nucleation data can say nothing
about what, if any, are the correlations between competing nucleation
processes. Thus we can show that, with data of our form, it is impossible to
disentangle the competing nucleation processes. We also find that the growth
rate and the shape of a crystal depends on when it nucleated. This is new
evidence that nucleation and growth are linked.Comment: 8 pages, plus 17 pages of supplementary materia
