Thermochemistry of calcium rare-earth silicate oxyapatites

The calcium rare-earth (RE) silicate oxyapatite, Ca2RE8(SiO4)(6)O-2 (RE = Yb, Er, Y, Dy, Nd, Gd, and Sm), powders were synthesized by the solid-state reaction method and characterized by X-ray diffraction (XRD), Raman spectroscopy, and elemental composition analysis. The thermodynamic properties of the oxyapatites have been investigated using high-temperature oxide melt calorimetry in molten 2PbO-B2O3 solvent at 805 degrees C. The energetics of the oxyapatites related to ionic substitution on two crystallographic sites, M(1) and M(2), are discussed. The enthalpy of formation from the oxides becomes more exothermic as the ionic potential decreases or the ionic radius of the REs increases, which indicates increasing energetic stability in this order.