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Physica C 392–396 (2003) 286–290

EXAFS observation of two distinct Bi–O distancesbelow Tc for a Ba0:6K0:4BiO3 single crystal

B.J. Kim a,*, Y.C. Kim a,*, Hyun-Tak Kim b, Kwang-Yong Kang b, J.M. Lee c

a Department of Physics, Pusan National University, San 30 JangJeundong, KumjungGu, Busan 609-735, South Koreab Telecommunication Basic Laboratory, ETRI, Taejon 305-350, South Korea

c Pohang Accelerator Laboratory, Pohang 790-784, South Korea

Received 6 January 2003; received in revised form 12 March 2003; accepted 24 March 2003

Abstract

In order to find two distinct Bi–O distances as evidence for the breathing mode distortion due to an electron–phonon

interaction below Tc, extended X-ray absorption fine structure of Bi L3-edge is measured from 14 to 300 K for a high

quality single-crystal Ba0:6K0:4BiO3. The two distinct Bi–O bond lengths are obtained from data analyzed by a two-shell

fit, and the Bi–O distances are similar to that in BaBiO3, independent of temperature. They come from a remnant phase

of BaBiO3. Bi–O peaks coming from the superconducting phase are not analyzed because the Bi–O peaks overlap.

Further, the Debye–Waller factor ðr2Þ for Bi–O bonds does not fit Einstein model with increasing temperature.

� 2003 Elsevier B.V. All rights reserved.

PACS: 74.72.Yg; 74.62.Bf

Keywords: BKBO; EXAFS; Breathing mode distortion

1. Introduction

The Ba1�xKxBiO3 (BKBO) has a perovskite

structure. The Ba0:6K0:4BiO3 is a superconductor

with Tc ¼ 31 K [1–3]. Its structure has been known

as a simple cubic without tilt of octahedral below

Tc according to neutron powder diffractions [4,5].

However, Ono et al. [6] first observed a tetragonal

structure by using X-rays with ceramic crystalsannealed in oxygen at a high pressure. Braden et al.

[7] also found the tetragonal structure by using

* Corresponding authors. Tel.: +82-51-510-3244; fax: +82-

51-513-7664.

E-mail addresses: bjunkim@pusan.ac.kr (B.J. Kim),

yckim@pusan.ac.kr (Y.C. Kim).

0921-4534/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/S0921-4534(03)00935-3

synchrotron X-rays for BKBO single crystals.Yacoby et al. [8] proposed the presence of a local

tilting of oxygen octahedra around pseudocubic

axes [1 1 0] or [1 1 1] on the angle of 4�–5� at

T 6 220 K in Ba0:6K0:4BiO3. These results indicate

that there is a local structural distortion even in the

superconducting BKBO arising from an BiO6 oc-

tahedra tilt. The local structural distortion is at-

tributed to a local charge ordering, such as Bi3þ

and Bi5þ. Thus, the structure of the supercon-

ducting phase below Tc remains still unclear.

It has been thought that the superconducting

mechanism of Ba0:6K0:4BiO3 is caused by the

electron–phonon interaction.The magnitude of its

superconducting gap was obtained from the tun-

neling and optical experiments [9,10], neutron

ed.

Fig. 1. The temperature dependence of zero-field-cooled (ZFC)

and field-cooled (FC) magnetic susceptibilities.

B.J. Kim et al. / Physica C 392–396 (2003) 286–290 287

scattering experiments [11], the diamagnetic

shielding susceptibility [12], and the ultrahigh res-

olution angle-integrated photoemission spectro-

scopy [13]. The electron–phonon interaction causes

a structural distortion into a non-cubic one in the

condensed state. For example, the parent insula-tor, BaBiO3 has been explained by the charge-

density-wave (CDW) of Bi3þ(6s2) and Bi5þ(6s0)

between the nearest neighbor sites. The CDW oc-

curs due to the breathing-mode distortion of oxy-

gen in the BiO6 octahedra. The breathing-mode

distortion results in two distinct Bi–O bond

lengths, as observed by neutron scattering [14,15]

and the extended X-ray-absorption fine-structure(EXAFS) [16–18] for BiBaO3.

On the other hand, because it has been revealed

that superconductivity occurs by the electron–

phonon interaction, it is natural that the interac-

tion causes two distinct Bi–O bond lengths which

are observable by experiments. The observation of

the bond lengths is direct evidence for the pairing

interaction such as CDW, which is the purpose ofthis EXAFS experiments.

In this paper, we perform EXAFS experiments

to observe both two distinct Bi–O distances as

evidence for the structure distortions and their

temperature dependence from 14 to 300 K for a

high quality single-crystal Ba0:6K0:4BiO3. In addi-

tion, EXAFS observation of two distinct Bi–O

distances is first shown below Tc.

2. Experimental

A Ba0:6K0:4BiO3 single crystal was grown by the

electrochemical method as reported elsewhere [19–

21]. The transition temperature is 31 K, as shown

in Fig. 1 and the dimensions are 3� 3� 1 mm3.The potassium concentration was found to be

x ¼ 0:4 by electron-probe microanalysis. EXAFS

experiments were performed at beam line 3C1 of

the Pohang Acceleratory Laboratory (PAL) op-

erated at an energy of 2.5 GeV and an average

current of �250 mA. Bi LIII edge absorption en-

ergy (13419 eV) has been used to investigate BiO6

octahedral structure in BKBO samples. For thesingle crystal, the incident beam had a 45� angle tothe surface of the crystal. IF was detected in the

fluorescence detector. We used free analysis pro-

gram provided by the Naval Research Laboratory

in the University of Washington [22].The name ofprogram is feffit.exe.

3. Results and discussion

Fig. 2 shows the distances from the absorber Bi

to its neighbors which are represented as peaks in rspace deduced from the Fourier transformation ofEXAFS spectra. The first peak denotes the nearest

neighbor Bi–O distance, the second peak repre-

sents the Bi–Ba(K) distance, and the third peak the

Bi–Bi distance. The magnitude of the peaks de-

creases with increasing temperature. The maxi-

mum peak positions of the Bi–Ba(K) and Bi–Bi

are not shifted with increasing temperature.

However the Bi–O peak has only slight changes atpeak position with temperature and has a broad

width and more fluctuation in shape at a higher

temperature. This may be because oxygen is very

sensitive to temperature on the contrary to Ba(K)

or Bi. This is caused by the vibration of the light

oxygen atoms bonded to the bismuth atom.

Fig. 3 shows the fitting results in r space to the

experimental data for (a) T ¼ 14 K and (b)T ¼ 300 K, to determine the structural parameters

from the Bi–O peak. The fitting range is 1.1–2.1 �AA.

A two-shell fit or double-well potential method has

been used to calculate two different Bi–O or Cu–O

Fig. 3. Two-shell fitting results for a single crystal. The solid

lines correspond to the experiment and the dashed lines to the

theoretical fit at (a) 14 K and (b) 300 K.

Fig. 2. The magnitude of the Fourier transformation for a

single crystal at 14 K (solid line), 30 K (dashed line) and 200 K

(dotted line).

288 B.J. Kim et al. / Physica C 392–396 (2003) 286–290

bond distances in oxide superconductors for ana-

lyzing EXAFS data [8,16,18,23–25]. The open

circles connected by solid lines correspond to the

experimental data and the dashed lines to the

theoretical fit. In Gaussian statistics, v2 for a good

fit is approximately equal to 1. In general, anEXAFS fit with v2 � 1 is common and under-

standable because of the numerous sources of

systematic error which is including detector re-

sponse, sample inhomogeneity, fluctuations in the

incident beam, and sample misalignment. We have

used v2 values in the range of about 10–30.

Fig. 4(a) and (b) shows theoretical fits of the

inverse Fourier transformation, vðkÞ � k3. The fitsagree well with the experimental data up to the

higher k at 14 and 300 K. Two parameters, Bi–O

distances ðr1; r2Þ and Debye–Waller factors ðr2Þ,are obtained from these fits.

Fig. 5 shows the temperature dependence of Bi–

O distances for the single crystal. r1 and r2 denotethe Bi–O distances such as Bi3þ–O bond and Bi5þ–

O, respectively, and are independent of tempera-ture. The average bond lengths of r1 and r2 are

about 2.20 and 2.08 �AA, respectively. More longer

r1 can be regarded as Bi3þ(6s2) and r2 as Bi5þ(6s0).Salem-Sugui et al. [18] analyzed the different Bi–

O bonds by using a single-shell fit by EXAFS ex-

periments in melt BKBO samples at x ¼ 0, 0.2, and

0.4 at room temperature. Heald et al. [16] obtained

two Bi–O distances of 2.11 and 2.28 �AA forBa0:6K0:4BiO3 powder sample at room tempera-

ture. In addition, for Pb-doped BaBi0:25Pb0:75O3

with Tc ¼ 13 K, two Bi–O distances of 2.09 and

2.21 �AA were also observed and did not changed

with temperature [17]. Their bond lengths are

similar to the lengths obtained in this research.

The observed temperature-independent r1 and

r2 are anomalous for the high quality single crys-tal. When the crystal is a homogeneous single-

phase metal with the cubic structure observed at

room temperature by neutron-powder diffractions

[4,5] and X-rays [6,7], the two bonds should be-

come a single bond. Whereas, when the crystal is

tetragonal at less than room temperature [6,7], the

bonds can be observed, but their lengths should

decrease with a decreasing temperature because ofthe decreasing volume [6,7] and have the temper-

ature dependence near Tc because of the volume

Fig. 4. Inverse Fourier transformation in the range of

r ¼ 1:1–2:1 �AA. The solid lines correspond to the experimental

data and the dashed lines to the theoretical fit at (a) 14 K and

(b) 300 K.

Fig. 5. Temperature dependence of (a) the two Bi–O distances

and (b) the Debye–Waller factors r2.

B.J. Kim et al. / Physica C 392–396 (2003) 286–290 289

expansion near Tc observed by Ono et al. [6].

Therefore, the observed temperature-independent

two-bonds can be explained by not the single-

metal phase but an insulating remnant CDW

phase of BaBiO3 where two-bond lengths are

independent of temperature before the insulator–

metal transition occurs at a high temperature [4];this is the reason why the crystal is inhomoge-

neous. The inhomogeneous single crystal is com-

posed of a metal phase of Bi4(6s1) and an

insulating phase of the CDW such as BaBiO3.

We introduce very important two experimental

examples supporting the inhomogeneity. Chainani

et al. [13] proposed an pseudogap by observation

of a suppression in intensity up to 70 meV bindingenergy observed by the photoemission spectros-

copy for a high quality superconducting

Ba0:67K0:33BiO3 single crystal. The energy corre-

sponds to the highest phonon energy related to the

breathing-mode phonon energy. Kim et al. [5]

observed satellite peaks in neutron powder dif-

fraction patterns for a high quality superconduct-

ing Ba0:60K0:40BiO3 powder with Tc ¼ 31 K. Thepeaks followed the temperature dependence of

peaks of BaBiO3. Thus, Chainani et al. [13] and

Kim et al. [5] suggested that the pseudogap comes

from a remnant CDW phase in the crystal. Fur-

thermore, the cause of two phases can be theoret-

ically explained by the metal–insulator instability

of the CDW-potential energy [26,27]. On the other

hand, the distinct bond lengths to occur in the su-

perconducting phase, given by the hypothesis of

this research, were not revealed by analysis of clear

Bi–O main peaks, because Bi–O bond peaks in

both the semiconducting CDW phase and the su-

perconducting (or metal) phase overlap. The fullwidth at half maximum of the Bi–O peaks is much

larger than that of the Bi–Ba and Bi–Bi peaks,

which is attributed to an overlap of peaks rather

than oxygen vibration; there are some peaks near

the main peaks of Bi–O, as shown in Fig. 2.

Fig. 5(b) shows the temperature dependence of

the Debye–Waller factor, r2, for the two Bi–O

distances which is the mean-square atomic dis-placement around Bi atoms. The temperature de-

pendence of r2 is anomalous because of an

anharmonic vibration of oxygen atom. This

290 B.J. Kim et al. / Physica C 392–396 (2003) 286–290

dependence does not follow Einstein�s model de-

pending upon temperature.

4. Conclusion

The two distinct Bi–O distances of 2.20 and 2.08�AA are observed and independent of temperature

below and above Tc, which is not related to su-

perconductivity. Bi–O peaks coming from the

superconducting phase cannot be revealed by two-

shell fit because of the Bi–O peaks overlap with the

peaks of BaBiO3. The phase of Ba0:6K0:4BiO3 sin-

gle crystal should be inhomogeneous.

Acknowledgement

This work was supported by a Korea Research

Foundation Grant (KRF 2000-051-DP0105).

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