Energetic Stability and Its Role in the Mechanism of Ionic Transport in NASICON-Type Solid-State Electrolyte Li1+xAlxTi2-x(PO4)3

We apply high-temperature oxide melt solution calorimetry to assess the thermodynamic properties of the material Li_1+xAl_xTi_2-x(PO₄)₃, which has been broadly recognized as one of the best Li-ion-conducting solid electrolytes of the NASICON family. The experimental results reveal large exothermic enthalpies of formation from binary oxides (ΔH_f,ox^°) and elements (ΔH_f,el^°) for all compositions in the range 0 ≤ x ≤ 0.5. This indicates substantial stability of Li_1+xAl_xTi_2-x(PO₄)₃, driven by thermodynamics and not just kinetics, during long-term battery operation. The stability increases with increasing Al³⁺ content. Furthermore, the dependence of the formation enthalpy on the Al³⁺ content shows a change in behavior at x = 0.3, a composition near which the Li⁺ conductivity reaches the highest values. The strong correlation among thermodynamic stability, ionic transport, and clustering is a general phenomenon in ionic conductors that is independent of the crystal structure as well as the type of charge carrier. Therefore, the thermodynamic results can serve as guidelines for the selection of compositions with potentially the highest Li⁺ conductivity among different NASICON-type series with variable dopant contents.

Rare-Earth Orthophosphates From Atomistic Simulations

Lanthanide phosphates (LnPO₄) are considered as a potential nuclear waste form for immobilization of Pu and minor actinides (Np, Am, and Cm). In that respect, in the recent years we have applied advanced atomistic simulation methods to investigate various properties of these materials on the atomic scale. In particular, we computed several structural, thermochemical, thermodynamic and radiation damage related parameters. From a theoretical point of view, these materials turn out to be excellent systems for testing quantum mechanics-based computational methods for strongly correlated electronic systems. On the other hand, by conducting joint atomistic modeling and experimental research, we have been able to obtain enhanced understanding of the properties of lanthanide phosphates. Here we discuss joint initiatives directed at understanding the thermodynamically driven long-term performance of these materials, including long-term stability of solid solutions with actinides and studies of structural incorporation of f elements into these materials. In particular, we discuss the maximum load of Pu into the lanthanide-phosphate monazites. We also address the importance of our results for applications of lanthanide-phosphates beyond nuclear waste applications, in particular the monazite-xenotime systems in geothermometry. For this we have derived a state-of-the-art model of monazite-xenotime solubilities. Last but not least, we discuss the advantage of usage of atomistic simulations and the modern computational facilities for understanding of behavior of nuclear waste-related materials.

A Spectroscopic and Computational Study of Cm3+ Incorporation in Lanthanide Phosphate Rhabdophane (LnPO4·0.67H2O) and Monazite (LnPO4)

This study investigates the incorporation of the minor actinide curium (Cm³⁺) in a series of synthetic La_{1- x}Gd _xPO₄ ( x = 0, 0.24, 0.54, 0.83, 1) monazite and rhabdophane solid-solutions. To obtain information on the incorporation process on the molecular scale and to understand the distribution of the dopant in the synthetic phosphate phases, combined time-resolved laser fluorescence spectroscopy and X-ray absorption fine structure spectroscopy investigations were conducted and complemented with ab initio atomistic simulations. We found that Cm³⁺ is incorporated in the monazite endmembers (LaPO₄ and GdPO₄) on one specific, highly ordered lattice site. The intermediate solid-solutions, however, display increasing disorder around the Cm³⁺ dopant as a result of random variations in nearest neighbor distances. In hydrated rhabdophane, and especially its La-rich solid-solutions, Cm³⁺ is preferentially incorporated on nonhydrated lattice sites. This site occupancy is not in agreement with the hydrated rhabdophane structure, where two-thirds of the lattice sites are associated with water of hydration (LnPO₄·0.67H₂O), implying that structural substitution reactions cannot be predicted based on the structure of the host matrix only.

New insights into phosphate based materials for the immobilisation of actinides

This paper focuses on major phosphate-based ceramic materials relevant for the immobilisation of Pu, minor actinides, fission and activation products. Key points addressed include the recent progress regarding synthesis methods, the formation of solid solutions by structural incorporation of actinides or their non-radioactive surrogates and waste form fabrication by advanced sintering techniques. Particular attention is paid to the properties that govern the long-term stability of the waste forms under conditions relevant to geological disposal. The paper highlights the benefits gained from synergies of state-of-the-art experimental approaches and advanced atomistic modeling tools for addressing properties and stability of f-element-bearing phosphate materials. In conclusion, this article provides a perspective on the recent advancements in the understanding of phosphate based ceramics and their properties with respect to their application as nuclear waste forms.

Using Eu(3+) as an atomic probe to investigate the local environment in LaPO4-GdPO4 monazite end-members

In the present study, we have investigated the luminescent properties of Eu(3+) as a dopant in a series of synthetic lanthanide phosphates from the monazite group. Systematic trends in the spectroscopic properties of Eu(3+) depending on the size of the host cation and the dopant to ligand distance have been observed. Our results show that the increasing match between host and dopant radii when going from Eu(3+)-doped LaPO4 toward the smaller GdPO4 monazite decreases both the full width at half maximum of the Eu(3+) excitation peak, as well as the (7)F2/(7)F1 emission band intensity ratio. The decreasing Ln⋯O bond distance within the LnPO4 series causes a systematic bathochromic shift of the Eu(3+) excitation peak, showing a linear dependence of both the host cation size and the Ln⋯O distance. The linear relationship can be used to predict the energy band gap for Eu(3+)-doped monazites for which no Eu(3+) luminescent data is available. Finally, mechanisms for metal-metal energy transfer between host and dopant lanthanides have been explored based on recorded luminescence lifetime data. Luminescence lifetime data for Eu(3+) incorporated in the various monazite hosts clearly indicated that the energy band gap between the guest ion emission transition and the host ion absorption transition can be correlated to the degree of quenching observed in these materials with otherwise identical geometries and chemistries.