An in-depth multi-technique characterization of rare earth carbonates – RE2(CO3)3.2H2O – owning tengerite-type structure

The present study has been conducted to contribute, once and for all, filling the lack of structural information for the whole class of rare earth (RE) carbonates owning the tengerite-(Y) structure. A complete structural characterization, carried out by Rietveld refinement of X-ray powder diffraction (XRD) data, was comparatively performed for the first time on several hydrated RE carbonates, having the general chemical formula RE2(CO3)3.xH2O, RE = Y, Gd, Tb, Dy, Ho and Er. All samples share the same space group and lattice parameters similar to tengerite-(Y); the structures are also closely related, consisting in all cases of a three-dimensional framework of nine-fold coordinated RE atom polyhedral, linked together by carbonate ions. In addition to relatively minor changes in fractional coordinates of atom sites and corresponding interatomic distances, the only perceivable difference lies in the lattice parameters affected by the ionic radius of RE3+. However, in the case of Er, which has the lowest cationic radius among the analyzed RE, the stabilization of tengerite structure is at its limit condition, because the carbonate groups are heavily distorted. Furthermore, FT-IR and Raman spectra confirm the main structural features obtained by Rietveld refinements. The observed morphology of the various samples is almost the same, being characterized by the presence of bidimensional rod-like particles grouped in agglomerates, typical of tengerite crystals, thus indicating that the crystallization mechanism occurring during the hydrothermal synthesis is the same, irrespective of the involved RE cation.