Hydrothermal synthesis and ab initio structure determination from powder data of a new three-dimensional mixed valence oxyfluorinated titanium phosphate with an open structure: TiIIITiIVF(PO4)2·2H2O or MIL-15

Abstract

Ti ^III Ti ^IV F(PO ₄ ) ₂ ·2H ₂ O (MIL-15) has been prepared hydrothermally (3 days, 483 K, autogenous pressure) in the presence of an organic base (diazabicyclo[2.2.2]octane). The structure of this compound has been determined ab initio from powder diffraction data and has been refined in the monoclinic space group: P2 ₁ /n (no. 14); the unit cell is a=10.935(1), b=14.447(1), c=5.105(1) Å, β=90.50(1)° and V=806.5(1) Å ³ , Z=4. Its three-dimensional network, built up from corner sharing TiX ₆ octahedra (X=F, O, H ₂ O) and tetrahedral PO ₄ groups, delimits seven-membered ring channels along the c-axis. The two crystallographically independent titanium atoms have different oxidation states (+III and +IV) and are connected via fluorine atoms while two water molecules are bound to the trivalent titanium atoms. Thermal decomposition of MIL-15 in two steps is described. Structural correlations with other titanium phosphates are presented.