Understanding the Stability of Salt-Inclusion Phases for Nuclear Waste-forms through Volume-based Thermodynamics

Abstract

Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.

Versatile Uranyl Germanate Framework Hosting 12 Different Alkali Halide 1D Salt Inclusions

“…The pore volume per channel per unit cell was determined by removing the ionic species in the channels and using the Calc Solv functionality in PLATON and dividing the resulting volume by Z , the number of formula units per unit cell . The salt-inclusion volumes were calculated by the summation of “thermochemical radii”, an adaptation of the Goldschmidt ionic radii used to describe complex salts such as those in SIMs . The percent-volume is derived from the thermochemical volume of the salt and the formula-unit volume obtained from XRD patterns .…”

“…DFT and VBT studies on uranyl salt-inclusion materials were recently reported by Moore et. al, who concluded that the [(UO 2 ) 3 ­(Ge 2 O 7 ) 2 ] 6– and analogous [(UO 2 ) 3 ­(Si 2 O 7 ) 2 ] 6– frameworks are thermodynamically stable . VBT methods predict the average formation enthalpies of the silicates and germanates to be −14781 and −13972 kJ/mol, respectively, while DFT methods predict −9365 kJ/mol and −7967 kJ/mol, respectively.…”

“…18 The salt-inclusion volumes were calculated by the summation of "thermochemical radii", 22 an adaptation of the Goldschmidt 23 ionic radii used to describe complex salts such as those in SIMs. 24 The percent-volume is derived from the thermochemical volume of the salt and the formula-unit volume obtained from XRD patterns. 25 The pore-size diameters are calculated using the Zeo++ software, which considers the largest included sphere or void within a crystalline porous material using Voronoii decomposition methods.…”

“…al, who concluded that the [(UO 2 ) 3 (Ge 2 O 7 ) 2 ] 6− and analogous [(UO 2 ) 3 (Si 2 O 7 ) 2 ] 6− frameworks are thermodynamically stable. 24 VBT methods predict the average formation enthalpies of the silicates and germanates to be −14781 and −13972 kJ/mol, respectively, while DFT methods predict −9365 kJ/mol and −7967 kJ/mol, respectively. The analogous uranyl silicate SIMs by DFT calculations are more negative by 16.1% when compared to the germanates, and 5.6% more negative as predicted by VBT.…”

Polyoxometalate chemistry at volcanoes: discovery of a novel class of polyoxocuprate nanoclusters in fumarolic minerals

“…The geometrical similarity of the Na array in arsmirandite and lehmannite to that observed in halite, NaCl, allows to describe both minerals as consisting of cubic-shape nanoclusters periodically integrated into deficient NaCl matrix, a feature that is quite uncommon for inorganic materials. There has been a recent interest in salt-inclusion compounds (SICs), which possess hierarchical structures consisting of porous metal-oxide frameworks with voids filled with simple ionic salts 53 . The examples of natural SICs are averievite, Cu 5 O 2 (VO 4 )•nMCl x (M = Cu, Cs, Rb, K) [54][55][56][57] , and aleutite, Cu 5 O 2 (AsO 4 )(VO 4 )•(Cu 0.5□0.5 )Cl 25 .…”