Sr2Nb6O13F8•4H2O and Sr3Nb2O2F12•2H2O: A Variant of Three-Dimensional Tungsten Bronze and a Polar Molecular Oxide Fluoride

Crystals of two strontium niobium oxyfluorides, Sr2Nb6O13F8 center dot 4H(2)O and Sr3Nb2O2F12 center dot 2H(2)O, have been grown in phase pure forms via hydrothermal reactions using SrCO3, Nb2O5, and an aqueous HF solution. Single-crystal X-ray diffraction suggests that Sr2Nb6O13F8 center dot 4H(2)O, crystallizing in the orthorhombic centrosymmetric space group, Pbam (No. 55), reveals a new variant of the three-dimensional tungsten bronze structure with three-, four-, and five-membered rings that are composed of corner-sharing NbO2(O/F)(2)F-2, NbO4(O/F)F, NbO3 (O/F)(3), and SrO3F6 groups. Sr3Nb2O2F12 center dot 2H(2)O with the noncentrosymmetric polar space group, Cmc2(1) (No. 36), however, reveals a molecular structure consisting of Nb(O/F)(2)F-5 pentagonal bipyramids and two unique Sr2+ cations interacting with F, O/F, and water molecules. Band gaps calculated by the Kubelka-Munk function based on the ultraviolet-visible diffuse-reflectance spectra of Sr2Nb6O13F8 center dot 4H(2)O and Sr3Nb2O2F12 center dot 2H(2)O are estimated to be ca. 3.22 and 4.11 eV, respectively, in which the values are related to the contents of electronegative F atoms and the Nb-O(F)-Nb bond angles influenced by structural distortion. An interesting phase transition reaction from Sr3Nb2O2F12 center dot 2H(2)O to thermodynamically more stable Sr2Nb6O13F8 center dot 4H(2)O occurs under a hydrothermal condition.