Structure and thermal expansion behavior of Ca4La6−xNdx(SiO4)4(PO4)2O2 apatite for nuclear waste immobilization

Abstract

In this study, Ca₄La_6−xNd_x(SiO₄)₄(PO₄)₂O₂ (x = 0, 1, 2, 3, 4, 5, and 6) apatites were explored for nuclear waste immobilization, and Nd³⁺ ions were used as the surrogate of radionuclides (such as Am³⁺, Cm³⁺, and Pu³⁺). The synthesized samples conform to the P6₃/m (176) symmetry in the hexagonal system according to the characterizations by means of X-ray diffraction, Raman spectra, and Fourier-transform infrared spectra. Rietveld analyses indicate that both Ca²⁺ and Ln³⁺ (La, Nd) cations are located at the M_4f and M_6h sites, which is different from earlier studies. The M_6h sites prefer to be occupied by Ln³⁺ (La, Nd) cations with higher valence. Besides, the content of the impurity phase Ca₃(PO₄)₂ reduces from 2.815 wt% to 0 with the incorporation of Nd³⁺ ions. These results demonstrate that apatites possess excellent ability to accommodate radionuclides with various valences and radii at the M_4f and M_6h sites. Moreover, we investigated the thermal expansion behavior by high-temperature X-ray diffraction. There is no phase transformation in the range of 298–1173 K, and the Ca₄La_6−xNd_x(SiO₄)₄(PO₄)₂O₂ apatites exhibit lower thermal expansion coefficients than other candidates that have been extensively studied. Furthermore, the thermal expansion coefficient gradually decreases with the accommodation of Nd³⁺ ions. All the results suggest that apatites are promising candidates for nuclear waste immobilization.