Enhanced dielectric responses in Mg-doped CaCu
3
Ti
4
O
12
M. Li, Gemei Cai, D. F. Zhang, W. Y. Wang, W. J. Wang, and X. L. Chen
a͒
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of
Sciences, P.O. Box 603, Beijing 100190, People’s Republic of China
͑Received 6 March 2008; accepted 12 August 2008; published online 7 October 2008͒
We report the effects of the Mg doping on the dielectric properties of CaCu
3
Ti
4
O
12
in the frequency
range of 200 Hz–200 kHz and in the temperature range of 58–473 K. It is found that the
incorporations of Mg
2+
result in increases in the dielectric constant by 12%–20% and decreases in
the dielectric loss by 41%–64% ͑with the minimum of 0.042 for CaCu
2.7
Mg
0.3
Ti
4
O
12
͒ at room
temperature and at 1 kHz. The x-ray photoemission spectroscopy measurements reveal that the
content of Cu
3+
increases with the increasing concentration of Mg
2+
. The enhanced dielectric
response may be related to the influence of Cu
3+
and/or Mg
2+
. In other words, Mg
2+
is an effective
ion to optimize the dielectric properties of CaCu
3
Ti
4
O
12
.©2008 American Institute of Physics.
͓DOI: 10.1063/1.2989124͔
I. INTRODUCTION
The discovery of ultrahigh dielectric response in body-
centered CaCu
3
Ti
4
O
12
͑CCTO͒ has triggered an extensive
research to understand the underlying physics behind this
peculiar phenomenon. Compared with its counterparts, this
perovskitelike compound is very unique in exhibiting dielec-
tric constants as high as 10
4
–10
5
.
1
For example, isostructural
compounds ACu
3
Ti
4
O
12
͑A=Na, Cd, Bi, rare-earth elements,
or vacancy͒ were reported to have dielectric constants less
than 4000.
2–4
Another series of isostructural compounds
ACu
3
M
4
O
12
͑M =Mn, Ru, Ge, Cr, Ga, Nb, Fe, Sb, Ta͒ offers
dielectric constants below 1000.
2,5,6
Theoretical calculations
reveal that CCTO provides dielectric constants only in the
range 40–50.
7
Therefore, the observed giant dielectric re-
sponse seems to originate from a nonintrinsic nature. Many
experimental results support that an internal barrier layer ca-
pacitor ͑IBLC͒ effect is responsible.
5,8–10
However, Zhu et
al.
11
ascribed this abnormal behavior to the local disorder at
the Ca and Cu sites, with a few Cu occupying the Ca site
based on a detailed investigation. Unlike the Cu ions at their
usual sites, the ones at the Ca sites will split the degeneracy
and result in a metal-like behavior, significantly enhancing
the dielectric responses. In addition, the occurrence of the
dielectric relaxations in both single crystals and polycrystal-
line samples indicates that the IBLC model is not the only
possible explanation.
12
Hence, the mechanism for the giant
dielectric response of CCTO needs to be understood further.
On the other hand, the dielectric loss ͑ϳ0.1 at 1 kHz͒ of
CCTO is a little high from an application point of view. That
still needs improvements to facilitate the device implemen-
tations. However, all attempts to lower the dielectric loss are
always accompanied with the decrease in dielectric constant
up to now.
13–16
The best results so far come from the La-
doped compounds, which exhibit dielectric losses less than
0.05, while with a decrease in dielectric constant to 7500.
16
It is known that the Cu ion is one of the most effective
intergranular dopants for barrier layer capacitors and can act
as an acceptor ion.
17
Yang
18,19
found that the addition of Cu
to BaTiO
3
or SrTiO
3
can greatly enhance the dielectric con-
stant via the barrier layer mechanism. In the CCTO system,
the concentration of Cu does affect the dielectric properties.
1
Doping Mn on Cu site suppresses the dielectric constant by
two orders.
20
In this paper, we report the effects of Mg doping on
dielectric responses of CCTO. It is found that the dielectric
constant is enhanced from 14 300 to above 16 000 by doping
Mg on Cu site at room temperature and at 1 kHz. Notably,
the dielectric loss is reduced from 0.119 for the pure sample
to below 0.071 for the doped samples. The x-ray photoemis-
sion spectroscopy ͑XPS͒ measurements show the existence
of Cu
3+
in both pure and doped CCTO. The content of Cu
3+
increases with the increasing doping level. The enhanced di-
electric properties in this system may be related to the influ-
ence of Cu
3+
and/or Mg
2+
.
II. EXPERIMENTAL DETAILS
CaCu
3−x
Mg
x
Ti
4
O
12
͑x=0, 0.1, 0.2, 0.3, 0.4, and 0.5͒
͑CCMT͒ powders were synthesized via the conventional
solid-state reaction method. High-purity CaCO
3
, CuO, TiO
2
,
and MgO were weighed according to the stoichiometric ra-
tios and mixed thoroughly in an agate mortar. The mixed
powders were calcined in air at 1000 ° C for 12 h and at
1100 °C for 24 h with an intermediate grinding. The cal-
cined samples were milled and pressed into pellets of 10 mm
in diameter and about 1 mm in thickness. Then the pellets
were sintered in air at 1100 ° C for 12 h. X-ray powder dif-
fraction ͑XRD͒ data were recorded on an x-ray diffracto-
meter ͑MXP21VAHF/M21X, MAC Science͒ with Cu K
␣
ra-
diation and a diffracted-beam graphite monochromator
operated at 50 kV and 200 mA. To measure the dielectric
properties, silver electrodes were painted on the samples’
surfaces. Dielectric constant and loss tangent data were col-
lected by HP-4274A and HP-4275A multifrequency LCR
meters in a temperature range of 58–473 K. The applied
a͒
Author to whom correspondence should be addressed. Tel.:
ϩ86-10-8264-9039. FAX: ϩ86-10-8264-9646. Electronic mail:
chenx29@aphy.iphy.ac.cn.
JOURNAL OF APPLIED PHYSICS 104, 074107 ͑2008͒
0021-8979/2008/104͑7͒/074107/4/$23.00 © 2008 American Institute of Physics104, 074107-1