Comprehensive Dissolution Study on Two Double Ce(IV) Phosphates with Evidence of Secondary CeO2 Nanoparticle Formation

Herein, we present a comprehensive study on the dissolution behaviour of two sodium-cerium(IV) phosphate phases synthesised hydrothermally from CeO₂ nanoparticles: crystalline Na₂Ce(PO₄)₂ and nanocrystalline NaCe₂(PO₄)₃. For the first time, experimental dissolution data were obtained for both compounds over a wide pH range (1.5-10) under long-term equilibration. The crystalline phase undergoes pH-dependent transformation, including recrystallisation at a near-neutral pH and the formation of secondary CeO₂ nanoparticles above pH 7. In contrast, the nanophase NaCe₂(PO₄)₃ exhibits exceptional structural and chemical stability, showing no signs of recrystallisation, phase transformation, or CeO₂ formation, even after extended ageing. The experimental results help refine the thermodynamic stability conditions for cerium phosphate and oxide phases, providing insights into the reversible transformation pathways between CeO₂ and Ce(IV) phosphates as governed by pH.