Crystallisation of phosphates revisited. A multi-step formation process for SrHPO${}_4$

SrHPO${}_4$ is used in a multitude of applications, including biomedicine, catalysts, luminescent materials, and batteries. However, the performance of these materials depends on the ability to control the formation and transformation of strontium phosphates. This work focuses on the application of in situ and ex situ measurements, including synchrotron-based X-ray diffraction (XRD) analysis, luminescence of Ce${}^{3+}$ and Eu${}^{3+}$ dopants, light transmission, reflectance, and thermogravimetry to track structural changes in SrHPO${}_4$ under different experimental conditions. Ex situ analysis of aliquots revealed favourable crystallisation of β-SrHPO${}_4$ through the formation of Sr${}_6$H${}_3$(PO${}_4$)${}_5$·2H${}_2$O as an intermediate. Furthermore, in situ analysis showed that the reaction mechanism evolves via the initial formation of amorphous strontium phosphate and Sr${}_5$(PO${}_4$)${}_3$OH, which subsequently transforms to γ-SrHPO${}_4$. Analysis of the luminescence properties of the lanthanide dopants provided insights into the coordination environments of the substituted Sr${}^{2+}$ sites.