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Enthalpies of formation of lanthanide oxyapatite phases

Risbud, A. S. and Helean, K. B. and Wilding, M. C. and Lu, P. and Navrotsky, A. (2001) Enthalpies of formation of lanthanide oxyapatite phases. Journal of Materials Research, 16 (10). pp. 2780-2783. ISSN 0884-2914.

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A family of lanthanide silicates adopts an oxyapatite-like structure with structural formula Ln9.33∎0.67(SiO4)6O2 (Ln 4 La, Sm, Nd, Gd, ∎ = vacancy). The enthalpies of solution, DHS, 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, ΔHs f-oxides (kJ/mol): La9.33∎0.67(SiO4)6O2 = −776.3 ± 17.9, Nd9.33∎0.67(SiO4)6O2 = −760.4 ± 31.9, Sm9.33∎0.67(SiO4)6O2 = −590.3 ± 18.6, and Gd9.33∎0.67(SiO4)6O2 = −446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ΔH0 f (kJ/mol): La9.33∎0.67(SiO4)6O2 = −14611.0 ± 19.4, Nd9.33∎0.67(SiO4)6O2 = −14661.5 ± 32.2, Sm9.33∎0.67(SiO4)6O2 = −14561.7 ± 20.8, and Gd9.33∎0.67(SiO4)6O2 = −14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases.

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Navrotsky, A.0000-0002-3260-0364
Additional Information:© 2001 Materials Research Society (Received 13 June 2001; accepted 2 August 2001) This work was supported by the Center for High Pressure Research, a National Science Foundation Science and Technology Center. The microprobe analysis was performed at the Department of Geology at the University of California, Davis.
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ID Code:5793
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Deposited On:02 Nov 2006
Last Modified:09 Mar 2020 13:19

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