Photon Factory Activity Report 2016 #34 (2017)
BL-7C/2015G672, 2016G582

Temperature Dependent Electronic Structure Study of CaCu3Ti4O12
by means of X-ray Raman Scattering
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1

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1

Kenjiro Nishiyama *, Seiya Nakamoto , Hojun Im , Takao Watanabe ,3,Shunsuke Nozawa , Nobuo Nakajima Toshiaki Iwazumi , and Yasuhisa Tezuka1**
1
Grad. Sch. of Sci. and Tech., Hirosaki Univ., Hirosaki, Aomori 036-8561, Japan2
Photon Factory, Inst. of Mats. Struct. Sci., Tsukuba, Ibaraki 305-0801, Japan
3
Grad. Sch. of Sci., Hiroshima Univ., Higashi-Hiroshima, Hiroshima 739-8526, Japan4Grad. Sch. of Eng., Osaka Pref. Univ., Sakai, Osaka 599-8531, Japan
1 Introduction
A-site ordered perovskite CaCu3Ti4O12 (CCTO) has attracted much attention recently, because it shows a giant
dielectric constant (~104) over a wide temperature range
from about 100 to 600K. Furthermore, the dielectric constant decreases to one-hundredth at the temperature below
about 100K without structural phase transition [1]. To
clarify the nature of the anomalous dielectric property,
electronic structure of CCTO should be investigated.
X-ray Raman scattering (XRS) is an effective method
studying electronic state of insulator material, since ituses photons in both excitation and detection. It has beenreported the XRS by core excitation reflects unoccupied
density-of-state [2]. The XRS spectra observed at Ti K
and Cu K edge were reported [2, 3]. In each spectra, scatterings by 2p3d and 2p4p core-excitations were observed,
where underline denote a core-hole state. We have reported XRS results of polycrystalline CCTO and its temperature dependence [4]. In this article, temperature dependent XRS spectra of single crystal CCTO observed atCu K edge are reported.
2 Experiment
In this study, single crystal CCTO(100) and (111) wereused. The XRS spectra were observed using X-ray emission spectrometer (ESCARGOT) at beamline BL-7C.
Scattered photons were analyzed using crystals Ge(444)
and (400) in the measurements on Cu K and Ti K edge,
respectively. One-dimensional multi-channel proportionalcounter was used to detect the dispersed photons. Theenergy resolution of XRS measurement was about 1 eV.
3 Results and Discussion
In this study, the XRS spectra of single crystal (100)and (111) were observed at both Ti K and Cu K absorption edge. Azimuth dependence and temperature dependence were observed in these samples.
Figure 1 shows the temperature dependence of XRS
spectra of CCTO(100) observed at the Cu K absorption
edge. The polarization of excitation photon was aligned tothe [110] axis, in which direction oxygen ion is located.
The spectra are plotted against energy-loss from excitation energy (Raman shift). The XRS spectra observed at45K (circles) and 300K (line) are shown. These spectra
were excited at the energy just below the Cu K absorption
edge, where quadrupole transition to Cu 3d state is observed, so that these are the Cu 3d resonant XRS spectra.

4CaCu3Ti4O12(100)Cu K XRSIntensity(arb.units)

2E//(110)
E//[110]45K
300K

P1(3d)930

P2LIII
P3(4p)

940

P4P1'P2'(3d)950

Raman Shift(eV)

LIIP3'P4'(4p)

960

970

Fig.1: XRS spectra of CCTO(100) observed at Cu K
absorption edge. Spectra at 300K (line) and 45K (dot)are shown.
The figure shows the excitation from Cu 2p3/2(LIII) and2p1/2(LII) core state, both of which shows unoccupied Cu
3d and 4p states. The peaks P1 and P1' are caused by theexcitation of Cu 2p3d, while the other peaks are causedby the excitation of Cu 2p4p. The 3d peaks (P1 and P1')
become weak at low temperature, while the 4p structure
does not change. Similar peaks with P1 and P1' havebeen observed in the several divalent Cu-compounds [5]and these are supposed to reflect 3d9 state. Thus, thetemperature dependence of Cu 3d peak in this study suggests the increasing of the electron in Cu 3d state at low
temperature. It would be caused by increasing hybridization between Cu and O ions.
Recently, temperature dependence of XRS were observed in detail and showed drastic decrease of Cu 3dpeak intensity at about 100K. The result suggests close
relation between dielectric constant and electronic state.
However, CCTO shows complicated variety on azimuth
angle. Further experiments and investigations would reveal the electronic state of CCTO.
References[1] A.P. Ramirez, et al., Solid State Commun. 115, 217(2000).
[2] Y. Tezuka, et al., J. Phys. Soc. Jpn. 83, 014707 (2014).
[3] Y. Tezuka, et al., J. Electron Spectrosc. Relat. Phenom.
(2017). in press.
[4] Y. Yokouchi, et al., PF Activity Report #32, (2015) B[5] G. Döring, et al., Phys. Rev. B70, 085115 (2004).
* h16ms211@hirosaki-u.ac.jp** tezuka@hirosaki-u.ac.jp