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Density functional calculations of the enthalpies of formation of rare-earth orthophosphates

  • James R. Rustad EMAIL logo
Published/Copyright: April 2, 2015
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American Mineralogist
From the journal American Mineralogist Volume 97 Issue 5-6

Abstract

Electronic structure calculations are carried out to estimate the enthalpies of formation of rareearth orthophosphates from their oxides. The calculated enthalpies of formation are systematically less exothermic than the measured values. The discrepancy is almost entirely in the electronic total energy calculated from density functional theory, and appears to be intrinsic to the generalized-gradient exchange-correlation functional used. However, comparison with electronic structure calculation of the enthalpies of formation of alkaline earth oxyacid carbonates, silicates, and sulfates suggests that near chemical accuracy can be obtained for the enthalpies of formation of most of the compounds in the phosphate system by applying a scaling factor obtained from the simpler alkaline earth oxyacids. The increasingly exothermic ΔHfox with increasing ionic radius (i.e., LaPO4 is more exothermic than ScPO4) results from the higher charge localization on the oxide anion (O2-) relative to the phosphate anion (PO43-), making it more favorable, in a relative sense, to pair the smaller cation with the oxide anion than with the phosphate anion. This effect is also manifested in ΔHfox of the other oxyacids, such as carbonates, silicates, and sulfates.

Keywords: Rare-earth elements; density functional calculations; thermodynamic properties; phosphate minerals
Received: 2011-7-12
Accepted: 2012-1-6
Published Online: 2015-4-2
Published in Print: 2012-5-1

© 2015 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am.2012.3948
Keywords for this article
Rare-earth elements; density functional calculations; thermodynamic properties; phosphate minerals

Articles in the same Issue

  1. Dana medal lecture: Tilts and tetrahedra: The origin of the anisotropy of feldspars
  2. Amorphous Materials: Determination of water content in silicate glasses using Raman spectrometry: Implications for the study of explosive volcanism
  3. Density functional calculations of the enthalpies of formation of rare-earth orthophosphates
  4. U-Pb ID-TIMS dating applied to U-rich inclusions in garnet
  5. Wassonite: A new titanium monosulfide mineral in the Yamato 691 enstatite chondrite
  6. Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy
  7. The morphology of the reaction front of the dissolutionprecipitation reaction rutile + wollastonite = titanite in time series experiments at 600 °C/400 MPa
  8. Precipitation of rutile and ilmenite needles in garnet: Implications for extreme metamorphic conditions in the Acadian Orogen, U.S.A.
  9. The genesis of mantle-derived sapphirine
  10. Experimental investigation of smectite interaction with metal iron at 80 °C: Structural characterization of newly formed Fe-rich phyllosilicates
  11. High-pressure study of a natural cancrinite
  12. Investigation of cation ordering in triclinic sodium birnessite via 23Na MAS NMR spectroscopy
  13. The dual behavior of the β-As4S4 altered by light
  14. Pressure-induced changes in local electronic structures of SiO2 and MgSiO3 polymorphs: Insights from ab initio calculations of O K-edge energy-loss near-edge structure spectroscopy
  15. Structural stability, cation ordering, and local relaxation along the AlNbO4-Al0.5Cr0.5NbO4 join
  16. Europium oxidation state and local structure in silicate glasses
  17. Kinetics of partial dehydroxylation in dioctahedral 2:1 layer clay minerals
  18. Determination of Fe3+/Fe using the electron microprobe: A calibration for amphiboles
  19. Characterization of smectite to NH4-illite conversion series in the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania
  20. Allanite-(Nd), CaNdAl2Fe2+(SiO4)(Si2O7)O(OH), a new mineral from Åskagen, Sweden
  21. Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, a new mineral species of the tourmaline supergroup from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy
  22. Letter. A nanoscopic approach to the kinetics of anhydrite (100) surface growth in the range of temperatures between 60 and 120 °C
  23. Letter. High-pressure phase transitions in Ca-Mn carbonates (Ca,Mn)CO3 studied by Raman spectroscopy
  24. Letter. Actinides in Geology, Energy, and the Environment: Thermally induced transformation of vorlanite to “protovorlanite”: Restoration of cation ordering in self-irradiated CaUO4
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