A comparative study of the thermodynamic stability of britholites

Abstract

The apatites have a structure stability and durability under radiation conditions in geological situation and in synthetic products. Thus, britholites, which are phospho-silicate-apatite-containing rare earth elements, are considered to be one of the possible candidates for actinides and fission product immobilization. In this work, a comparative study of the thermochemical stability of four apatite-solid-solutions were performed, which are: Lanthanum-enclosing silicate-fluorapatites (LaF) Ca(10\u2009−\u2009x)Lax(PO4)6\u2009−\u2009x(SiO4)xF2 Lanthanum-enclosing silicate-oxyapatites (LaO) Ca(10\u2009−\u2009x)Lax(PO4)6\u2009−\u2009xSiO4)xO□ Neodymium-enclosing silicate-fluorapatite (NdF) Ca(10\u2009−\u2009x)Ndx(PO4)6\u2009−\u2009x(SiO4)xF2 Neodymium-enclosing silicate-oxyapatites (NdO) Ca(10\u2009−\u2009x)Ndx(PO4)6\u2009−\u2009x(SiO4)xO□ Lanthanum-enclosing silicate-fluorapatites (LaF) Ca(10\u2009−\u2009x)Lax(PO4)6\u2009−\u2009x(SiO4)xF2 Lanthanum-enclosing silicate-oxyapatites (LaO) Ca(10\u2009−\u2009x)Lax(PO4)6\u2009−\u2009xSiO4)xO□ Neodymium-enclosing silicate-fluorapatite (NdF) Ca(10\u2009−\u2009x)Ndx(PO4)6\u2009−\u2009x(SiO4)xF2 Neodymium-enclosing silicate-oxyapatites (NdO) Ca(10\u2009−\u2009x)Ndx(PO4)6\u2009−\u2009x(SiO4)xO□ with 0 ≤x≤6 is the substitution rate and □ is a vacancy. It was found that the absolute value of the solution enthalpies at infinite dilution increases with the substitution rate in all studied britholites. This is in agreement with the evolution of the dissociation energies of these apatites, proofing the gain in interatomic cohesion with the double substitution (Ca2+; PO43−) ↔ (Ln3+; SiO44−; Ln: La or Nd). In addition, the entropy of oxybritholite indicates a slight disorder compared to fluorbritholites, and the standard Gibbs free energy of formation reveals the particular stability of oxybritholite, either containing lanthanum or neodymium.