Monoclinic Form of the Rhabdophane Compounds: REEPO₄·0.667H₂O

Hydrated rhabdophane with a general formula REEPO₄·<i>n</i>H₂O (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 <i>C</i>2 space group with <i>a</i> = 28.0903(1) Å, <i>b</i> = 6.9466(1) Å, <i>c</i> = 12.0304(1) Å, β = 115.23(1)°, and <i>V</i> = 2123.4 (1) ų 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 <i>C</i>2 space group with <i>a</i> = 12.14426 (1) Å, <i>b</i> = 7.01776(1) Å, <i>c</i> = 6.34755(1) Å, β = 90.02 (1)°, <i>V</i> = 540.97(1) ų, and <i>Z</i> = 6