Ko Euna, Jo Hongil, Ok Kang Min
Department of Chemistry, Sogang University, Seoul 04107, Republic of Korea.
Inorg Chem. 2021 Jun 7;60(11):7914-7921. doi: 10.1021/acs.inorgchem.1c00425. Epub 2021 May 19.
Crystals of two strontium niobium oxyfluorides, SrNbOF·4HO and SrNbOF·2HO, have been grown in phase pure forms via hydrothermal reactions using SrCO, NbO, and an aqueous HF solution. Single-crystal X-ray diffraction suggests that SrNbOF·4HO, crystallizing in the orthorhombic centrosymmetric space group, (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 NbO(O/F)F, NbO(O/F)F, NbO(O/F), and SrOF groups. SrNbOF·2HO with the noncentrosymmetric polar space group, 2 (No. 36), however, reveals a molecular structure consisting of Nb(O/F)F pentagonal bipyramids and two unique Sr 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 SrNbOF·4HO and SrNbOF·2HO 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 SrNbOF·2HO to thermodynamically more stable SrNbOF·4HO occurs under a hydrothermal condition.
通过使用碳酸锶、五氧化二铌和氢氟酸水溶液进行水热反应，已经生长出了两种氟氧化铌锶晶体，即SrNbOF·4H₂O和SrNbOF·2H₂O，且为纯相形式。单晶X射线衍射表明，SrNbOF·4H₂O结晶于正交中心对称空间群（编号55），揭示了一种三维钨青铜结构的新变体，其具有由共用角的NbO(O/F)F、NbO(O/F)F、NbO(O/F)和SrOF基团组成的三元、四元和五元环。然而，具有非中心对称极性空间群2（编号36）的SrNbOF·2H₂O揭示了一种分子结构，该结构由Nb(O/F)F五角双锥体和两个与F、O/F及水分子相互作用的独特Sr阳离子组成。基于SrNbOF·4H₂O和SrNbOF·2H₂O的紫外可见漫反射光谱，通过Kubelka-Munk函数计算得出的带隙分别估计约为3.22和4.11 eV，其中这些值与电负性F原子的含量以及受结构畸变影响的Nb-O(F)-Nb键角有关。在水热条件下，发生了从SrNbOF·2H₂O到热力学上更稳定的SrNbOF·4H₂O的有趣相变反应。
