4 6 8
Cu Mo Se
8
semiconductors with low lattice thermal conductivity. The
disorder induced by substitution and partial filling of Chevrel
phases has been shown to significantly decrease their thermal
diffraction, quantitative electron microprobe analysis, and
single-crystal X-ray diffraction. We determined the composi-
tion of this phase to be Cu
a new structure type and represents the first example of an
extended structure with Chevrel-like Mo Se layers that are
4 6 8
Mo Se . This compound adopts
4
conductivity. These fillings and cluster core substitutions
can also be used to tune the electronic properties from
metallic to semiconducting, as discussed below.
6
8
not joined to each other through Mo-Se bonding. The only
interlayer connections are through Cu-Se bonding. This
results in two Mo atoms on each cluster having square-planar
Se coordination, while the other four have the usual square
pyramidal coordination. In addition to the synthesis and
The unfilled Chevrel phase Mo
band-structure calculations on the extended structure, as
well as molecular orbital (MO) calculations on the Mo Se
6 8
Se is metallic. However,
5
,9
6
8
1
0
unit (vide infra), show that it can be made semiconducting
by the addition of four electrons per cluster (or unit cell).
By adding four (or nearly four) electrons per unit cell, using
filling or substitutions, semiconducting Chevrel phases have
been realized in several cases. These include the selenides
4 6 8
crystal structure of Cu Mo Se , we discuss the electronic
consequences of the separation of the Mo Se layers.
6 8
Experimental Section
4
7
3
Ti0.88Mo
6
Se
8
, Mo
4
Ru
2
Se
8
, and Mo
2
Re
4
Se
8
.
6 8 6 8
Synthesis. Mo S and Mo Se cannot be prepared directly from
the elements,14 so the starting materials used in our intercalation
studies for the sulfide and selenide systems were Cu Mo (Q )
S, Se). For the telluride system, Mo Te was used since it can be
One interesting case in which a semiconducting compound
has not been attained is the copper-filled Chevrel phase
selenides. Although not semiconducting, the copper-filled
compound Cu3.1Mo Se shows the best thermoelectric per-
6 8
formance at high temperature of any Chevrel phase studied
to date. Efforts to improve this material by increasing the
Cu concentration have not been successful. This is not true
4 6 8
for the sulfides, and Cu Mo S has been synthesized by the
electrochemical intercalation of Cu. One reason for the
limited Cu content seen in the Se compounds may be the
high temperatures (>1100 °C) at which these materials are
typically prepared. This led us to investigate lower-temper-
2
6 8
Q
6
8
produced by direct reaction of Mo and Te. These compounds were
synthesized from the elements: Cu (Fisher, electrolytic powder),
Mo (Aldrich, 99.9%, -100 mesh), S (Cominco, 99.99%), Se
4
(
unspecified source, 99.999%), and Te (Johnson Matthey, 99.9999%).
The Mo (Cu) powder was reduced in forming gas at 1000 °C (300
C) for about 24 (3) h and subsequently stored and handled inside
°
11
an argon-filled glovebox. The S, Se, and Te were used as received.
Stoichiometric mixtures (typically 2-3 g total) were sealed in
evacuated silica tubes and heated over 1 day to 400 °C and held at
this temperature for 1 day. The tubes were then shaken to mix the
reaction products but not opened. The tubes were then heated to
various temperatures, from 900 to 1200 °C, held there for 3 days,
and then cooled naturally to room temperature with the furnace
power off. The resulting powdered products were almost single-
6 8
ature routes toward Cu-filled Mo Se .
There have been numerous investigations of the addition
of guest atoms to these materials near room temperature.
Electrochemical cells have been used to intercalate Li, Mg,
Na, Zn, Cd, and Cu into Chevrel phase sulfides and
2 6 8 6 8
phase Cu Mo Q or Mo Te with lattice constants matching closely
2
those reported in the literature. A small amount (∼5%) of MoQ
2
12
selenides. One study by Selwyn and McKinnon investigated
the intercalation of Li into already partially filled Cu Mo
Se
(0 < y < 2.5).13 They found, in addition to a series of
impurity was present in some samples.
y
6
-
It is common to prepare partially Cu-filled Chevrel phases and
then remove the Cu to obtain unfilled Mo Q , which cannot be
8
6
8
1
4
Li/Cu-filled phases, an unidentified Cu-rich phase. They also
showed that this phase could be produced by the intercalation
prepared from the elements for Q ) S, Se, as noted above. One
way in which this is done is through reaction with iodine dissolved
in acetonitrile (AN).15 The Chevrel phase is oxidized by the I
solution and forms CuI, which is soluble in AN (3.4 wt% ).
in
of Cu into Cu
into the structure. The authors were unable to prepare the
copper-rich compound as a single phase (Cu1.7Mo Se was
y 6 8
Mo Se , proving that no Li was incorporated
2
1
6
As a means for adding Cu to the Chevrel phase, we essentially
performed this reaction in reverse. Copper was intercalated through
CuI dissolved in AN from an excess supply of elemental Cu in
electrical contact with the Chevrel phase. In the presence of excess
Cu, this reaction should proceed until it is no longer energetically
favorable to add more Cu to the Chevrel phase (assuming that the
diffusion rate of Cu in the Chevrel phase remains large enough).
A special device was constructed for this reaction, designed to
ensure the required electrical contact between the Cu reservoir and
the sample.
6
8
always present as an impurity), and were unable to index
the observed powder X-ray diffraction peaks. In this paper,
we report the crystal structure of this copper-rich Chevrel
phase.
Unlike many previous authors’ work, our low-temperature
studies were not performed in an electrochemical cell but in
a specially designed apparatus described below. We char-
acterized the products of our reactions using powder X-ray
Our Cu intercalation apparatus is shown in Figure 1. It consists
of a glass tube which is threaded on one end and sealed with a flat
glass plate on the other. The glass plate is held in place with Torr-
Seal epoxy. The sample (typically 200-500 mg) is placed between
two accurately weighed cylindrical copper blocks, machined from
(
9) Roche, C.; Chevrel, R.; Jenny, A.; Pecheur, P.; Scherrer, H.; Scherrer,
S. Phys. ReV. B 1999, 60 16442.
(
(
10) Hughbanks, T.; Hoffmann, R. J. Am. Chem. Soc. 1983, 105, 1150.
11) Fischer, C.; Gocke, E.; Stege, U.; Sch o¨ llhorn, R. J. Solid State Chem.
993, 102, 54.
12) Dahn, J. R.; McKinnon, W. R.; Coleman, S. T. Phys. ReV. B 1985,
1, 484. Levi, M. D.; Gizbar, H.; Lancry, E.; Gofer, Y.; Levi, E.;
1
(
3
(14) Belin, S.; Chevrel, R.; Sergent, M. J. Solid State Chem. 1999, 145,
159.
(15) Tarascon, J. M.; Waszczak, J. V.; Hull, G. W., Jr.; DiSalvo, F. J.;
Blitzer, L. D. Solid State Comm. 1983, 47, 973.
(16) Janz, G. J.; Tomkins, R. P. T. Nonaqueous Electrolytes Handbook II;
Academic Press: New York, 1973.
Aurbach, D. J. Electroanal. Chem. 2004, 569, 211. Gocke, E.;
Schramm, W.; Dolscheid, P.; Schoellhorn, R. J. Solid State Chem.
1
987, 70, 71.
13) Selwyn, L. S.; McKinnon, W. R. J. Phys. C: Solid State Phys. 1988,
1, 1905.
(
2
Inorganic Chemistry, Vol. 45, No. 6, 2006 2719