Crystal chemistry of RE(CO₃)OH

Rare earth carbonate hydroxides, RE(CO₃)OH, were hydrothermally synthesized from formic acid and the hydroxide gels of Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Y. An orthorhombic phase with a kozoite-type structure was obtained for RE = Nd and Sm. Another orthorhombic modification of the kozoite-type structure was obtained for RE = Eu, Gd, Tb, Dy, Ho, Er, Tm, and Y. The latter phase has a lower symmetry (space group: P2₁2₁2₁) in comparison to the true kozoite-type structure (Pnma). A new tetragonal phase (space group: P4₂/nmc) was found for RE = Tm and Yb.

The crystal structures of RE(CO₃)OH were refined for Pnma (RE = Nd and Sm), P2₁2₁2₁ (RE = Eu, Gd, Tb, Dy, Ho, Er, Tm and Y), and P4₂/nmc phases (RE = Tm and Yb) using single-crystal X-ray diffraction intensity data. The distinct features of the differences among the three structures are the coordination numbers of the RE³⁺ ions: 10, 9, and 8 for the Pnma, P2₁2₁2₁, and P4₂/nmc phases, respectively. A systematic comparison of the two orthorhombic structures revealed a dynamic variation in the coordination environment of the RE³⁺ ions accompanied by variations in their ionic radii. Although the interatomic RE—O distances tend to decrease with the lanthanide contraction, exceptions were observed for two of the RE—O distances. The mutual close proximity of CO₃^2– anions caused by the lanthanide contraction led to repulsion between the CO₃^2– anions, which decreased the symmetry of the configuration of CO₃^2– anions around the central RE³⁺ ion, and some oxygen atoms of CO₃^2– moved away from the first coordination shell of the RE³⁺ ions due to the elongation of the RE-O distances.

The crystal structures of the tetragonal RE(CO₃)OH are distinct from those of the orthorhombic phases. They consist of ladders of RE³⁺ ions 8-coordinated by the CO₃^2– and OH^– anions arranged in the form of a double cross. The powder XRD patterns of the tetragonal RE(CO₃)OH are identical to that of a synthetic material previously reported as Tm₆(OH)₄(CO₃)₇.