Thermodynamic non-ideality and disorder heterogeneity in actinide silicate solid solutions

Abstract

Non-ideal thermodynamics of solid solutions can greatly impact materials degradation behavior. We have investigated an actinide silicate solid solution system (USiO4–ThSiO4), demonstrating that thermodynamic non-ideality follows a distinctive, atomic-scale disordering process, which is usually considered as a random distribution. Neutron total scattering implemented by pair distribution function analysis confirmed a random distribution model for U and Th in first three coordination shells; however, a machine-learning algorithm suggested heterogeneous U and Th clusters at nanoscale (~2 nm). The local disorder and nanosized heterogeneous is an example of the non-ideality of mixing that has an electronic origin. Partial covalency from the U/Th 5f–O 2p hybridization promotes electron transfer during mixing and leads to local polyhedral distortions. The electronic origin accounts for the strong non-ideality in thermodynamic parameters that extends the stability field of the actinide silicates in nature and under typical nuclear waste repository conditions.

Crystal Chemistry and Thermodynamics of HREE (Er, Yb) Mixing in a Xenotime Solid Solution

“…45 Furthermore, it is interesting to compare our result on mixing of HREEs in xenotime with mixing of early actinides (Th and U) in an isostructural system (zircon), the latter of which shows a strong influence on the thermodynamics of mixing from ΔH mix rather than ΔS mix that originated from the relatively stronger covalent characters of actinides. 57,63 We then can conclude that mixing HREEs in xenotime, described by a regular solution model, is close to ideal due to the largely pure ionicity, which may be deviated more from ideality when early actinides and/or LREEs are presented for mixing (i.e., HREE (x) An (1−x) PO 4 , where An = Th, U, Np, or Pu).…”

“…Note that the calorimetrically determined W x value includes two contributions: (1) the elastic term that originates from the volume of mixing due to mismatch of sizes and (2) the chemical term considering bonding and local mixing features. 63,128 The elastic contribution of W x can be empirically approximated based on the unit cell volume and Young's modulus, which has been used by Migdisov et al 31 to estimate the heats of mixing of HREE xenotimes:…”

“…Using eq , the interaction parameter can be obtained to be W x = 20.3 ± 0.8 kJ/mol, suggesting that the solid solution is enthalpically not favorable to form (Figure a). Note that the calorimetrically determined W x value includes two contributions: (1) the elastic term that originates from the volume of mixing due to mismatch of sizes and (2) the chemical term considering bonding and local mixing features. , The elastic contribution of W x can be empirically approximated based on the unit cell volume and Young’s modulus, which has been used by Migdisov et al to estimate the heats of mixing of HREE xenotimes: where E̅ is the average Young’s modulus of all REE xenotime endmembers (168.8 GPa) and Δ V is the difference between the molar volumes of the two endmembers, 42.453 ± 0.009 cm 3 /mol for ErPO 4 and 41.646 ± 0.005 cm 3 /mol for YbPO 4 , both experimentally determined from XRD (Table S6). Using these values and eq , the elastic W x term was thus volumetrically derived to be 0.4 kJ/mol.…”

“…Xenotime also has the ability to accommodate trivalent elements, the dominant valence state of the late actinides (Am–Es), − which were predicted to be thermodynamically unstable in orthosilicates. − All of the actinide orthophosphates (Pu–Es) synthesized in the air have been reported to adopt the monazite structure, ,,− yet there are also reports that Np and Pu were found to be stabilized in the xenotime structure (Y ( x ) An (1– x ) PO 4 , where An = Np or Pu) under reducing environments . This stabilization within the xenotime structure could most likely be attributed to the favorable energetics of mixing, similar to what is found with uranothorite (Th ( x ) U (1– x ) SiO 4 ). ,, …”

“…52 This stabilization within the xenotime structure could most likely be attributed to the favorable energetics of mixing, similar to what is found with uranothorite (Th (x) U (1−x) SiO 4 ). 57,62,63 Thus, studying the crystal chemistry and thermodynamics of a xenotime solid-solution series has both fundamental and applicable significance. In this work, we report the crystal chemistry, high-temperature material behaviors, and thermodynamic stability of an Er−Yb binary xenotime solid-solution series (Er (x) Yb (1−x) PO 4 ) with hydrothermal origins.…”