The disordered cluster compound CaMo5(Mo0.38Ti 0.62)O10

Article abstract of DOI:10.1107/S0108270108002989

The title compound, calcium penta-molybdenum titanium deca-oxide, is isomorphous with the AMo5(Ti0.7Mo0.3)O10 (A = Sr and Eu) compounds. The smaller size of calcium induces a higher molybdenum content on the capping sites of the bioctahedral Mo10 clusters, leading to more Mo11 and Mo12 clusters in the crystal structure. The oxygen framework derives from the stacking of close-packed layers along the a direction in the ABAC sequence. The Ca ²⁺ ions occupy large cavities which result from the fusion of two cubo-octahedra and are surrounded by ten O atoms. The Ti⁴⁺ ion is octa-hedrally coordinated by the O atoms.

Full text of DOI:10.1107/S0108270108002989

inorganic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
ment and atom Mo5 of the shared edge by four O atoms,
forming a ¯attened tetrahedron. The capping atom Mo6 is in a
distorted octahedral environment of O atoms, as previously
observed for the bicapped Mo6 cluster in NdMo8O14
(Gougeon et al., 1991). The MoÁ Á ÁMo distances within the
ISSN 0108-2701
Ê
clusters range from 2.5927 (4) to 2.8744 (5) A. The main
The disordered cluster compound
CaMo5(Mo0.38Ti0.62)O10
differences between the present Ca and the Sr/Eu compounds
concern the distances between the Mo5 atoms of the shared
Ê
edge [2.8744 (5) and 2.85 A in the Ca and Sr/Eu compounds,
respectively] and those between apical atoms Mo4 and Mo5
P. Gougeon* and P. Gall
Ê
[2.5927 (4) and 2.6178 A in the Ca and Sr/Eu compounds,
respectively]. These variations are in agreement with previous
®ndings in the in®nite chains of edge-shared Mo6 octahedra,
which show that the bond distance between the Mo atoms of
the shared edge is shorter when the apical±apical interaction is
weaker and longer when the interaction between the apex Mo
atoms is stronger (Gall et al., 2002).
Â
Sciences Chimiques de Rennes, UMR CNRS No. 6226, Universite de Rennes I,
Â
Â
Avenue du General Leclerc, 35042 Rennes CEDEX, France
Correspondence e-mail: patrick.gougeon@univ-rennes1.fr
Received 13 December 2007
Accepted 27 January 2008
Online 16 February 2008
From Fig. 2(a), it is also interesting to note that the Mo
clusters are arranged in layers perpendicular to the a axis (see
below). As the shortest distance between Mo clusters is
The title compound, calcium pentamolybdenum titanium
decaoxide, is isomorphous with the AMo5(Ti0.7Mo0.3)O10
(A = Sr and Eu) compounds. The smaller size of calcium
induces a higher molybdenum content on the capping sites of
the bioctahedral Mo10 clusters, leading to more Mo11 and
Mo12 clusters in the crystal structure. The oxygen framework
derives from the stacking of close-packed layers along the a
direction in the . . . ABAC . . . sequence. The Ca2+ ions occupy
large cavities which result from the fusion of two cubo-
octahedra and are surrounded by ten O atoms. The Ti4+ ion is
octahedrally coordinated by the O atoms.
Ê
Ê
3.1475 (4) A within one layer and greater than 3.65 A between
adjacent layers, the structure can be considered to be two-
dimensional with respect to the Mo network, although it is
three-dimensional overall. The MoÐO distances range from
Ê
1.898 (4) to 2.149 (3) A, as usually observed in reduced
molybdenum oxides. The oxygen framework derives from the
stacking of close-packed layers along the a direction in the
1
3
. . . ABAC . . . sequence. The B (y ' ) and C (y ' ) layers are
4
4
entirely occupied by O atoms and thus have the composition
[O24]. In the A layers (y ' 0.0 and 12), one third of the O atoms
are missing or substituted by the Ca ions in an ordered way.
Consequently, the latter layers can be formulated as
Comment
&
&
[O16Ca4 4], where stands for the oxygen vacancies. In this
close packing, all the tetrahedral interstices are empty, while
Among the molybdenum cluster compounds in solid-state
chemistry, the AMo5(Ti0.7Mo0.3)O10 (A = Sr and Eu)
compounds (Tortelier et al., 2001) constitute an interesting
family. Indeed, contrary to other reduced molybdenum
compounds in which the Mo clusters are well ordered, the
AMo5(Ti0.7Mo0.3)O10 compounds present
a
disordered
mixture of bioctahedral Mo10, Mo11 (monocapped Mo10 clus-
ters) and Mo12 clusters (bicapped Mo10 clusters). Another
intriguing feature of the latter compounds is the absence of
solid solution between the Mo and Ti atoms and thus the
quasi-invariance of the Mo:Ti ratio on the capping sites. We
present here the crystal structure of the title isomorphous
calcium compound, which has an Mo:Ti ratio slightly different
to those observed for the Sr and Eu compounds.
As observed for the Sr and Eu compounds, the crystal
structure of the title compound is based on a random mixture
of Mo10, Mo11 (monocapped Mo10 clusters) and Mo12
(bicapped Mo10 clusters) clusters due to the partial occupation
of the capping Mo6 position (Fig. 1). The Mo10 cluster results
from the edge-sharing of two octahedral Mo6 clusters and is
similar to those previously observed in the R16Mo21O56 (R =
La to Nd) compounds (Gall & Gougeon, 1993; Gall et al.,
1999). Consequently, atoms Mo1, Mo2, Mo3 and Mo4 are
surrounded by ®ve O atoms in a square-pyramidal environ-
Figure 1
The Mo±O cluster unit in CaMo5(Mo0.38Ti0.62)O10. The Mo atoms in black
which fully occupy their site form the bioctahedral Mo10 cluster. The Mo
atoms in light grey which cap the Mo10 cluster have an occupancy factor
of 0.382 (3). Displacement ellipsoids are drawn at the 90% probability
level.
i30 # 2008 International Union of Crystallography
DOI: 10.1107/S0108270108002989
Acta Cryst. (2008). C64, i30±i32

inorganic compounds
half of the octahedral interstices are ®lled by the Mo1, Mo2,
Mo3, Mo4 and Mo5 atoms, which then form bioctahedral Mo10
clusters. In addition, one tenth of the octahedral sites are
occupied statistically by atoms Mo6, which cap the faces of the
bioctahedral Mo10, and by atoms Ti1 (Fig. 2).
The Ca2+ ions occupy large cavities which result from the
fusion of two cubooctahedra. They are surrounded by ten O
atoms with six nearest neighbours at distances ranging from
Ê
2.354 (2) to 2.657 (3) A (thick grey lines in Fig. 3) and four
second nearest neighbours at distances between 2.850 (3) and
Ê
3.053 (3) A (thin grey lines in Fig. 3). In the Sr and Eu
compounds, the cation-to-oxygen distances range from 2.50 to
Ê
3.23 A with no clear gap.
Ê
The Ti atom, which occupies at 62% a position 0.362 (2) A
from that of the capping Mo6 atom, obviously has the same
O-atom environment as the latter atom. It is thus surrounded
by six O atoms, forming a highly distorted octahedron. The
Ê
TiÐO distances vary between 1.848 (4) and 2.178 (4) A, with a
Ê
mean value of 2.00 A. The latter value is in good agreement
Ê
with the distance of 2.005 A expected from the sum of the
ionic radii of O2 and Ti4+ in octahedral coordination,
according to Shannon & Prewitt (1969). The oxidation state of
+4 for the Ti atoms was also con®rmed from the TiÐO bond
lengths using the relationship of Brown & Wu (1976) [s = (dTiÐ
O/1.806) 5.2] which leads to a value of +3.8.
Ê
The shortest MoÁ Á ÁTi distance is 2.904 (3) A and this is
Ê
identical to the distances observed in the Eu [2.903 (2) A] and
Ê
Sr [2.905 (2) A] compounds. Such values preclude the exis-
tence of metallic TiÐMo bonds and thus the formation of
heteronuclear clusters.
Bond-valence calculations (Brown & Wu, 1976) performed
on the Mo atoms led to values of 3.16, 3.23, 2.89, 2.73, 2.35 and
4.39 v.u. for atoms Mo1, Mo2, Mo3, Mo4, Mo5 and Mo6,
respectively. This yields an average molybdenum oxidation
state of +2.98 leading to about 34 electrons which are delo-
calized over the cluster. As expected, the oxidation state of the
capping Mo6 atom is higher than those of the other Mo atoms.
This is principally because atom Mo6 has the lowest coordi-
nation by other Mo atoms and therefore the fewest electrons
available for metal-to-metal bonding.
Experimental
Figure 2
(a) A view of CaMo5(Mo0.38Ti0.62)O10 along the b axis. Displacement
ellipsoids are drawn at the 90% probability level. (b) A view of
CaMo5(Mo0.38Ti0.62)O10 along the a axis. Displacement ellipsoids are
drawn at the 90% probability level.
Single crystals of CaMo5(Mo0.38Ti0.62)O10 were obtained by the
reaction of TiO2, MoO3, CaMoO4 and Mo with the nominal
composition CaMo6Ti2O15. CaMoO4 was prepared by heating a
stoichiometric mixture of CaCO3 and MoO3 in an open porcelain
crucible at 1073 K for 24 h. Mo powder was heated under a ¯ow of H2
at 1273 K for 6 h prior to use. The initial mixture (ca 5 g) was cold
pressed and loaded into a molybdenum crucible, which was sealed
under low argon pressure using an arc welding system. The charge
was heated at a rate of 300 K h 1 to 1673 K and this temperature was
1
held for 48 h. The vessel was then cooled at a rate of 100 K h to
1373 K and ®nally furnace cooled. Subsequently, single-phase
powders were obtained for compositons CaMo5(Mo1 xTix)O10, with x
ranging from 0.55 to 0.65.
Crystal data
3
Ê
CaMo5(Mo0.38Ti0.62)O10
Mr = 746.06
Orthorhombic, Pbca
V = 1548.09 (4) A
Z = 8
Mo Kꢀ radiation
1
Ê
Figure 3
The environment of the Ca atoms (see Comment). The symmetry codes
are as in Table 1.
a = 8.97790 (10) A
Ê
ꢁ = 9.77 mm
T = 293 (2) K
b = 11.4442 (2) A
Ê
c = 15.0673 (3) A
0.06 Â 0.05 Â 0.05 mm
ꢀ
Acta Cryst. (2008). C64, i30±i32
Gougeon and Gall
CaMo5(Mo0.38Ti0.62)O10 i31

inorganic compounds
Data collection
The ®nal re®nement cycles included the atomic coordinates, the
anisotropic displacement parameters for all atoms and site-occu-
pancy factors for the Mo6 and Ti1 sites, the sum of which was
constrained to unity. Re®nement of the occupancy factors of the O
atoms did not reveal defects in oxygen. Because of the disordering of
the Mo6 and Ti1 atoms, a reciprocal-space reconstruction of
different planes was carried out, as well as long-exposure rotations
along the three axes on a single crystal on the Nonius KappaCCD
diffractometer. In both cases, no superlattice re¯ections or diffuse
lines were observed.
Nonius KappaCCD diffractometer
Absorption correction: analytical
(de Meulenaer & Tompa, 1965)
Tmin = 0.642, Tmax = 0.698
43304 measured re¯ections
4205 independent re¯ections
2960 re¯ections with I > 2ꢂ(I)
Rint = 0.072
Re®nement
R[F2 > 2ꢂ(F2)] = 0.035
wR(F2) = 0.057
S = 1.09
166 parameters
1 restraint
3
Ê
Áꢃmax = 1.75 e A
3
Ê
1.90 e A
4205 re¯ections
Áꢃmin
=
Data collection: COLLECT (Nonius, 1998); cell re®nement:
COLLECT; data reduction: EVALCCD (Duisenberg, 1998);
program(s) used to solve structure: SIR97 (Altomare et al., 1999);
program(s) used to re®ne structure: SHELXL97 (Sheldrick, 2008);
molecular graphics: DIAMOND (Brandenburg, 2001); software used
to prepare material for publication: SHELXL97.
Table 1
Selected bond lengths (A).
Ê
Ca1ÐO8i
2.354 (2)
2.425 (3)
2.432 (3)
2.557 (3)
2.645 (2)
2.657 (3)
2.850 (3)
2.948 (3)
3.048 (3)
3.053 (3)
1.989 (2)
2.002 (3)
2.013 (2)
2.034 (2)
2.147 (2)
2.6829 (4)
2.7564 (19)
2.7660 (4)
2.7752 (4)
2.7869 (4)
2.004 (2)
2.004 (3)
2.005 (3)
2.038 (2)
2.079 (2)
2.6891 (18)
2.7534 (4)
2.7657 (4)
2.7852 (4)
2.034 (2)
2.053 (2)
Mo3ÐO10
Mo3ÐO7
2.068 (2)
2.074 (2)
2.086 (2)
2.5927 (4)
2.6944 (4)
2.7252 (4)
2.921 (2)
2.063 (2)
2.070 (2)
2.074 (2)
2.095 (2)
2.106 (2)
2.7406 (4)
2.7886 (4)
2.028 (2)
2.049 (2)
2.071 (2)
2.078 (2)
2.8744 (5)
1.898 (4)
1.905 (3)
1.950 (3)
2.026 (3)
2.042 (3)
2.149 (3)
1.848 (4)
1.870 (4)
1.891 (4)
2.095 (4)
2.104 (4)
2.178 (4)
Ca1ÐO2ii
Ca1ÐO3iii
Ca1ÐO4
Mo3ÐO6v
Mo3ÐMo4
Mo3ÐMo5
Ca1ÐO7iv
Ca1ÐO6
Ca1ÐO9iv
Ca1ÐO7
Ca1ÐO10i
Ca1ÐO1v
Mo1ÐO1vi
Mo1ÐO2vii
Mo1ÐO9
Mo3ÐMo5vii
Mo3ÐMo6vii
Mo4ÐO8
Mo4ÐO5
Mo4ÐO4
Mo4ÐO7
Mo4ÐO1ix
Mo4ÐMo5
Mo4ÐMo5vii
Mo5ÐO8vii
Mo5ÐO6
Mo5ÐO9
Mo5ÐO7
Mo5ÐMo5vii
Mo6ÐO2
Mo6ÐO10vii
Mo6ÐO3
Mo6ÐO1
Mo6ÐO5x
Mo6ÐO9xi
Ti1ÐO5
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: IZ3044). Services for accessing these data are
described at the back of the journal.
References
Mo1ÐO3vii
Mo1ÐO4viii
Mo1ÐMo2vii
Mo1ÐMo6vii
Mo1ÐMo5
Mo1ÐMo4vii
Mo1ÐMo3
Mo2ÐO6
Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C.,
Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J.
Appl. Cryst. 32, 115±119.
Brandenburg, K. (2001). DIAMOND. Version 2.1e. Crystal Impact GbR,
Bonn, Germany.
Brown, I. D. & Wu, K. K. (1976). Acta Cryst. B32, 1957±1959.
Duisenberg, A. J. M. (1998). PhD thesis, University of Utrecht, The
Netherlands.
Gall, P., Barrier, N., Gautier, R. & Gougeon, P. (2002). Inorg. Chem. 41, 2879±
2885.
Gall, P., Gautier, R., Halet, J.-F. & Gougeon, P. (1999). Inorg. Chem. 38, 4455±
4461.
Gall, P. & Gougeon, P. (1993). Acta Cryst. C49, 659±663.
Gougeon, P., Potel, M. & Sergent, M. (1991). Acta Cryst. C47, 1791±
1794.
Meulenaer, J. de & Tompa, H. (1965). Acta Cryst. 19, 1014±1018.
Nonius (1998). COLLECT. Nonius BV, Delft, The Netherlands.
Shannon, R. D. & Prewitt, C. T. (1969). Acta Cryst. B25, 925±946.
Sheldrick, G. M. (2008). Acta Cryst. A64, 112±122.
Tortelier, J., Gougeon, P., Gautier, R. & Berjoan, R. (2001). Inorg. Chem. 40,
2292±2297.
Mo2ÐO10vii
Mo2ÐO2
Mo2ÐO4
Mo2ÐO5i
Mo2ÐMo6
Mo2ÐMo4
Mo2ÐMo5
Mo2ÐMo3vii
Mo3ÐO8
Ti1ÐO1ix
Ti1ÐO9xii
Ti1ÐO3ix
Ti1ÐO10xiii
Ti1ÐO2ix
Mo3ÐO3vii
Symmetry codes: (i)
x ‡ 12; y ‡ 12; z; (ii)
x
12; y; z ‡ 32; (iii)
x
1; y; z; (iv)
x; y; z ‡ 1; (v) x ‡ 12; y 21; z; (vi) x; y ‡ 21; z 12; (vii) x ‡ 1; y; z ‡ 1; (viii)
x ‡ 12; y; z 12; (ix) x ‡ 1; y
;
z ‡ 32; (x) x ‡ 1; y ‡ 21; z ‡ 23; (xi) x ‡ 21, y ‡ 12,
1
2
z ‡ 1; (xii) x ‡ 12; y; z ‡ 12; (xiii) x;
y
12; z ‡ 12.
ꢀ
i32 Gougeon and Gall
CaMo5(Mo0.38Ti0.62)O10
Acta Cryst. (2008). C64, i30±i32