Phase tailoring and temperature‐induced phase evolution of SrZP‐(Ce, Nd, Sm)PO ₄ dual‐phase ceramic waste forms

Abstract In this study, a series of (1‐ x )Sr 0.5 Zr 2 (PO 4 ) 3 ‐ x (Ce 1‐2 y Nd y Sm y )PO 4 (abbreviated as SrZP‐(Ce, Nd, Sm)PO 4 , x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1; y = 0, 1/12, 2/12, 3/12, 4/12, 5/12, 6/12) dual‐phase ceramic waste forms were fabricated through a one‐step microwave sintering technique. The phase evolution, micromorphology, and densification behavior of these ceramics were systematically investigated using XRD, Raman spectroscopy, SEM‐EDS, and density measurement. The results confirmed that the dual‐phase ceramics exclusively consisted of SrZP and (Ce, Nd, Sm)PO 4 phases, which coexisted stably and independently, enabling simultaneously effective immobilization of simulated fission products (Sr 2+ ) and actinides radionuclides (Ce 3+ , Nd 3+ , and Sm 3+ ). The ceramics exhibited a homogeneous distribution of both phases, a compact micromorphology, and relative densities achieving a maximum of 98.9%. It was demonstrated that alterations in the phase content ( x value) induced only negligible variations in the lattice parameters of both crystalline phases, indicating minimal structural perturbations. Rietveld refinement analysis revealed a gradual reduction in the lattice parameters of the (Ce, Nd, Sm)PO 4 phase with increasing y value, consistent with the anticipated immobilization of the simulated actinide radionuclides. In addition, by integrating XRD and Raman analyses, the temperature‐induced phase evolution of the representative sample SrZP‐Ce 1/3 Nd 1/3 Sm 1/3 PO 4 was comprehensively characterized. The results indicated that the Ce 1/3 Nd 1/3 Sm 1/3 PO 4 phase preferentially formed over the SrZP phase during the dual‐phase ceramic preparation. Furthermore, the Kissinger model based on TG‐DSC results at various heating rates ( β T = 10, 15, 20, and 25 K/min) provided additional thermodynamic evidence, in which the Ce 1/3 Nd 1/3 Sm 1/3 PO 4 phase possessed a lower apparent activation energy (353.35 kJ/mol) than the SrZP phase (501.33 kJ/mol). Besides, the SrZP‐Ce 1/3 Nd 1/3 Sm 1/3 PO 4 dual‐phase ceramic waste form demonstrated high chemical stability, with normalized leaching rates of all elements remaining relatively low ( LR Sr ∼ 10 −4 g·m −2 ·d −1 , LR Ce ∼ 10 −7 g·m −2 ·d −1 , LR Nd ∼ 10 −7 g·m −2 ·d −1 , and LR Sm ∼ 10 −6 g·m −2 ·d −1 ).