Enthalpies of formation of lanthanide oxyapatite phases
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K.B. Helean Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616
M.C. Wilding Department of Geology, University of California at Davis, Davis, California 95616
P. Lu Department of Physics, Harvard University, Cambridge, Massachusetts 02138
A. Navrotskya) Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616 (Received 13 June 2001; accepted 2 August 2001)
A family of lanthanide silicates adopts an oxyapatitelike structure with structural formula Ln 9.33䊐0.67(SiO4)6O2 (Ln ⳱ La, Sm, Nd, Gd, 䊐 ⳱ vacancy). The enthalpies of solution, ⌬HS, 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, ⌬H 0f-oxides (kJ/mol): La9.33䊐0.67(SiO4)6O2 ⳱ −776.3 ± 17.9, Nd 9.33䊐0.67(SiO4)6O2 ⳱ −760.4 ± 31.9, Sm9.33䊐0.67(SiO4)6O2 ⳱ −590.3 ± 18.6, and Gd 9.33䊐0.67(SiO4)6O2 ⳱ −446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ⌬H 0f (kJ/mol): La9.33䊐0.67(SiO4)6O2 ⳱ −14611.0 ± 19.4, Nd 9.33䊐0.67(SiO4)6O2 ⳱ −14661.5 ± 32.2, Sm9.33䊐0.67(SiO4)6O2 ⳱ −14561.7 ± 20.8, and Gd 9.33䊐0.67(SiO4)6O2 ⳱ −14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases.
Lanthanide oxyapatites, Ln9.33䊐0.67(SiO4)6O2 (Ln ⳱ La, Sm, Nd, Gd, 䊐 ⳱ vacancy), have many interesting applications. Lanthanides are used as sintering aids during silicon nitride synthesis resulting in Ln oxyapatite formation at grain triple junctions in silicon nitride ceramics.1 In addition, there is increasing interest in Gd-containing compounds because of their high luminescence efficiency when doped with other rare-earth ions.2– 4 Many of these properties can be attributed to the unique oxyapatite structure that contains oxygen atoms located in the hexagonal tunnels parallel to the c axis. These oxygen atoms are bonded to Ln cations but are not bonded to Si and are therefore isolated from Si tetrahedra.5 Ln oxyapatites are potentially useful for modeling the release of actinides from ceramic nuclear waste forms.6,7 Thermodynamic measurements are critical in
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J. Mater. Res., Vol. 16, No. 10, Oct 2001 Downloaded: 25 Apr 2015
evaluating the phase stability of the Ln oxyapatites. The lack of experimental data motivated the present work on the correlation of enthalpies of formation to changes in the ionic radii of the lanthanide in the oxyapatite structure. The Ln oxyapatite phases were synthesized using a solid-state sintering process. A mechanical mixture of Ln2O3 (Ln ⳱ La, Nd, Sm, Gd) and SiO2-quartz according to the stoichiometric ratio 7Ln2O3:9SiO2 was prepared in a glove
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