Electronic and thermoelectric properties of CuCoO
2
: Density functional calculations
D. J. Singh
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6032, USA
Received 26 May 2007; revised manuscript received 17 July 2007; published 8 August 2007
Density functional calculations are used to elucidate the electronic structure and some transport properties of
CuCoO
2
. We find an electronic structure with similarities to Na
x
CoO
2
, although it is much less two dimen-
sional. In particular, there are narrow manifolds of t
2g
and e
g
states. Application of Boltzmann transport theory
to the calculated band structure shows high thermopowers comparable to Na
x
CoO
2
for both p- and n-type
doping.
DOI: 10.1103/PhysRevB.76.085110 PACS numbers: 72.15.Jf, 71.20.-b, 74.25.Fy, 73.50.Jt
Na
x
CoO
2
and some closely related misfit cobaltate com-
pounds constitute the presently known high figure of merit
1
ZT oxide thermoelectrics suitable for power generation at
temperatures characteristic of waste heat.
2–6
These materials
are based on triangular sheets of octahedrally coordinated Co
and are invariably p type. The transport properties generally
reflect the layered crystal structure in that they are highly two
dimensional; the high thermoelectric figure of merit exists
for transport along the layers, with much lower performance
for perpendicular transport.
3
This complicates practical ap-
plications since either single crystals or a highly oriented
material is needed. In addition, in order to develop all oxide
thermoelectric devices, an n-type oxide material with com-
parable ZT needs to be found for the n-type legs.
The essential ingredient in the high ZT of Na
x
CoO
2
is the
existence of a high thermopower S 100 V/K at 300 K
increasing to 200 V/K at a high temperature, even
though the material has a high metallic carrier density of
0.3 holes per Co. This is remarkable because, in general,
materials with comparable values of S are doped semicon-
ductors with much lower carrier concentrations. This unusual
thermoelectric behavior has been explained in two ways: a
many body approach which yields a high temperature limit-
ing value of S and relies on strong correlations and the par-
ticular electronic configuration of Co to produce a high
thermopower
7
and a conventional Boltzmann transport ap-
proach for metals
8,9
applied to the local density approxima-
tion band structure of Na
x
CoO
2
,
10–12
as may be done for
ordinary thermoelectrics.
13
Within this approach, the essen-
tial ingredient in the thermopower is the position of the
Fermi energy near the top of a very narrow set of bands of a
Co t
2g
character. In spite of the fact that both these bands and
the higher lying e
g
bands are very flat, the Co d orbitals
hybridize strongly with p states from the neighboring O ions.
This happens because of the topology of the Co-O sheets,
which have nearly 90° Co-O-Co bond angles. Considering
that the essential ingredient is the presence of narrow d elec-
tron bands and that this is related to structural features, it is
perhaps not surprising that similarly high thermopowers are
also predicted for n-type doping into the e
g
bands and for
related rhodates. However, both Na
x
CoO
2
and the related
rhodates have anomalously large crystal field gaps, in excess
of 1 eV, between the t
2g
and e
g
manifolds, again reflecting
the combination of narrow bands and strong hybridization.
14
Therefore, it may be very difficult to dope these compounds
n-type with mobile electrons to realize the high thermopow-
ers that are predicted if this can be done. As such, it is of
interest to examine other compounds with related bonding
topologies in order to find other candidate oxide thermoelec-
trics with properties complementary to those of Na
x
CoO
2
.
The purpose of this study is to examine one promising can-
didate, specifically CuCoO
2
.
CuCoO
2
occurs in a rhombohedral structure,
15
space
group R3
¯
m 166, related to a series of other Cu–transition
element oxides with a similar structure.
16
Within this family,
the rhodate, Cu
1-x
Ag
x
Rh
1-y
Mg
y
O
2
has been recently investi-
gated as a potential thermoelectric material.
17
The structure
Fig. 1 has CoO
2
sheets similar to Na
x
CoO
2
, but these are
connected quite differently perpendicular to the layers. In
particular, Na
x
CoO
2
has CoO
2
sheets separated by a Na layer
with partially filled sites, while the sheets in CuCoO
2
are
joined by Cu atoms, which form bridges between O ions in
the adjacent sheets. This provides a three dimensionally con-
nected structure that may be more isotropic from an elec-
tronic point of view as well.
The calculations were done in the local density approxi-
mation LDA using the general potential linearized aug-
mented plane wave LAWP method,
18
as implemented in
two codes, which were cross-checked. These were an in-
house code and the WIEN2K code.
19
Local orbitals
20
were
used to relax the linearization of the d bands and to accu-
rately treat the semicore states of Cu and Co as well as the
O2s state. The calculations of the thermopower were per-
Co
O
Cu
O
O
O
O
O
Co
Co
Co
Cu
Cu
FIG. 1. Color online Crystal structure of CuCoO
2
. Co is
shown as dark blue spheres, Cu as light green, and O as small dark
red spheres. The left panel shows a view along the c axis, while the
right panel has the c axis in the vertical direction. The rhombohe-
dral unit cell contains 1 f.u.
PHYSICAL REVIEW B 76, 085110 2007
1098-0121/2007/768/0851104 ©2007 The American Physical Society 085110-1