Lanthanide Phosphates Prepared by Direct Precipitation and Hydrothermal Synthesis: Structure and Behavior during Heating

Phosphates isostructural with the mineral monazite—NdPO₄, GdPO₄, and a La_0.3Nd_0.5Sm_0.1Eu_0.1PO₄ solid solution modeling the composition of the lanthanide components of radioactive waste—and YbPO₄, crystallizing in the xenotime structure, have been prepared via direct precipitation from acid solutions. Under hydrothermal conditions, we have prepared the crystalline hydrate NdPO₄·0.67Н₂О isostructural with the mineral rhabdophane and the YbPO₄ phosphate with the xenotime structure. The powders range in crystallite size from 13 to 65 nm. The particle morphology and size have been shown to depend on the synthesis process. During heating to 1170 K, the phase composition of the powders remained unchanged. Their average 900-K thermal expansion coefficients lie in the range (5.6–9.6) × 10^–6 K^–1, so these phosphates can be regarded as having medium thermal expansion.