Structural, electronic, mechanical and optical properties of Pu₁₋ₓThₓO₂ fluorite ceramics: A first-principles study

Fluorite-type actinide mixed oxide ceramics exhibit exceptional structural stability and composition-tunable physical properties, making them promising candidates for advanced nuclear and high-temperature applications. In this work, the structural, electronic, mechanical, optical, and thermoelectric properties of Pu₁−ₓThₓO₂ (x = 0, 0.25, 0.5, 0.75, 1) solid solutions are systematically investigated using first-principles density functional theory within the modified Becke–Johnson generalized gradient approximation (mBJ-GGA), as implemented in WIEN2k, combined with BoltzTraP2 transport calculations. The lattice parameters increase monotonically with Th content, while the bulk modulus shows a non-linear compositional dependence with a minimum at x = 0.25, indicating enhanced compressibility induced by cation substitution. Electronic structure analysis reveals a transition from conducting/metal-like PuO₂ to wide-gap insulating ThO₂, with intermediate compositions exhibiting tunable band gaps between 0.95 and 2.81 eV. Elastic constants confirm mechanical stability for all compositions and indicate overall ductile behavior, accompanied by pronounced elastic softening at intermediate Th concentrations. Optical properties show composition-dependent dielectric response and absorption, while thermoelectric calculations at 300 K predict Seebeck coefficients up to ∼400 μV/K near band edges. These results provide atomistic insight into composition–property relationships in Pu–Th fluorite oxides and clarify how thorium incorporation modulates structural rigidity and electronic behavior.