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. http://resolver.caltech.edu/CaltechAUTHORS:RISjmr01
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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.
|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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|Deposited On:||02 Nov 2006|
|Last Modified:||26 Dec 2012 09:15|
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