Crystal structure of a new polar borate Na2Ce2[BO2(OH)][BO3]2 · H2O with isolated boron triangles

Resumo

Crystals of a new polar borate Na₂Ce₂[BO₂(OH)][BO₃]₂ · H₂O were prepared by hydrothermal synthesis. The crystals are orthorhombic, a = 7.2295(7) Å, b = 11.2523(8) Å, c = 5.1285(6) Å, Z = 2, sp. gr. C2mm (Amm2), R = 0.0253. The formula of the compound was derived from the structure determination. The Ce and Na atoms are coordinated by nine and six O atoms, respectively. The Ce position is split, and a small amount of Ce is incorporated into the Na1 site with the isomorphous substitution for Na. The anionic moieties exist as isolated BO₃ and BO₂(OH) triangles. The planes of the BO₂(OH) triangles with mm2 symmetry are parallel to the ab plane. The planes of the BO₃ triangles with m symmetry are perpendicular to the ab plane and are rotated in a diagonal way. The splitting of the Ce positions and the polar arrangement of the BO₂(OH) triangles, water molecules, and Na atoms are observed along the polar a axis. The new structure is most similar to the new borate NaCa₄[BO₃]₃ (sp. gr. Ama2), in which triangles of one type are arranged in a polar fashion along the c axis. Weak nonlinear-optical properties of both polar borates are attributed to the quenching of the second-harmonic generation due to the mutually opposite orientation of two-thirds of B triangles in the unit cell.