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 CO32- 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+).