Crystal Chemistry of NaRE(CO ₃ ) ₂ across Lanthanide Series, Y, and Sc

A series of NaRE(CO 3 ) 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(CO 3 ) 2 reveals three distinct structure types dictated by the RE 3+ species: an orthorhombic phase ( Pmc 2 1 ) for larger REs (La–Nd), a monoclinic phase ( P 2 1 / c ) for smaller ones (Tb–Lu, Y), and a trigonal dolomite-type (CaMg(CO 3 ) 2, R 3̅) uniquely for Sc. Intermediate rare earths (Sm–Gd) failed to crystallize into NaRE(CO 3 ) 2 under our conditions. Structural evolution arises from lanthanide contraction and the rigidity of CO 3 2– units. As the RE 3+ 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(CO 3 ) 2 broadens the structural family of rare-earth carbonates and exemplifies heterovalent substitution (Na + + Sc 3+ for Ca 2+ + Mg 2+ ).