High-temperature phase transformations of hydroxylapatite and the formation of silicocarnotite in the hydroxylapatite–quartz–lime system studied in situ and in operando by Raman spectroscopy

Abstract Silica-/calcium phosphate ceramics are of high interest in various aspects. On the one hand, they play an important role in medical applications due to their excellent biocompatibility. Therefore, detailed knowledge of the formation and stability properties of the high-temperature products ensures production under controlled conditions. On the other hand, they were identified as sinter deposits in industrial kilns, where it can indicate problems caused by too high combustion temperatures during the thermal combustion processes. Here, we report the results of two Raman heating studies to ~ 1300 °C in 10 °C-steps with nano-crystalline hydroxylapatite (HAp) and tricalcium phosphate (TCP), and a Raman heating study of natural silicocarnotite (to ~ 1200 °C, 50 °C-steps). The Raman experiments were complemented with thermal analyses. The Raman spectra of nano-crystalline HAp recorded at high temperatures revealed the stepwise loss of adsorbed water and surface-bound OH groups until ~ 570 °C. Significant loss of structural OH started at ~ 770 °C and was completed at ~ 850 °C, when HAp transformed to β-TCP. Between ~ 1220 and ~ 1270 °C, β-TCP was found to transform to α-TCP. The room temperature Raman spectrum of silicocarnotite is characterized by an intense v₁(PO₄) band at 951 ± 1 cm⁻¹ that shifts to ~ 930 cm⁻¹ at ~ 1200 °C. Using hyperspectral Raman imaging with a micrometer-scale spatial resolution, we were able to monitor in operando and in situ the solid-state reactions in the model system Ca₁₀(PO₄)₆(OH)₂-SiO₂-CaO, in particular, the formation of silicocarnotite. In these multi-phase experiments, silicocarnotite was identified at ~ 1150 °C. The results demonstrate that silicocarnotite can form by a reaction between β-TCP and α′_L-Ca₂SiO₄, but also between β-TCP and CaSiO₃ with additional formation of quartz.