PREDICTED CORNER SHARING TITANIUM SILICATES Armel Le Bail

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 3 D 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 Ti. O 6 octahedra and Si. O 4 tetrahedra are reported here. Models with real counterparts 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 Ti. O 5 polyhedra (the survey of the Ti. O 5/Si. O 4 combinations by GRINSP is in progress). A vast majority (>70%) of the hypothetical models proposed by GRINSP has the general formula [Ti. Sin. O(3+2 n)]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. Model PCOD 2200207 (Si 3 Ti. O 9)2 - : a = 7. 22 Å; b = 9. 97 Å; c =12. 93 Å, SG P 212121 PCOD 2200042 [Ti. Si 2 O 7]2 identified as Nenadkevichite Na. Ti. Si 2 O 7 2 H 2 O PCOD 2200033 : [Ti. Si 4 O 11]2 identified as Narsarsukite Na 2 Ti. Si 4 O 11 Known as K 2 Ti. Si 3 O 9. H 2 O (isostructural to mineral umbite): a = 7. 1362 Å; b = 9. 9084 Å; c =12. 9414 Å, SG P 212121 (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 H 2 O are not taken into account in the model prediction. Highest quality (? ) model First nine models Next nine models Models with largest porosity PCOD 3200086 : 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] [Si 6 Ti. O 15]2 - , cubic, SG = P 4132, 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. CONCLUSIONS Structure and properties prediction is THE challenge of this XXIth century in crystallography. Advantages are obvious (less serendipity and fishing-type syntheses). 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. We have to establish databases of predicted compounds, preferably open access on the Internet. [3] Rocha, J. & Anderson, M. W. (2000). Eur. J. Inorg. Chem. 5, 801 -818. If we are unable to do that, we have to stop pretending to understand master the crystallography laws. [4] http: //www. crystallography. net/pcod/ [5] Küppers, H. , Liebau, F. , Spek, A. L. (2006), J. Appl. Cryst. 39, 338 -346.