New layer megaborate Sm3[B13O22(OH)3](OH) · 3H2O and its place in the structural systematics

Crystals of a new aqueous rare earth borate Sm3[B13O22(OH)3](OH) · 3H2O, space group P2/c, are obtained under hydrothermal conditions. The structure is determined by the heavy-atom method without preliminary knowledge of the chemical formula. The anionic radical is a boron-oxygen sheet in which two corrugated layers are related by centers of inversion. An independent layer is akin to pentaborate layers; it differs from the layer in (Nd0.925Na0.075)Nd[B9O15(OH)2]Cl0.85 · 2.65H2O by an additional branch in the form of a 4: (3[2Δ + 1T] + 1Δ) group. The intersheet space and large holes of the sheet accommodate Sm atoms, (OH) groups, and water molecules. The new Sm-borate and the related Nd-borate are polyborates (megaborates) with complex anionic radicals. In the Sm-borate, the new two-dimensional [B13O22(OH)3]∞∞ complex anionic radical of the 13{(4: [2T + 2Δ])∞ + (5: [3T + 2Δ] + 4: (3[2δ + 1T] +1δ))∞}∞∞ formula is built of three different blocks, unlike the [B9O15(OH)2]∞∞ radical of the 9{(4: [2T + 2Δ])∞ + (5: [3T + 2Δ])∞}∞∞ formula in the Nd-borate, which consists of two blocks. The rule of the inverse relationship between the polymerization degrees of the boron-oxygen radical and rare earth polyhedra holds for both borates.