Photon Factory Activity Report 2022 #40 (2023)

BL-7C/2021G601

Temperature Dependent Electronic Structure ofA-site Ordered Perovskite CaCu3Ti4O12
Masanori SHIMAMURA1*, Hojun IM1, Takao WATANABE1, Shunsuke NOZAWA2,Nobuo NAKAJIMA3, Toshiaki IWAZUMI4, 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 Met. Univ., Sakai, Osaka 599-8531, Japan
1 Introduction
CaCu3Ti4O12(CCTO) has A-site ordered perovskitestructure (A’A3B4O12). One of the four A-site (Cu) in theperovskite structure (ABO3) is replaced by Ca-ion, so that
the TiO6 cluster in CCTO is tilted from crystallographicaxis. CCTO exhibits a huge dielectric constant of ε ~ 104in the temperature range of 100 to 600 K, while thedielectric constant decreases suddenly below about 100 K
without a structural phase transition [1]. Since CCTO does
not undergo structural phase transitions on the dielectricanomaly, it is important to study the electronic structure tounderstand the dielectric properties. In this study, resonant
X-ray Raman scattering (XRS) [2-4] of CCTO wasmeasured. Core excitations such as 2p3d and 2p4p, where
underline denote a hole state, were observed. These spectra
reflect unoccupied density-of-states. Temperaturedependent XRS spectra were observed at Cu K absorptionedge.
2 ExperimentSingle crystal CCTO (100) was used in this experiment.
The XRS spectra were measured using X-ray emission
spectrometer (Escargot) at beamline BL-7C of Photon
Factory. XRS measurements were carried out attemperatures of 15, 30, 80 and 120 K as well as room
temperature. 80 and 120 K are below and just above thedielectric anomaly temperature, respectively. Helium
circulation cryostat was used in these measurements.
3 Results and Discussion
Figure 1 shows the Cu K XAS spectrum of CCTO (100)measured by partial photon yield (PPY) method. Cu 3dstates were observed at the pre-edge region, while the mainstructure shows Cu 4p state. The vertical bar denotes
excitation energy in the XRS measurement.
Figure 2 shows the temperature dependent XRS spectraof CCTO (100) excited at 8980.2 eV. X-ray Emission
spectra are plotted against energy loss (Raman shift). FourXRS peaks were observed with each core level of Cu 2p3/2
and 2p1/2. The P1 and P2 are originated from Cu 2p3d
excitations, while the P3 and P4 are originated from Cu
2p4p excitations. These spectra were normalized with P3
peak intensity that reflects 4p state and does not changewith excitation energy and temperature. The temperaturedependence measurements show that the intensity of P1(P1’) decreases around 100K, then it increases again at low

Fig.1: Cu K XAS spectrum of CCTO (100). The verticalbars indicate the excitation energy of the XRS
measurement.

Fig.2: Temperature dependence of the Cu K resonant Xray Raman scattering spectra of CCTO (100) excited at8980.2 eV.
temperature. Since the XRS spectra reflect the unoccupied
density of states, the result shows increase of Cu 3d statearound dielectric anomaly temperature. Other hand, Ti 3dpeak was observed decreasing continually with decreasing
temperature in Ti K XRS measurements. These results
suggest the relation between electronic state and dielectric
properties.
References[1] A.P. Ramirez, et al., Solid State Commun., 115, 217(2000).
[2] Y. Tezuka, et al., Phys. Rev. B104, 235148 (2021).
[3] Y. Tezuka, et al., J. Phys. Soc. Jpn. 83, 014707 (2014).
[4] Y. Tezuka, et al., J. Electron Spectrosc. Relat. Phenom.
220, 114-117 (2017).
* h22ms106@hirosaki-u.ac.jp** tezuka@hirosaki-u.ac.jp