Crystal Chemistry of NaRE(CO₃)₂ across Lanthanide Series, Y, and Sc

A series of NaRE(CO₃)₂ compounds (RE = Ce-Lu, Y, Sc) was synthesized under high-pressure and high-temperature and characterized by single-crystal X-ray diffraction and Raman spectroscopy. Combining these data with previously reported results for NaLa(CO₃)₂ reveals three distinct structure types dictated by the RE³⁺ species: an orthorhombic phase (Pmc2₁) for larger REs (La-Nd), a monoclinic phase (P2₁/c) for smaller ones (Tb-Lu, Y), and a trigonal dolomite-type (CaMg(CO₃)₂, R3̅) uniquely for Sc. Intermediate rare earths (Sm-Gd) failed to crystallize into NaRE(CO₃)₂ under our conditions. Structural evolution arises from lanthanide contraction and the rigidity of CO₃^2- units. As the RE³⁺ radius decreases, RE-O bonds contract, while C-O bonds remain inflexible, causing incomplete bond adjustment and accumulating strain. This strain elevates lattice energy until a phase transition alleviates it, enabling reduced coordination numbers. In the monoclinic phase, positional disorder of Na⁺ serves as an additional strain-relief mechanism, accommodating a less symmetric coordination environment. The findings elucidate how lanthanide contraction and carbonate rigidity dictate phase transitions and cation disorder, offering critical insights into coordination flexibility in rare-earth chemistry. Moreover, the dolomite-type NaSc(CO₃)₂ broadens the structural family of rare-earth carbonates and exemplifies heterovalent substitution (Na⁺ + Sc³⁺ for Ca²⁺ + Mg²⁺).