Microwave-Assisted Hydrothermal Synthesis of Ceric-Ammonium Phosphates (NH4)2Ce(PO4)2⋅H2O and NH4Ce2(PO4)3

The possibility of preparation of crystalline double cerium(IV) phosphates (NH4)2Ce(PO4)2⋅H2O and NH4Ce2(PO4)3 under the conditions of microwave-assisted hydrothermal synthesis has been analyzed. It has been shown that these phosphates in a single-phase state can be obtained in the temperature range of 130–190°С with a synthesis duration of ≥5 min, while the phase composition of the synthesis products is determined by the molar ratio of ammonia and phosphoric acid in the reaction mixture. Short-term (5 min) low-temperature (130°С) hydrothermal synthesis under microwave heating leads to the preparation of (NH4)2Ce(PO4)2⋅H2O and NH4Ce2(PO4)3 with a particle size of ~70 and ~200 nm, respectively. At higher temperatures and treatment times (190°C and 24 h), the particle size of these phases increases to ~200 and ~500 nm, respectively. For the first time, the value of the optical band gap for (NH4)2Ce(PO4)2⋅H2O was determined to be 2.8 and 3.1 eV for indirect and direct transitions, respectively.