La(OH)₂I(H₂O): Closing a gap in rare earth hydroxide halide structural chemistry

UCl₃ type La(OH)₂I can be stabilized by additional water molecules during a hydrothermal synthesis from hydroiodic acid and lanthanum carbonate hydrate at 453 K to form La(OH)₂I(H ₂O). The new rare earth (RE) hydroxide halide hydrate crystallizes monoclinically, space group C2/m with lattice parameters of a = 19.691(3), b = 4.136(1), c = 6.286(1) Å, β = 108.45(1)° and V = 485.6(2) ų, wR2 = 0.0695, 648 F² values and 32 variables. La centered, distorted, tricapped, trigonal prisms formed by iodide, (OH ^-)- and (H₂O) groups are connected via common edges in [001]-direction and common faces in [010]-direction to built up a zigzag like layered arrangement. Hydrogen bonding between the water molecules and iodide ions of adjacent La(OH)₂I layers stabilize the UCl₃ related structure, which was only observed for the lighter homologues La(OH)₂X (X = Cl, Br) so far. DTA/TG and IR measurements substantiated the occurrence of (H₂O)- and (OH^-)-groups and semiquantitative EDX analyses proved a 1:1 composition for La:I in La(OH)₂I(H₂O).