Synthesis and structural investigation of churchite-type REPO4·2H2O (RE = Y, Gd, Dy) nanocrystals

Churchite-type REPO4·2H2O (RE ​= ​Gd to Lu & Y) materials are a metastable polymorph of the rare-earth phosphate family and adopts a 2D layered structure with the water molecules occupying the interlayer regions. The synthesis and structural properties of churchite materials are not well-studied in comparison to those of other polymorphs of the rare-earth phosphate family and this article aims to address this aspect by carrying out a detailed synthetic and structural investigation of churchite-type REPO4·2H2O (RE = Y, Gd, Dy) materials. Phase-pure REPO4·2H2O materials were synthesized via precipitation method at temperatures ranging from room temperature (~22 °C) to 50 °C. The microstructure of these materials was determined using Optical microscopy, SEM, and TEM. Microstructural analysis using optical microscopy and SEM indicated that the REPO4·2H2O materials appear as needle like particles but in reality are agglomerates of single crystal nanorods as revealed by TEM analysis. The effect of temperature on their structure was determined using TGA, DSC, and ex-situ XRD. TGA-DSC results show that the interlayer water is removed from the churchite structure via an endothermic process in the 250° - 300 ​°C range. Ex-situ XRD experiments reveal that the churchite structure is unstable upon removal of water and undergoes a structural transformation to a thermodynamically stable xenotime (REPO4) structure at temperatures exceeding 200 ​°C.