Effect of rare earth ion substitution on phase decomposition of apatite structure

The paper describes an investigation of phase decomposition of apatite lattice doped with rare earth ions (cerium, samarium, and holmium) at temperatures ranging from 25 to 1200 ºC. The rare-earth ion-doped apatite minerals were synthesized using sol-gel method. In situ high-temperature powder X-ray diffraction (XRD) was used to observe phase changes and the lattice parameters were analyzed to ascertain the crystallographic transformations. The expansion coefficient of the compounds was determined, and it was found that the c-axis was the most expandable due to relatively weak chemical bonds along the c-crystallographic axis. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to examine the decomposition properties of the materials. Due to rare earth ion doping, the produced materials had slightly variable decomposition behaviour. The cerium and samarium ions were present in multiple oxidation states (Ce³⁺, Ce⁴⁺, Sm³⁺, Sm²⁺), whereas only Ho³⁺ ions were observed. Rare earth ion substitution affects tri-calcium phosphate proportion during decomposition by regulating concentrations of vacancies. X-ray photoelectron spectroscopy (XPS) analysis indicated that cerium and samarium ion-doped apatite yielded only 25% tricalcium phosphate during decomposition. This finding advances our understanding of apatite structures, with implications for various high-temperature processes like calcination, sintering, hydrothermal processing, and plasma spraying.