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Enthalpies of formation of lanthanide oxyapatite phases

Published online by Cambridge University Press:  31 January 2011

A. S. Risbud ,
K. B. Helean ,
M. C. Wilding ,
P. Lu  and
A. Navrotsky
A. S. Risbud
Affiliation:
Department of Applied Physics, California Institute of Technology, MC 128–95, Pasadena, California 91125
K. B. Helean
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616
M. C. Wilding
Affiliation:
Department of Geology, University of California at Davis, Davis, California 95616
P. Lu
Affiliation:
Department of Physics, Harvard University, Cambridge, Massachusetts 02138
A. Navrotsky*
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616
*
a)Address all correspondence to this author.[email protected]
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Abstract

A family of lanthanide silicates adopts an oxyapatitelike structure with structural formula Ln9.33 0.67(SiO4)6O2 (Ln = La, Sm, Nd, Gd,   = vacancy). The enthalpies of solution, ΔHS, 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, ΔH0 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.

Type
Rapid Communications
Information
Journal of Materials Research , Volume 16 , Issue 10 , October 2001 , pp. 2780 - 2783
Copyright
Copyright © Materials Research Society 2001

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