In pursuit of the rhabdophane crystal structure: from the hydrated monoclinic LnPO 4 .0.667H 2 O to the hexagonal LnPO 4 (Ln = Nd, Sm, Gd, Eu and Dy)

Abstract The dehydration process of the hydrated rhabdophane LnPO 4 .0.667H 2 O (Ln = La to Dy) was thoroughly studied over the combination of in situ high resolution synchrotron powder diffraction and TGA experiments. In the case of SmPO 4 .0.667H 2 O (monoclinic, C2), a first dehydration step was identified around 80 °C leading to the formation of SmPO 4 .0.5H 2 O (Monoclinic, C2) with Z =12 and a =17.6264(1) A, b =6.9704(1) A, c =12.1141(1) A, β=133.74(1) °, V =1075.33(1) A 3 . In agreement with the TGA and dilatometry experiments, all the water molecules were evacuated above 220 °C yielding to the anhydrous form, which crystallizes in the hexagonal P3 1 21 space group with a =7.0389(1) A, c =6.3702(1) A and V =273.34(1) A 3 . This study was extended to selected LnPO 4 .0.667H 2 O samples (Ln= Nd, Gd, Eu, Dy) and the obtained results confirmed the existence of two dehydration steps before the stabilization of the anhydrous form, with the transitory formation of LnPO 4 .0.5H 2 O.