NEW STRONTIUM BORATE HALIDES - SR5(BO3)3X (X = F, BR)

Two new strontium borate halides, Sr5(BO3)3F and Sr5(BO3)3Br, have been synthesized and structurally characterized by single-crystal X-ray diffraction techniques. The two compounds are structurally similar, 3-dimensional frameworks of interconnecting Sr polyhedra and orthoborate groups but do not crystallize with the same symmetry. The fluoride forms in the centrosymmetric orthorhombic space group, Pnma, with cell parameters a = 7.220 (2) angstrom, b = 14.092 (2) angstrom, c = 9.810 (1) angstrom, and Z = 4. Three distinct Sr-centered 0 environments exist in this structure: distorted SrO7F undecahedra, irregular SrO6F coordination sites, and distorted SrO8 square antiprisms, monocapped by a very weak Sr-O interaction of 3.04 angstrom. A trio of Sr atoms caps each F atom to form a shallow pyramid. Sr5(BO3)3Br crystals are also orthorhombic in the noncentrosymmetric space group C222(1) with a = 10.002 (2) angstrom, b - 14.197 (1) angstrom, c = 7.458 (2) angstrom, and Z = 4. Four Sr-centered polyhedra join to each other: distorted SrO7Br2 tricapped trigonal prisms, irregular octacoordinate SrO6Br2 undecahedra, and two dissimilar SrO8 distorted square antiprisms. Six Sr nearest neighbors bond to each Br atom and define Sr octahedra that share triangular faces along the c direction to form zigzag chains.