Hydrothermal synthesis and ab initio structure determination from powder data of a new three-dimensional mixed valence oxyfluorinated titanium phosphate with an open structure: (TiTiF)-Ti-III-F-IV(PO4)(2)center dot 2H(2)O orMIL-15

(TiTiF)-Ti-III-F-IV(PO4)(2). 2H(2)O (MIL-15) has been prepared hydrothermal ly (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 dete rmined nb initio from powder diffraction data and has been refined in the m onoclinic space group: P2(1)/n (no. 14); the unit cell is a = 10.935(1), b = 14.447(1), c = 5.105(1) Angstrom, beta = 90.50(1)degrees and V = 806.5(1) Angstrom(3), Z=4. Its three-dimensional network, built up from corner shar ing TiX6 octahedra (X = F, O, H2O) and tetrahedral PO4 groups, delimits sev en-membered ring channels along the c-axis. The two crystallographically in dependent titanium atoms have different oxidation states (+ III and + IV) a nd 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 pr esented.