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

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.