A. Navrotsky

Enthalpies of formation of Ce-pyrochlore, Ca0.93Ce1.00Ti2.035O7.00, U-pyrochlore, Ca1.46U4+0.23U6+0.46Ti1.85O7.00 and Gd-pyrochlore, Gd2Ti2O7: three materials relevant to the proposed waste form for excess weapons plutonium

High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, ΔH0f (kJ/mol), for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (−3656.0±5.6), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−3610.6±4.1) and Gd2Ti2O7 (−3822.5±4.9). Enthalpy of drop solution data, ΔHds, were used to calculate enthalpies of formation with respect to an oxide phase assemblage, ΔH0f−ox: CaO+MO2+2TiO2=CaMTi2O7 or Gd2O3+2TiO2=Gd2Ti2O7, and an oxide/perovskite phase assemblage, ΔH0f−pv+ox: CaTiO3+MO2+TiO2=CaMTi2O7, where M=Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with ΔH0f−ox (kJ/mol) (Gd2Ti2O7)=−113.4±2.8; ΔH0f−ox(Ca1.46U4+0.23U6+0.46Ti1.85O7.00)=−123.1±3.4; ΔH0f−ox(Ca0.93Ce1.00Ti2.035O7.00)=−54.1±5.2. U-pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (ΔH0f−pv+ox=−5.1±4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (ΔH0f−pv+ox=+21.0±5.5 kJ/mol). A significant metastability field was defined with respect to an oxide/perovskite phase assemblage. However, the proposed waste form baseline composition lies in the stable regions of the phase diagrams.

Understanding the stability of fluorosulfate Li-ion battery cathode materials: a thermochemical study of LiFe1−xMnxSO4F (0 ≤ x ≤ 1) polymorphs

Isothermal acid solution calorimetry was employed to investigate the relative thermochemical stabilities of two polymorphs of the LiFe1−xMnxSO4F (0 ≤ x ≤ 1) solid solution series: triplite and tavorite. These compounds have shown promise as lithium-ion battery cathodes, and a fuller understanding of their thermodynamics will aid in synthesis and their practical application. The linear energetic trends among triplites and tavorites indicate greater stabilization of each of these structures from the binary components with increase in manganese content and suggest a negligible heat of mixing of Fe and Mn ions. The tavorite phase, formed for x < 0.2, appears energetically more stable than the triplite. The formation of the disordered triplite structure appears to be entropy driven, and the factors that increase the disorder of the system (e.g. rapid phase formation) favor the triplite structure. Further, the free energy change associated with the tavorite to triplite transformation obtained by calculating configurational entropies using measured enthalpies was almost zero (−1.3 ± 0.8 kJ mol−1) at ambient temperature but becomes exothermic at 500 °C (−4.3 ± 0.8 to −6.8 ± 0.8 kJ mol−1). This suggests that both tavorite and triplite (with random cation distribution) are equally stable at ambient temperature but the tavorite to triplite transformation is thermodynamically favored at high temperatures because of entropy.

Enthalpy of formation and thermodynamic insights into yttrium doped BaZrO3

The enthalpies of formation from binary oxide components at 25 °C of Ba(Zr1−xYx)O3−δ, x = 0.1 to 0.5 solid solutions are measured by high temperature oxide melt solution calorimetry in a molten solvent, 3Na2O·4MoO3 at 702 °C. The enthalpy of formation is exothermic for all the compositions and becomes less negative when increasing yttrium content from undoped (−115.12 ± 3.69 kJ mol−1) to x = 0.5 (−77.09 ± 4.31 kJ mol−1). The endothermic contribution to the enthalpy of formation with doping content can be attributed to lattice distortions related to the large ionic radius difference of yttrium and zirconium and vacancy formation. For 0.3 ≤ x ≤ 0.5, the enthalpy of formation appears to level off, consistent with an exothermic contribution from defect clustering. Raman spectra indicate changes in short range structural features as a function of dopant content and, suggests that from x = 0.3 to 0.5 the defects begins to cluster significantly in the solid solution, which corroborates with the thermodynamic data and the drop-off in proton conductivity from x > 0.3.

Thermodynamics of thorium substitution in yttrium iron garnet: comparison of experimental and theoretical results

The thermodynamic stability of Th-doped yttrium iron garnet (Y3Fe5O12, YIG) as a possible actinide-bearing material has been investigated using calorimetric measurements and first-principles electronic-structure calculations. Yttrium iron garnet with thorium substitution ranging from 0.04 to 0.07 atoms per formula unit (Y3−xThxFe5O12, x = 0.04–0.07) was synthesized using a citrate–nitrate combustion method. High-temperature oxide melt solution calorimetry was used to determine their enthalpy of formation. The thermodynamic analysis demonstrates that, although the substitution enthalpy is slightly endothermic, an entropic driving force for the substitution of Th for Y leads to a near-zero change in the Gibbs free energy. First-principles calculations within the density functional theory (DFT) indicate that the main limiting factors for Th incorporation into the YIG structure are the narrow stability domain of the host YIG and the formation of ThO2 as a secondary phase. Nevertheless, the defect formation energy calculations suggest that by carefully tuning the atomic and electronic chemical potentials, Th can be incorporated into YIG. The thermodynamic results, as a whole, support the possible use of garnet phases as nuclear waste forms; however, this will require careful consideration of the repository conditions.

Possible correlation between enthalpies of formation and redox potentials in LiMSO4OH (M = Co, Fe, Mn), Li-ion polyanionic battery cathode materials

The thermodynamic stabilities of lithium hydroxysulfates of general formula LiMSO4OH (M = Co, Fe, Mn) with layered and tavorite structures have been investigated using isothermal acid solution calorimetry. These compounds have been explored as sustainable F-free alternatives to F-based flurosulfate cathode materials. The energetic trends for layered LiMSO4OH (M = Co, Fe and Mn) samples generally showed a decrease in stability with an increase in ionic radius (Co2+ to Mn2+), reflecting weaker M–O bonds and increasing structural distortions. The low symmetry tavorite LiFeSO4OH with a structure containing corner-shared octahedral chains is less stable than layered LiFeSO4OH with a more symmetric edge-shared octahedral structure. Structural distortions within the metal octahedra as well as changes in sulfate bonding and symmetry of the SO42− groups appear to control the thermodynamic and electrochemical behavior of LiMSO4OH (M = Co, Fe and Mn) materials. Both redox potential and thermodynamic stability of layered LiMSO4OH (M = Co, Fe and Mn) can be correlated to the lowering of the sulfate bonding symmetry in the structure from C3v to C2v.