Mutual polymorphic transformation of CsMgPO4·6H2O induced by solution pH†

Abstract

This work compares the relative stability of the two forms of CsMgPO₄·6H₂O from theoretical and experimental approaches. The results show that the cubic form is more stable as it has a higher decomposition temperature and heat compared with the hexagonal form and also a larger lattice energy. However, the hexagonal form is more stable in solution within the pH range of 7.00–9.35 while the cubic form is more stable within the pH range of 9.35–11.00 according to the experimental solubility data. The intersection phenomenon of solubility curves is attributed to the pH regulation which changed the configuration of PO₄³⁻ affecting the effective concentration of each crystal form. Based on the solubility curves of the two forms, the mutual polymorphic transformation of CsMgPO₄·6H₂O induced by the solution pH was predicted and confirmed experimentally. The mechanism turned out to be the solution-mediated transformation and the rate-determining step was determined to be the growth of the stable crystal form. The parameter of solution pH had a great impact on the stability of the two forms and deduced the mutual polymorphic transformation of CsMgPO₄·6H₂O, which may serve as a good reference for polymorphic compounds.