Influence of iron on the synthesis and stability of yttrium silicate apatite

The stability of yttrium silicate apatite has been investigated by studying the influence of iron as a "stabilising cation" and also by using different synthesis routes. The formation of apatite in samples has been followed by X-ray diffraction and by ²⁹Si MAS NMR spectroscopy. The apatite phase appears to be stable at high temperatures (≈1700°C) especially when heated in a nitrogen atmosphere; it can also occur in a metastable state when heated in air at lower temperatures; ≈1600°C if prepared from a Y₂O₃-SiO₂ mixture or in the range 950°C < T < 1150°C if synthesised by the sol-gel process. Longer heat-treatments result in its decomposition into Y₂Si₂O₇ and Y₂SiO₅. Iron appears to have two roles depending on the temperature; it stabilises the apatite phase at high temperatures when produced by the sol-gel route and catalyses the decomposition of sol-gel derived apatite at low temperatures.