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

Monazite- (REPO4; RE = La to Gd) and xenotime- (REPO4; RE = Dy to Lu & Y) type 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 · 0.667 H2O; RE = La to Dy) and churchite (REPO4 · 2 H2O; 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 · 2 H2O; 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 has shown 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 · 2 H2O (...