Self-healing ThSiO4-ZrSiO4 system under conditions relevant to underground nuclear waste repositories

Two series of Th 1- x Zr x SiO 4 phases were synthesized hydrothermally under weakly basic (pH = 8) and strongly acidic (pH = 1) conditions. Changes in pH were found to have a significant effect on experimental phase diagrams. Synthesis at pH = 8 favors the formation of Th-rich phases with resulting Th 1- x Zr x SiO 4 solid solution for x = 0 – 0.5. Contrary, synthesis at pH = 1 results in the formation of pure end-members of the ThSiO 4 -ZrSiO 4 pseudo-binary system separated by multiple miscibility gaps. Phases formed both under basic and acidic conditions were found to retain water, which can be discharged from the structure upon heating. A different high-pressure ( HP ) behaviour was found for Th-rich and Zr-rich solid solutions. While Th-rich Th 0.9 Zr 0.1 SiO 4 and Th 0.6 Zr 0.4 SiO 4 phases retain their stoichiometry and crystal structure upon compression at HP, a significant reduction of the Th occupancy related to a decrease of the Th-O distances is observed for the Th-poor Th 0.26 Zr 0.74 SiO 4 phase at P > 8 GPa, with the subsequent formation of a Th-rich amorphous phase. The Th diffusion between the crystalline and amorphous phases was found to be fully reversible. This unique self-healing property makes these phases promising candidates for nuclear applications under extreme pressure and temperature conditions, in particular those found in underground repositories.