Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.14, pp.1277-1283, 2011
jmmce.org Printed in the USA. All rights reserved
1277
Fluorine Doping Effect in the New Superconducting
Y
3
Ba
5
Cu
8
O
y
Compound
K. Srinivasan
a, b
, George Thomas C.
b*
, P. Padaikathan
c
a
Department of Physics, Govt. First Grade College, K. R. Puram,
Bangalore – 560 036, India
b
Department of Physics, Christ University, Bangalore – 560 029, India
c
Department of Materials Engineering, Indian Institute of Science,
Bangalore – 560 012, India
* Corresponding author: [e-mail]
ABSTRACT
The newly identified yttrium based high critical temperature (T
c
) superconductor of
composition Y
3
Ba
5
Cu
8
O
y
(Y358) and the fluorine doped Y
3
Ba
5
Cu
8
O
y-0.2
F
0.2
(Y358+F)
compounds were synthesized by solid state reaction technique. The micro structural and
morphological features of the synthesized samples were investigated by X-Ray Diffraction
(XRD), Scanning Electron Microscope (SEM) and Energy Dispersive Spectrum (EDS)
analysis. The XRD of the samples revealed a structure similar to Y
1
Ba
2
Cu
3
O
y-δ
(Y123)
compound with about three fold larger ‘c’ axis. The SEM image of the fluorine doped sample
(Y358+F) showed a needle like inter growth which was different from that of pure (Y358)
sample. The T
c
of the sample, measured by self inductance method showed an onset of
superconductivity at 92 K for the pure and 98 K for the fluorine doped sample. On further
heating the fluorinated sample for 24 hours, the needle like intergrowth had disappeared as
seen from SEM micrograph and the T
c
onset of the sample had increased to 110 K with a two
step transition.
Keywords: High T
c
superconductor, solid state reaction, XRD, SEM, T
c
measurement,
fluorine doping.
1. INTRODUCTION
The field of high critical temperature (T
c
) superconductors is scientifically very much
interesting and technologically worth exploring. Before 1986, no materials were found
superconducting above 23 K. In 1986, the promising first high T
c
superconductor was
discovered by Bednorz and Muller [1] in (La-Ba)-Cu-O (La214) system with a record T
c
of
around 30 K. Wu et al. [2] reported a T
c
of around 92 K in the well known Y
1
Ba
2
Cu
3
O
7
(Y123) compound. For the past two decades, researchers have carried out several
investigations on YBCO family and synthesized a number of compounds such as

1278 K. Srinivasan, George Thomas C Vol.10, No.14
Y
1
Ba
2
Cu
4
O
16
(Y124, T
c
= 84 K) and Y
2
Ba
4
Cu
7
O
15
(Y247, T
c
= 40 K) [3-4]. In La214
compound, there is only one CuO
2
plane. Y123 has one CuO chain and two CuO
2
planes, both
the planes being symmetrically equivalent. Y124 has one CuO double chain and one in-
equivalent plane. Y247 has one CuO chain, one CuO double chain and two in-equivalent
CuO
2
planes [5]. More recently, Aliabadi et al. [6] have reported a new superconductor
Y
3
Ba
5
Cu
8
O
18
(Y358) of T
c
above 100 K with the same crystalline structure as that of Y123
but CuO
2
planes are increased to five and CuO chains are increased to three. According to
Aliabadi et al. [6], to have improvement in T
c
, one should pump more holes from the chains
to the oxygen sites of the planes. Fisk et al. [7] and Horet et al. [8] have shown that in YBCO
system superconductors, substitution at the rare earth site or alkaline-earth site would not
change T
c
considerably. But substitution at the oxygen site is very crucial in the formation of
superconductors. Ovshinsky et al. [9] have reported substantial increase in T
c
of the
fluorinated Y123 system with a T
c
onset of around 155 K. The present study is an attempt to
prepare pure and fluorinated Y358 superconductors and to study their T
c
dependence.
2. MATERIALS AND METHODS
Samples of the nominal composition Y
3
Ba
5
Cu
8
Oy (sample-A) and Y
3
Ba
5
Cu
8
O
y-0.2
F
0.2
(sample-B) were prepared by standard solid-state reaction technique. Appropriate
stoichiometric ratios of high purity powders of Y
2
O
3
, BaCO
3
, CuO and CuF
2
were mixed
thoroughly and finely ground. The samples were taken in ceramic crucibles and calcined in
air at 810°C for 14 hours using a tubular furnace. The powders were re-ground and pressed in
the form of pellets of 10 mm diameter and about 2 mm thickness under a pressure of 5000
kg/cm
2
. The pellets were then sintered at 950°C for 20 hours and furnace cooled to room
temperature. The fluorine substituted sample (sample-B) was found to be slightly melted at
the surface, but the pellets could be retrieved as such. All the pellets were re-ground, re-
pelletized and further heated at 925°C for 24 hours in air followed by furnace cooling to
room temperature. Another 24 hours of heating at 925°C was given to only the fluorine
substituted sample and this sample was labeled as B′.
A portion of the sintered pellets were crushed into fine powder and used for X-ray diffraction
(XRD) analysis. The XRD measurements were carried out using a compact PHILIPS Pro
Analytical automated diffractometer with copper K
α
source (λ = 1.5405 Å) in the two theta
range from 10º to 70º. The T
c
of the sample was measured by self inductance method using
Colpitt’s oscillator and a frequency counter. A Chromel-Alumel thermocouple was used for
the measurement of temperature with an accuracy of ±1ºC. The SEM and EDS measurements
were carried out on all the samples using Quanta ASM 840A machine.
3. RESULTS AND DISCUSSIONS
The X-ray diffractogram of Y358 (sample-A) and Y358 + F (sample-B), sintered for 20 hours
at 950°C + 24 hours at 925°C are shown in Fig. 1 along with the XRD pattern of Y123. It is
seen that, most of the peaks of Y358 and Y358 + F are identical [10, 11] with that of Y123.
Fig. 2 shows a XRD pattern of sample B′, sintered for 20 hours at 950°C + 48 hours at
925°C. The peak at 2θ ≈ 27.8º is identified as due to the impurity phase Ba
2
Cu
3
O
5
which is

Vol.10, No.14 Fluorine Doping Effect 1279
Fig. 1. XRD pattern of Y123, Y358+F and Y358
Fig. 2. XRD Pattern of sample- B′
marked by symbol ‘+’ in the pattern. The small impurity peaks at 2θ ≈ 30º, 35.5º, 36.2º,
47.5º, 48.5º, 62º and 65.4 º, are marked by the symbol ‘*’ which could not be identified as
that belonging to neither Y358 system nor that of the starting materials. This observation was
similar to that reported by Udomsamuthirun et al. [12]. The remaining peaks of the sample B′
were indexed in an orthorhombic cell with lattice parameters a = 3.902 Å, b = 3.824 Å and
c = 30.690 Å and unit cell volume of 457.931 Å
3
. The c value of the sample B′ is almost 3
times the c value of Y123 sample. Similar results were reported by Aliabadi et al. [6] but a
prominent (002) peak at about 2θ ≈ 15º was not reported by them.

1280 K. Srinivasan, George Thomas C Vol.10, No.14
The sintered pellets of Y358 + F are found to be very hard compared to that of un-doped
Y358 pellets. To compare the microstructure pattern of the prepared samples, SEM
micrographs were recorded on polished surfaces. The SEM micrograph of pure Y358
(sample-A) is shown in Fig. 3. A few black spots in the micrograph represent pores. Fig. 4
shows the SEM image and EDS spectra of the sample doped with fluorine Y358 + F (sample-
B). The fiber like inter growth may be attributed due to the partial melting of the sample
during the first sintering at 950°C for 20 hours. The EDS spectra clearly indicate the presence
of fluorine in the sample. Fig. 5 shows the SEM and EDS spectra of the fluorinated sample
sintered for another 24 hours (total 48 hrs) at 925°C (sample-B′). Comparing Fig. 3, 4 and 5,
it is evident that the grain size of the fluorine substituted sample has decreased. All the
samples were porous and also the fluorine addition has increased the hardness of the pellets.
Fig. 3. SEM Micrograph of sample-A
The frequency versus temperature plot of sample-A is shown in Fig. 6 and that of sample B
and B′ in Fig. 7. As seen from the Fig. 6, the T
c
onset is at 92 K for a pure sample with a
frequency change of about 2 kHz showing a strong superconducting transition. Addition of
fluorine had increased the onset T
c
to a value 98 K with a frequency change of 0.5 kHz. (Fig.
7) This lowering of the frequency change indicates that the amount of fluorine substitution (x
= 0.2) may not be the optimal value required to fluorinate the entire sample. This sample
when further heated at 925°C for a period of 24 hours, improved the T
c
onset to 110 K with a
two step transition as seen from the T
c
curve of sample-B′ in Fig. 7. This means that the
phase formation is incomplete in sample-B and on further heating, more single phase
compound is formed which is inferred from the XRD of the sample-B′. Also there is a two
step transition, one at 110 K and the other around 98 K indicating the presence of the sample-
B phase still.

Vol.10, No.14 Fluorine Doping Effect 1281
Fig. 4. SEM Micrograph and EDS spectra of sample-B
Fig. 5. SEM Micrograph and EDS spectra of sample- B′
It can be further inferred that by optimizing the amount of fluorine and the sintering duration,
Y358 phase with T
c
onset around 110 K or even more may be possible to be obtained by
fluorine doping.

1282 K. Srinivasan, George Thomas C Vol.10, No.14
8090100 110120 130140 150
58.2
58.3
58.4
58.5
58.6
58.7
58.8
58.9
59.0
Frequency (kHz)
Temperature (K)
Fig. 6. Tc curve of sample-A
8 09 01 0 01 1 01 2 01 3 01 4 01 5 0
5 8 .2
5 8 .3
5 8 .4
5 8 .5
5 8 .6
5 8 .7
5 8 .8
5 8 .9
5 9 .0
Frequency (kHz)
Tem perature (K)
Sample -B
Sample -B
I
Fig. 7. Tc curve of sample-B and B′
4. CONCLUSIONS
The pure (Y
3
Ba
5
Cu
8
O
y
) and the fluorine doped (Y
3
Ba
5
Cu
8
O
y-0.2
F
0.2
) compounds were
prepared by solid state reaction technique. Even though the crystalline structures of both the
samples were similar to that of Y123 compound, there was a remarkable increase in T
c
of the
fluorinated sample. This study confirms the formation of Y358 composition almost as a
single phase. The SEM studies carried out on the samples indicate the systematic variation in
the morphology and the microstructure in terms of pores present and the grain size. Marginal
fluorination of the Y358 sample seems to improve the T
c
onset value by about 18 K. This is

Vol.10, No.14 Fluorine Doping Effect 1283
because the fluorine addition caused optimization of oxygen excess in the sample. By
optimizing the fluorine content and the duration of the heat treatment, Y 358 phase with T
c
onset of 110 K can be obtained by fluorine doping.
ACKNOWLEDGEMENTS
The authors wish to thank S. B. Gudennavar, B. Manoj and P.R. Chethna for their assistance
and the fruitful discussions. One of the authors (KS) wish to express his sincere gratitude to
the University Grants Commission, New Delhi, India for their research support under FIP-XI
plan period.
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