AbstractKnowledge about the mechanisms involved in the structural development of solid materials at the atomic level is essential for designing rational synthesis protocols for these compounds, which may be used to improve desired technical properties, such as light emission, conductivity, magnetism, porosity or particle size, and may allow the tailored design of solid materials to generate the aforementioned properties. Recent technological advancements have allowed the combination of synchrotron-based

In this work, in situ luminescence analysis was applied for the first time for monitoring the phase transitions of calcium phosphate (CaP) and confirmed by synchrotron in situ X-ray diffraction in addition to in situ infrared spectroscopy, with simultaneous measurements of pH and ion conductivity. Applying doped Ce 3+ and Eu 3+ as local coordination sensors, the high sensitivity of their emission spectra upon the changes in the coordination sphere of the doped cation sites enabled to detect the formation of amorphous calcium phosphate (ACP) and Ca 5 (PO 4 ) 3 OH, besides their subsequent transitions to CaHPO 4 ·2H 2 O and Ca 8 H 2 (PO 4 ) 6 ·5H 2 O under real reaction conditions. Calcium phosphates are widely present in mammals and understanding their phase transitions is important to comprehend the conversion between healthy and diseased tissues. In situ luminescence measurements are advantageous for allowing monitoring these phase transitions in a fast and sensitive fashion also in conventional laboratories, independent of synchrotron radiation.