Sr 8 MgB 18 O 36 : a New Alkaline-Earth Borate with a Novel Zero- Dimensional (B 18 O 36 ) 18- Anion Ring Wenjiao Yao, †,‡ Xingxing Jiang, †,‡ Hongwei Huang, § Tao Xu, †,‡ Xiaoshan Wang, †,‡ Zheshuai Lin,* ,† and Chuangtian Chen † † Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § School of Materials Science and Technology, China University of Geosciences, Beijing 100083, People’s Republic of China ‡ University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China * S Supporting Information ABSTRACT: A new polyborate, Sr 8 MgB 18 O 36 , has been synthesized. Its crystal structure was determined from single-crystal X-ray diffraction data, and characterizations were made by differential scanning calorimetry, Fourier transform IR, UV-vis-near-IR diffuse-reflectance, and first-principles calculations. The structure of Sr 8 MgB 18 O 36 contains a novel isolated anionic groupan 18-membered ring (B 18 O 36 ) 18- , which is the first found in borates. B orates have been an important research focus for many years, and they have given rise to numerous applications because they possess many attractive properties, such as high electrical conductivity in ion conductors, 1 efficient frequency conversion capability in nonlinear optical crystals, 2 and large birefringence in controlling the polarization of illuminating lasers. 3 Therefore, it is of importance to explore new borate compounds with novel structural features. The structural diversity of the borates can be attributed to the coordination of boron as it bonds with either three or four O atoms to form a BO 3 triangle or BO 4 tetrahedron. These can be further linked by sharing O atoms to form isolated rings and cages or to construct infinite chains, sheets, or networks. In addition, the cations may significantly influence the packing patterns of anionic groups, 4,5 leading to new phases with interesting stoichiometries, structures, and properties. In this Communi- cation, we report the synthesis and crystal structure of a new binary alkaline-earth-metal mixed borate, Sr 8 MgB 18 O 36 . The structure of the new compound is built from isolated (B 18 O 36 ) 18- rings, a MgO 6 octahedron, and SrO n (n = 6, 7) polyhedra. The (B 18 O 36 ) 18- building block is the first found in borates. The novel (B 18 O 36 ) 18- group expands the structural diversity of borates. The properties of the crystal were characterized by a variety of experimental and computational methods. Polycrystalline samples of Sr 8 MgB 18 O 36 were obtained by a traditional solid-state reaction with a stoichiometric mixture of SrCO 3 , MgO, and H 3 BO 3 . 6 Single crystals were grown through spontaneous crystallization from different flux methods. 7 The powder X-ray diffraction (PXRD) patterns of polycrystalline samples display excellent agreement with the theoretical simulations from the single-crystal structure. The resulting colorless and transparent crystals from the flux method were chosen for further characterization. The FT-IR spectrum validates the existence of BO 3 and BO 4 groups in the Sr 8 MgB 18 O 36 structure, which are also commonly found in other borates. 8 Differential scanning calorimetric measurement reveals that this compound remains stable up to the phase transition temperature of 1052 °C. UV-vis-near-IR diffuse- reflectance data demonstrate that its UV cutoff wavelength is below 200 nm. All measured data are displayed in the Supporting Information. The structure of the Sr 8 MgB 18 O 36 crystal was determined from single-crystal X-ray di ffraction data. 9 The atomic coordinates, related anisotropic displacement parameters, and selected bond lengths (Å) and angles are summarized in Tables S2-S4 in the Supporting Information. Sr 8 MgB 18 O 36 crystallizes in the centrosymmetric uniaxal trigonal system with space group R3̅c, and its structure is illustrated in Figure 1. It features a complicated three- Received: May 10, 2013 Published: July 12, 2013 Figure 1. Structure of Sr 8 MgB 18 O 36 viewed along the b axis: (a) unit cell; (b) schematic diagram. Blue boxes are the basic [B 18 O 36 ] 18- building blocks, purple octahedra are MgO 6 groups, and dark- and light-green balls in part b indicate the Sr atoms in the different ac planes. Communication pubs.acs.org/IC © 2013 American Chemical Society 8291 dx.doi.org/10.1021/ic401167z | Inorg. Chem. 2013, 52, 8291-8293