The energetics of rare-earth element (REE)-substituted zircon (ZrSiO4) is investigated by combining classical molecular static and density functional theory (DFT)-based calculations. Two charge balancing mechanisms, (1) 2ZrZr× → 2REEZr′ + VO•• and (2) ZrZr ×+ SiSi×→ REEZr′ + PSi• (REE = La, Gd, and Yb), are considered. Substitutions of REE3+ increase the overall unit cell volume of zircon proportional to the ionic radius mismatch between Zr4+ and REE3+ in eightfold coordination. The formation enthalpy (ΔHf) of REExZr1–xSiO4–x/2 solid solutions (mechanism (1)) suggests their lack of stability at higher concentrations (x > 1/16). REEPO4 exhibits a nearly linear trend in ΔHf as a function of zircon concentration in the ZrSiO4-REEPO4 solid solution (mechanism (2)) due to the significantly higher energetic stability of REEPO4 than that of ZrSiO4. The ΔHf for mechanism (2) is almost 1 order of magnitude higher than that of mechanism (1). The REEZr′–VO•• clustering (leading to the corner-shared SiO4 tetrahedral chain formation) and REEZr′–REEZr′/REEZr′–PSi• clustering are identified as the REE3+ solubility process in mechanisms (1) and (2), respectively. The DFT-calculated mixing enthalpy (ΔHmix) indicates the formation of a complete solid solution in ZrSiO4–GdPO4 and ZrSiO4–YbPO4. In terms of binary oxides, these solid solutions are expected to be thermodynamically stable with minimal driving force for either exsolution or intermediate compound formation. Analysis of relative defect formation energies indicates that the substitution of REE3+ for Zr4+ is charge-compensated preferentially via the ZrZrx + SiSix → REEZr′ + PSi• in O-poor conditions, leading to an inhomogeneous spatial distribution of REE3+ and P5+ ions within the zircon structure. Moreover, the solution energies become more negative with REE3+ and P5+ concentrations. The relevance of these results lies in establishing zircon as a high-capacity, single-phase crystalline host for trivalent actinides (represented by them and surrogates), ensuring thermodynamic stability, maximum waste loading, and uniform chemical durability for long-term nuclear waste sequestration.
Char et al. (Tue,) studied this question.