Revisiting synthesis of pure chernobylite solid solutions for thermal stability investigations

A multiparametric study was developed to optimize the hydrothermal synthesis of chernobylite solid solutions (Zr_1-xU_xSiO₄), a phase first discovered in the Elephant's foot of the Chernobyl nuclear power plant following the 1986 nuclear accident. The goal was to obtain phase-pure samples for thermodynamic investigation. Optimal synthesis conditions were determined starting from uranium (IV) and zirconium (IV) chloride precursors, using a hydrothermal treatment for seven days at 250 °C with a reaction medium maintained within a pH range of 1.4 to 1.8, which minimized the formation of secondary phases Under these conditions, solid solutions of chernobylite were synthesized up to x = 0.80. However, the systematic presence of residual oxide phases in the samples required the development of a purification process involving alternating leaching steps in basic and acid media. This process removed amorphous silica and selectively dissolved residual oxide phases, respectively. This protocol yielded pure Zr_1-xU_xSiO₄ samples over a wide range of compositions (x ≤ 0.6). The resulting powders consisted of spherical agglomerates of approximately 300 µm in diameter, with specific surface areas ranging from 19 to 22 m² g^-1. Finally, the thermal stability of the chernobylite solid solutions was evaluated using heat treatments between 1000 °C and 1300 °C. All samples, except for those most enriched in uranium, proved to be thermally stable even at 1300 °C.