Prsentation PowerPoint

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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_at_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, areof great interest,
particularly with respect to host-guestchemistry,
ion-exchange and adsorption properties,and
shape selective catalytic activity. The large
classof titanium silicates is represented by
more than about 70minerals, mainly with mixed
cation frameworks 2.They display very exciting
crystal chemistry and open anattractive outlook
to synthesize them and their analogues.Many
synthetic homologues of minerals have been
reportedas well as some new titanium silicates
showing openframeworks or bidimensional
structures 3.
PREDICTIONS The present predictions have led to
the inclusion of morethan 1000 structure-types
into the PCOD (PredictedCrystallography Open
Database) 4. The list excludesstructures where
edge- or face-sharing polyhedra wouldoccur, and
also the structures built up from TiO5polyhedra
(the survey of the TiO5/SiO4 combinations
byGRINSP is in progress). A vast majority (70)
of thehypothetical models proposed by GRINSP has
thegeneral formula TiSinO(32n)2- . The most
numerous modelsare those with n 1, 2, 3, 4 and
6, with respectively93, 179, 174, 205 and 158
models corresponding to thesatisfaction of a
reliability criterion R Models with real counterparts
Model PCOD2200207 (Si3TiO9)2- a 7.22 Å b
9.97 Å c 12.93 Å, SG P212121
PCOD2200042 TiSi2O72- identified as
Nenadkevichite ? NaTiSi2O7?2H2O
PCOD2200033 TiSi4O112-identified as
NarsarsukiteNa2TiSi4O11
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 apertures9 x 9 x 9
VP calculated by PLATON 5
Si6TiO152- , 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
themautomatically, appropriately, for electrical
neutrality. Using bond valence rules or/and
energy calculationsto 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/gri
nsp/ 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.