More than 50 uranyl minerals, phases containing U6+ as the uranyl UO2 2+ cation, and hydroxide, carbonate, phosphate, and silicate anions, H2O, and alkali and alkaline earth cations, occur in nature ...and as corrosion products of spent nuclear fuel. Despite their importance and the need to understand their thermodynamics to predict uranium solubility, fate, and transport in the environment, reliable thermodynamic data have only been available recently. This paper summarizes recent studies of enthalpies of formation using high temperature oxide melt solution calorimetry and Gibbs free energies from solubility experiments. Standard state thermochemical parameters (at 25 °C and 1 bar) are tabulated and the stability and transformation sequences of these phases are discussed. The enthalpies of formation from oxides are discussed in terms of crystal structure and Lewis acid–base interactions.
Thermochemistry of Lanthana- and Yttria-Doped Thoria Aizenshtein, Michael; Shvareva, Tatiana Y.; Navrotsky, Alexandra
Journal of the American Ceramic Society,
December 2010, Volume:
93, Issue:
12
Journal Article
Peer reviewed
Lanthanide‐doped thoria is relevant both to nuclear energy and to solid oxide fuel cell technology. It is also a simple model system in which oxygen vacancy concentration is directly proportional to ...doping with no complication from oxidation–reduction reactions, ordered phases, or phase transitions in the tetravalent oxide. Despite this relevance, only few thermodynamic data are available for such systems. In the present study, LaxTh1−xO2−0.5x (0<x<0.50) and YxTh1−xO2−0.5x (0<x<0.18) have been synthesized, characterized, and systematically studied for the first time using high‐temperature oxide melt solution calorimetry. Two competing effects contribute to the energetics: endothermic deviation from ideality related to cation size difference and partial stabilization of the system due to the exothermic formation of defect clusters. The former can be described by endothermic regular solution interaction parameters of 119.3 ± 11.4 kJ/mol for the yttria system and 27.8 ± 4.1 kJ/mol for the lanthana system. The latter can be described by exothermic vacancy clustering or defect association energies of –116.3 ± 23.3 kJ/mol for yttria‐doped thoria and –98.6 ± 19.7 kJ/mol for lanthana‐doped thoria.
A comprehensive understanding of chemical interactions between water and actinide dioxide surfaces is critical for safe operation and storage of nuclear fuels. Despite substantial previous research, ...understanding the nature of these interactions remains incomplete. In this work, we combine accurate calorimetric measurements with first-principles computational studies to characterize surface energies and adsorption enthalpies of water on two fluorite-structured compounds, ThO2 and CeO2, that are relevant for understanding the behavior of water on actinide oxide surfaces more generally. We determine coverage-dependent adsorption enthalpies and demonstrate a mixed molecular and dissociative structure for the first hydration layer. The results show a correlation between the magnitude of the anhydrous surface energy and the water adsorption enthalpy. Further, they suggest a structural model featuring one adsorbed water molecule per one surface cation on the most stable facet that is expected to be a common structural signature of water adsorbed on actinide dioxide compounds.
Nanoscale uranyl peroxide clusters containing UO22+ groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. ...There is an extensive family of clusters containing 28 uranium atoms (U28 clusters), with an encapsulated anion in the center, for example, UO2(O2)3−x(OH)x4−, Nb(O2)43−, or Ta(O2)43−. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U28 compounds, one of which is uranyl centered and the other is peroxotantalate centered. The (Ta(O2)4‐centered U28 capsule is energetically more stable than the (UO2)(O2)3‐centered capsule. These data, along with our prior studies on other uranyl–peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali–uranyl–peroxide systems. It was suggested that the energetic role of charge‐balancing alkali ions and their electrostatic interactions with the negatively charged uranyl–peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the (Ta(O2)4‐centered U28 capsule is more stable than the uranyl‐centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali‐anion interactions was shown to be important at all length scales of uranyl–peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.
Nanocapsules: Uranyl–peroxide polyoxometalates were templated by encapsulated cations and anions; the stability gained by this encapsulation was quantified. Calorimetry and calculations agree that increasing cation/anion interactions lead to thermodynamic and electronic orbital stabilization. In the broader landscape, the related uranyl–peroxide monomers and extended solids do not have such extensive cation/anion interactions, and thus lack this enthalpic gain (see figure).
Novel open-framework alkali metal uranyl periodates, having the formula A(UO2)3(HIO6)(OH)(O)(H2O)·1.5H2O (A = Li, Na, K, Rb, Cs), have been prepared through mild hydrothermal synthesis. These ...isostructural compounds contain distorted UO7 pentagonal bipyramids that are linked through a uranyl (UO2 2+) to uranyl cation−cation interaction. This interaction arises from a single axial uranyl oxygen coordinating at an equatorial site of an adjacent uranyl unit. These uranium oxide polyhedra are further bound by IO6 distorted octahedra creating an open-framework structure whose channels contain the alkali metal cations.
Boltwoodite and uranophane are uranyl silicates common in oxidized zones of uranium ore deposits. An understanding of processes that impact uranium transport in the environment, especially pertaining ...to the distribution of uranium between solid phases and aqueous solutions, ultimately requires determination of thermodynamic parameters for such crystalline materials. We measured formation enthalpies of synthetic boltwoodites, K(UO
2)(HSiO
4)·H
2O and Na(UO
2)(HSiO
4)·H
2O, and uranophane, Ca(UO
2)
2(HSiO
4)
2·5H
2O, by high temperature oxide melt solution calorimetry. We also studied the aqueous solubility of these phases from both saturated and undersaturated conditions at a variety of pH. The combined data permit the determination of standard enthalpies, entropies and Gibbs free energies of formation for each phase and analysis of its potential geological impact from a thermodynamic point of view.
High‐temperature solution calorimetry in molten 3Na2O•4MoO3 solvent at 975 K is used to measure the enthalpy of mixing of (CaxSr1‐x)TiO3 and (BaxSr1‐x)TiO3 solid solutions. Both solid solutions show ...positive enthalpy of mixing, however, the (BaxSr1‐x)TiO3 shows less deviation from ideality, with interaction parameter λ = 8.5 ± 3.8 kJ/mol, than (CaxSr1‐x)TiO3 with interaction parameter λ = 25.3 ± 2.0 kJ/mol. The smaller interaction parameter of the (BaxSr1‐x)TiO3 system is attributed to the volume mismatch of the end‐members. The larger interaction parameter of (CaxSr1‐x)TiO3 may be related to larger ionic radius difference of Ca and Sr and may also reflect the additional energy needed to overcome the lattice distortion of the orthorhombic CaTiO3 to incorporate it into the cubic perovskite SrTiO3. (BaxCa1‐x)TiO3 has a large positive heat of mixing and limited solid solubility as derived from its phase diagram. This study indicates the tendency of SrTiO3 and BaTiO3 to separate from a CaTiO3 host matrix, which may be significant in perovskite‐bearing nuclear waste ceramics incorporating 90Sr and 137Ba.
High‐temperature solution calorimetry in molten
3Na
2
O
•
4MoO
3
solvent at 975 K is used to measure the enthalpy of mixing of (
Ca
x
Sr
1‐
x
)
TiO
3
and (
Ba
x
Sr
1‐
x
)
TiO
3
solid solutions. Both ...solid solutions show positive enthalpy of mixing, however, the (
Ba
x
Sr
1‐
x
)
TiO
3
shows less deviation from ideality, with interaction parameter λ = 8.5 ± 3.8 kJ/mol, than (
Ca
x
Sr
1‐
x
)
TiO
3
with interaction parameter λ = 25.3 ± 2.0 kJ/mol. The smaller interaction parameter of the (
Ba
x
Sr
1‐
x
)
TiO
3
system is attributed to the volume mismatch of the end‐members. The larger interaction parameter of (
Ca
x
Sr
1‐
x
)
TiO
3
may be related to larger ionic radius difference of
Ca
and
Sr
and may also reflect the additional energy needed to overcome the lattice distortion of the orthorhombic
CaTiO
3
to incorporate it into the cubic perovskite
SrTiO
3
. (
Ba
x
Ca
1‐
x
)
TiO
3
has a large positive heat of mixing and limited solid solubility as derived from its phase diagram. This study indicates the tendency of
SrTiO
3
and
BaTiO
3
to separate from a
CaTiO
3
host matrix, which may be significant in perovskite‐bearing nuclear waste ceramics incorporating
90
Sr
and
137
Ba
.
