Synthesis, Crystal Structure, and Enthalpies of Formation of Churchite-type REPO₄·2H₂O (RE = Gd to Lu) Materials

Monazite (REPO4; RE = La to Gd) and xenotime (REPO4; RE = Tb to Lu & Y) materials have been proposed as host matrices for the immobilization of actinides. Aqueous alteration of monazite and xenotime minerals could result in the formation of rhabdophane (REPO4·0.667H2O; RE = La to Dy) and churchite (REPO4·2H2O; RE = Gd to Lu & Y) phases, respectively. Among these structure types, the structure and properties of churchite materials are not well-understood, and this study aims to bridge this gap by providing a comprehensive insight into the structure and thermochemical properties of churchite materials. Churchite materials (REPO4·2H2O; RE = Gd to Lu) were synthesized by a low-temperature precipitation route, and their crystal structures were determined by powder X-ray diffraction (XRD). Examination of the powder XRD data showed that the churchite materials crystallize in the monoclinic crystal system (space group C2/c). The enthalpies of formation (ΔH°f,ox) of churchite-type REPO4·2H2O (RE = Gd to Yb) determined by high-temperature oxide melt solution calorimetry are more negative than their anhydrous counterparts (i.e., xenotime structure) and indicate that the formation of churchite is more exothermic than the xenotime phase. However, the churchite materials are likely to have a more negative entropy of formation (ΔS°f,ox) due to the presence of water molecules, resulting in a less negative Gibbs free energy of formation (ΔG°f,ox) than the xenotime structure. Therefore, churchite materials are expected to be stable at lower temperatures. For the unique case of GdPO4 and GdPO4·nH2O materials, which could adopt all the above-discussed structure types, the ΔG°f,ox of monazite from oxides was observed to be more negative than those of xenotime, rhabdophane, and churchite thereby suggesting the following order of stability: Gd-churchite < Gd-rhabdophane < Gd-xenotime < Gd-monazite.