In pursuit of the rhabdophane crystal structure: from the hydrated monoclinic LnPO₄.0.667H₂O to the hexagonal LnPO₄ (Ln = Nd, Sm, Gd, Eu and Dy)

The dehydration process of the hydrated rhabdophane LnPO₄.0.667H₂O (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 SmPO₄.0.667H₂O (monoclinic, C2), a first dehydration step was identified around 80 °C leading to the formation of SmPO₄.0.5H₂O (Monoclinic, C2) with Z =12 and a =17.6264(1) Å, b =6.9704(1) Å, c =12.1141(1) Å, β=133.74(1) °, V =1075.33(1) ų. 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 P3₁21 space group with a =7.0389(1) Å, c =6.3702(1) Å and V =273.34(1) ų. This study was extended to selected LnPO₄.0.667H₂O 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 LnPO₄.0.5H₂O. - Graphical abstract: The dehydration process of the rhabdophane SmPO₄.0.667H₂O was studied over combination of in situ high resolution synchrotron powder diffraction and TGA techniques, a first dehydration was identified around 80 °C leading to the formation of SmPO₄.0.5H₂O (Monoclinic, C2). Then above 220 °C, the anhydrous form of the rhabdophane SmPO₄ was stabilized and crystallizes in the hexagonal P3₁21 space group. - Highlights: • In situ synchrotron powder diffraction was carried out during the dehydration of the rhabdopahe LnPO₄.0.667H₂O. • The heat of the rhabdophane LnPO₄.0.667H₂O leads to LnPO₄.0.5H₂O then to anhydrous rhabdophane LnPO₄. • LnPO₄.0.5H₂O (monoclinic, C2) and LnPO₄ (Hexagonal, P3₁21) were solved over the use of direct methods.