From Th-Rhabdophane to Monazite-Cheralite Solid Solutions: Thermal Behavior of Nd_1–2xTh_xCa_xPO₄·nH₂O (x = 0–0.15)

The synthesis and thermal behavior of Nd_1–2xTh_xCa_xPO₄­·nH₂O (x = 0–0.15) solid solutions crystallizing in the rhabdophane structure type were studied by combining in situ high-temperature powder X-ray diffraction (HT-PXRD), thermogravimetric analysis, and dilatometry techniques. The samples prepared under hydrothermal conditions (T = 110 °C, t = 4 days) were found to follow the same dehydration process as the NdPO₄­·0.667H₂O end-member. The temperature of maximum dehydration rate slightly increased from 210 ± 3 °C to 214 ± 3 °C with the Ca–Th coupled substitution rate. In parallel, the evolution of the unit cell parameters was also monitored by collecting in situ HT-PXRD data. The phase transition into the monazite-type structure occurred at a higher temperature (from 650 to 850 °C) and was correlated to a temperature of maximum linear shrinkage rate increasing from 693 ± 1 °C to 762 ± 1 °C with the Ca–Th content. Therefore, the process of conversion leading to the stabilization of Nd_1–2xTh_xCa_xPO₄ was successfully achieved.