Phase structure evolution and chemical durability studies of Gd_1−xYb_xPO₄ ceramics for immobilization of minor actinides

Rare earth phosphates (REPO₄) are considered as promising materials to immobilize the high-level radioactive waste. In the paper, Gd_1−xYb_xPO₄ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0) series were prepared via the solid-state reaction using Gd as the surrogate for minor actinide Am and Cm, and the structure, phase transformation and morphology were characterized by XRD, Raman, TEM and SEM. With the calcination temperature ranging from 1200 to 1500 °C, the coexisted phases of monazite- and xenotime-type structure are observed in Gd_0.9Yb_0.1PO₄. The monophasic xenotime-type Gd_0.9Yb_0.1PO₄ is obtained at 1600 °C. The monazite → xenotime transformation of Gd_1−xYb_xPO₄ depends on the substitution of Gd³⁺ by Yb³⁺ ions. With the decrease in Gd³⁺ content, the gradual changes of FWHM and the hypsochromic shift of P-O symmetrical stretching vibration (Vs) are observed in the Raman spectra, indicating that the distortion of PO₄ tetrahedra occurs during the phase transformation process. The chemical durability test is measured by the ASTM product consistency test leaching method, and the normalized mass loss (NL_Gd) of Gd and (NL_Yb) of Yb is extremely low and shown in the order of 10⁻⁶–10⁻⁷ g/m² for all the ceramics.