Monoclinic Form of the Rhabdophane Compounds: REEPO4·0.667H2O

Hydrated rhabdophane with a general formula REEPO4·nH2O (REE: La → Dy) has been always considered to crystallize in the hexagonal system. A recent re-examination of this system by the use of synchrotron powder data of the SmPO4·0.667 H2O compound led to a structure crystallizing in the monoclinic C2 space group with a = 28.0903(1) Å, b = 6.9466(1) Å, c = 12.0304(1) Å, β = 115.23(1)°, and V = 2123.4 (1) Å3 with 24 formula units per unit cell. The structure consists of infinite channels oriented along the [101] direction and formed by the connection of Sm-polyhedra and P-tetrahedra by sharing O-edges. The water molecules filling the space have been localized for the first time. The monoclinic form of the hydrated rhabdophane was confirmed by studying the series with REE: La → Dy. Moreover, the dehydration of SmPO4·0.667H2O led to the stabilization of an anhydrous form SmPO4 in the C2 space group with a = 12.14426 (1) Å, b = 7.01776(1) Å, c = 6.34755(1) Å, β = 90.02 (1)°, V = 540.97(1) Å3, and Z = 6.