Crystal Chemical Substitution at Ca and La Sites in CaLa4(SiO4)3O To Design the Composition Ca1–x M x La4‑xRE x (SiO4)3O for Nuclear Waste Immobilization and Its Influence on the Thermal Expansion Behavior

The oxysilicate apatite host CaLa4(SiO4)3O has been explored for immobilization of radioactive nuclides. Divalent ion, trivalent rare earth ion, and combined ionic substitutions in the silicate oxyapatite were carried out to optimize the simulated wasteform composition. The phases were characterized by powder X-ray diffraction, FT-IR, TGA, SEM-EDS, and HT-XRD techniques. The results revealed the effect of ionic substitutions on the structure and thermal expansion behavior. The investigation resulted in the formulation of simulated wasteforms such as La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-1) and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-2). In comparison to the average axial thermal expansion coefficients of the hexagonal unit cell of the parent CaLa4(SiO4)3O measured in the temperature range 298–1073 K (α′ a = 9.74 × 10–6 K–1 and α′ c = 10.10 × 10–6 K–1), rare earth ion substitution decreases the thermal expansion coefficients, as in the case of La3.4Ce0.1Pr0.1­Nd0.1Sm0.1Gd0.1­Y0.1(SiO4)3O (α′ a = 8.67 × 10–6 K–1 and α′ c = 7.94 × 10–6 K–1). However, the phase Ca0.8Sr0.1Pb0.1­La3.4Ce0.1Pr0.1­Nd0.1Sm0.1Gd0.1­Y0.1(SiO4)3O shows an increase in the values of thermal expansion coefficients: α′ a = 11.74 × 10–6 K–1 and α′ c = 11.70 × 10–6 K–1.