Structure and thermal expansion behavior of CaLaNd(SiO)(PO)O apatite for nuclear waste immobilization

In this study, CaLaNd(SiO)(PO)O (x = 0, 1, 2, 3, 4, 5, and 6) apatites were explored for nuclear waste immobilization, and Nd ions were used as the surrogate of radionuclides (such as Am, Cm, and Pu). The synthesized samples conform to the P6/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 Ca and Ln (La, Nd) cations are located at the M and M sites, which is different from earlier studies. The M sites prefer to be occupied by Ln (La, Nd) cations with higher valence. Besides, the content of the impurity phase Ca(PO) reduces from 2.815 wt% to 0 with the incorporation of Nd ions. These results demonstrate that apatites possess excellent ability to accommodate radionuclides with various valences and radii at the M and M 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 CaLaNd(SiO)(PO)O 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 Nd ions. All the results suggest that apatites are promising candidates for nuclear waste immobilization.