Rare-Earth Orthophosphates From Atomistic Simulations

Abstract

Lanthanide phosphates ( LnPO 4 ) 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.

Electrode and Electrolyte Materials From Atomistic Simulations: Properties of LixFEPO4 Electrode and Zircon-Based Ionic Conductors

“…The PBEsol functional is specifically selected because it correctly reproduces slowly varying electron density limit and results in good structural parameters of solids (Perdew et al, 2008). This is important for consideration of, for instance, thermodynamics of solid solutions (Li et al, 2014;Ji et al, 2019a). Following our broad experience on computation of lanthanide orthophosphates and zirconates (e.g., Blanca-Romero et al, 2014;Li et al, 2015;Beridze et al, 2016) we applied the self-consistent DFT+U approach.…”

“…Intercalation of Li ions into FePO 4 results in formation of a solid solution between Li cations and vacant sites (Phan et al, 2019). Depending on the sizes of mixing species, a solid solution compounds could form a thermodynamically stable solid solution or a compound with a mixture of two phases, each rich in one of the cations Ji et al, 2019a). The second case indicates formation of a temperature dependent miscibility gap.…”

“…The second case indicates formation of a temperature dependent miscibility gap. The two endmember phases may be of the same type, like in the case of Li x FePO 4 , or different phases, like in the monazite-xenotime system (Mogilevsky, 2007;Ji et al, 2019a). The formation of miscibility gap is correlated with the maximum solubility (Ji et al, 2019a).…”

“…The two endmember phases may be of the same type, like in the case of Li x FePO 4 , or different phases, like in the monazite-xenotime system (Mogilevsky, 2007;Ji et al, 2019a). The formation of miscibility gap is correlated with the maximum solubility (Ji et al, 2019a). Such a temperature-dependent maximum solubility offers opportunity to use such a system as geothermometer (Andrehs and Heinrich, 1998;Mogilevsky, 2007).…”

“…In the last two decades, atomistic modeling became a widely used research technique in various research fields, including energy materials (Chroneos et al, 2013;Jahn and Kowalski, 2014;Wu et al, 2019). We used it intensively over the past decade for computation of, for instance, various physical and chemical properties of orthophosphate-and zircon-based ceramics (e.g., Kowalski et al, 2015;Ji et al, 2019a). This is because steady advancements in high performance computing and computational software, especially in ab initio methodsbased codes, allows nowadays for computation of complex systems containing hundreds of atoms from first principles (Jahn and Kowalski, 2014).…”