Crystallization Pathways of Cerium(IV) Phosphates Under Hydrothermal Conditions: A Search for New Phases with a Tunnel Structure

Hydrothermal crystallization pathways of amorphous ceric phosphate gels were found to be determined by the ammonia concentration in a reaction medium. This allows for highly selective hydrothermal synthesis of various finely crystalline ceric phosphates, including Ce(PO4)(HPO4)0.5(H2O)0.5, (NH4)2Ce(PO4)2(H2O), and previously unknown NH4Ce2(PO4)3. The structure of the latter compound was solved from powder X‐ray diffraction data. It appeared to be isostructural to ammonium thorium phosphate, NH4Th2(PO4)3; in this crystal structure, large channels (5.07 × 3.79 Å) located along the c‐axis are occupied by NH4+ ions. A selective hydrothermal synthesis was proposed for crystalline ceric phosphates Ce(PO4)(HPO4)0.5(H2O)0.5, (NH4)2Ce(PO4)2(H2O), and previously unknown NH4Ce2(PO4)3.