Crystal Structure of New Polar Rare Earth Borates Na_2.62Ln_2.12(Bo₃)₃ (Ln = Pr, Nd, Sm) Containing Isolated Bo₃ Groups

Three new rare earth borates, Na2.62Ln2.12(BO3)3 (Ln = Pr, Nd, Sm), were synthesized via a high-temperature solution reaction using a BaCO3–H3BO3–NaF flux and structurally characterized by single-crystal X-ray diffraction analysis. The compounds crystallize in the orthorhombic space group Amm2 with lattice parameters a = 5.0985(10) Å, b = 11.180(2) Å, c = 7.1483(14) Å, and a unit cell volume of 407.48(14) Å3 (Z = 2) for Na2.62Pr2.12(BO3)3, a = 5.0983(10) Å, b = 11.181(2) Å, c = 7.1491(14) Å, and a unit cell volume of 407.51(14) Å3 (Z = 2) for Na2.62Nd2.12(BO3)3, and a = 5.08630(10) Å, b = 11.0993(2) Å, c = 7.04200(10) Å, and a unit cell volume of 397.552(12) Å3 (Z = 2) for Na2.62Sm2.12(BO3)3. The new sodium rare earth borates crystallize in a three-dimensional framework consisting of zig-zag chains of lanthanide LnO9 polyhedra and mixed Na/LnO8 polyhedra linked to sodium NaO7 and NaO6 distorted polyhedra and trigonal planar BO3 groups. The new rare earth borates Na2.62Ln2.12(BO3)3 (Ln = Nd, Pr, Sm) are isostructural with the reported structures of Na3Ln2(BO3)3 (Ln = La, Nd, Sm), Na2.67Tb2.11B3O9 and Na3-3δEu2+δ(BO3)3 (δ = 0.16), however, there are some structural differences that are discussed. The Fundamental Building Block (FBB) consists of two isolated BO3 trigonal planar units that can be written as 2: [(2Δ)].