Enthalpies of formation of lanthanide oxyapatite phases
|Title||Enthalpies of formation of lanthanide oxyapatite phases, Journal of Materials Research 16, 2780-2783|
|Publication Type||Journal Article|
|Year of Publication||2001|
|Authors||Risbud, A. S., K. B. Helean, M. C. Wilding, Peter J. Lu, and A. Navrotsky|
A family of lanthanide silicates adopts an oxyapatitelike structure with structural formula Ln9.330.67(SiO4)6O2 (Ln = La, Sm, Nd, Gd, = vacancy). The enthalpies of solution, ÄHS, for these materials and their corresponding binary oxides were determined by high-temperature oxide melt solution calorimetry using molten 2PbO • B2O3 at 1078 K. These data were used to complete thermodynamic cycles to calculate enthalpies of formation from the oxides, ÄH0f-oxides (kJ/mol): La9.330.67(SiO4)6O2 = -776.3 ± 17.9, Nd9.330.67(SiO4)6O2 = -760.4 ± 31.9, Sm9.330.67(SiO4)6O2 = -590.3 ± 18.6, and Gd9.330.67(SiO4)6O2 = -446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ÄH0f (kJ/mol): La9.330.67(SiO4)6O2 = -14611.0 ± 19.4, Nd9.330.67(SiO4)6O2 = -14661.5 ± 32.2, Sm9.330.67(SiO4)6O2 = -14561.7 ± 20.8, and Gd9.330.67(SiO4)6O2 = -14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases.