Structural trends in the lanthanide trihydroxides

Abstract

The hydrous lanthanide oxides and hydroxides comprise a complex series of both amorphous and crystalline compounds. One important member of this system is the hexagonal form of M(OH) 3 where M studied here are La, Nd, Sm, Gd, Tb, Dy, Ho, Y and Er. The lattice constants and structure for these nine isotypic compounds has been determined by three-dimensional X-ray diffraction. The space group is P6 3/ m or P6 3 with two formula units per cell. The metal is 9-coordinate (tri-capped trigonal prism) with two independent metal-oxygen bond distances and three independent oxygenz.sbnd;oxygen bond distances. The metaloxygenoxygen bond distances both exhibit monotonic decreases proceeding from La to Er. The metal to oxygen distances range from 2.588(3) Å to 2.403(3) Å for MO 2 and from 2.551(3) Å to 2.439(3) Å for MO 1. At the low atomic number end of the series the MO 1, bond distance is shorter than the MO 2 bond distance. The two bond distances become the same at the spherically symmetric gadolinium and switch in relative magnitude as you proceed to erbium. The oxygenoxygen bond distances range from 3.033(5) Å to 2.856(5) Å for O 1O 2, 2.992(6) Å to 2.826(7) Å for O 2O 2 and 2.900(6) Å to 2.710(6) Å for the second O 1O 2 bond distance. Approximate positions for the hydrogen atoms have been found for the M(OH) 3 compounds Gd, Dy, Ho, Y and Er. A set of crystal radii for the lanthanides La, Nd, Sm, Gd, Tb, Dy, Ho and Er has been derived.