Structure and thermal expansion behavior of Ca 4 La 6-x Nd x (SiO 4 ) 4 (PO 4 ) 2 O 2 apatite for nuclear waste immobilization.

In this study, Ca4La6-xNdx(SiO4)(4)(PO4)(2)O-2 (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 P6(3)/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 Ln(3+) (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 Ln(3+) (La, Nd) cations with higher valence. Besides, the content of the impurity phase Ca-3(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 Ca4La6-xNdx(SiO4)(4)(PO4)(2)O-2 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.