PREDICTED CORNER SHARING TITANIUM SILICATES Armel Le Bail Université du Maine, Laboratoire des Oxydes et Fluorures, CNRS UMR 6010, Avenue O. Messiaen, 72085 Le Mans Cedex 9, France Email : alb@cristal.org INTRODUCTION As a consequence of the increase in computer power and due to the obvious interest in relying more on planning than on serendipity for chemical synthesis, times are coming for the systematic prediction of the crystal structures of inorganic compounds. The recent publication of the GRINSP (Geometrically Restrained Inorganic Structure Prediction) code [1] for the building of N-connected 3D frameworks (N = 3, 4, 5, 6 and binary combinations N/N’) allows for the exploration of single or mixed frameworks. Hypothetical GRINSP models built up from corner-sharing TiO6 octahedra and SiO4 tetrahedra are reported here. TITANOSILICATES Mixed frameworks, minerals and synthetic compounds, are of great interest, particularly with respect to host-guest chemistry, ion-exchange and adsorption properties, and shape selective catalytic activity. The large class of titanium silicates is represented by more than about 70 minerals, mainly with mixed cation frameworks [2]. They display very exciting crystal chemistry and open an attractive outlook to synthesize them and their analogues. Many synthetic homologues of minerals have been reported as well as some new titanium silicates showing open frameworks or bidimensional structures [3]. PREDICTIONS The present predictions have led to the inclusion of more than 1000 structure-types into the PCOD (Predicted Crystallography Open Database) [4]. The list excludes structures where edge- or face-sharing polyhedra would occur, and also the structures built up from TiO5 polyhedra (the survey of the TiO5/SiO4 combinations by GRINSP is in progress). A vast majority (>70%) of the hypothetical models proposed by GRINSP has the general formula [TiSinO(3+2n)]2- . The most numerous models are those with n = 1, 2, 3, 4 and 6, with respectively 93, 179, 174, 205 and 158 models corresponding to the satisfaction of a reliability criterion R < 0.02. Models with real counterparts Model PCOD2200207 (Si3TiO9)2- : a = 7.22 Å; b = 9.97 Å; c =12.93 Å, SG P212121 PCOD2200042 [TiSi2O7]2- identified as Nenadkevichite  NaTiSi2O72H2O PCOD2200033 : [TiSi4O11]2- identified as Narsarsukite Na2TiSi4O11 Known as K2TiSi3O9.H2O (isostructural to mineral umbite): a = 7.1362 Å; b = 9.9084 Å; c =12.9414 Å, SG P212121 (Eur. J. Solid State Inorg. Chem. 34, 1997, 381-390) Average discrepancy on cell parameters : 0.6%. Not too bad if one considers that K et H2O are not taken into account in the model prediction. First nine models Next nine models Highest quality (?) model Models with largest porosity PCOD3200086 : P = 70.2%, FD = 10.6, DP = 3 (dimensionality of the pore/channels system) Next nine models Ring apertures 9 x 9 x 9 VP calculated by PLATON [5] [Si6TiO15]2- , cubic, SG = P4132, a = 13.83 Å Opened doors, limitations GRINSP limitation : exclusively corner-sharing polyhedra. Opening the door potentially to > 50.000 hypothetical compounds. More than 10.000 should be included into PCOD [4] before the end of 2006. Then, their powder patterns will be calculated and possibly used for search-match identification. Expected improvements Edge, face, corner-sharing, mixed. Hole detection, filling them automatically, appropriately, for electrical neutrality. Using bond valence rules or/and energy calculations to define a new cost function. Etc, etc, etc. CONCLUSIONS Structure and properties prediction is THE challenge of this XXIth century in crystallography. Advantages are obvious (less serendipity and fishing-type syntheses). We have to establish databases of predicted compounds, preferably open access on the Internet. If we are unable to do that, we have to stop pretending to understand and master the crystallography laws. REFERENCES [1] Le Bail, A. (2005). J. Appl. Cryst. 28, 389-395. GRINSP : http://www.cristal.org/grinsp/ [2] Pyatenko, Y.A., Voronkov, A.A. & Pudovkina, Z.V. (1976). The Mineralogical Crystal Chemistry of Titanium in Russian), Nauka, Moscow. [3] Rocha, J. & Anderson, M.W. (2000). Eur. J. Inorg. Chem. 5, 801-818. [4] http://www.crystallography.net/pcod/ [5] Küppers, H., Liebau, F., Spek, A.L. (2006), J. Appl. Cryst. 39, 338-346.