Superconductivity in the tetragonal phase of La 1.9 Bi 0.1 CuO 4+ δ annealed under high oxygen pressure
ABSTRACT We have investigated the relation between the crystal structure and superconductivity in La1.9Bi0.1CuO4+d, in which the phase separation observed in La2CuO4+d is suppressed. A phase diagram in theT-d plane is given for La1.9Bi0.1CuO4+d with excess oxygen. For very smalld values, the crystal structure is orthorhombic, and an orthorhombic-tetragonal phase transition occurs markedly atd ~ 0.03 in the measured temperature range between 13 and 293 K. Superconductivity is observed in the range of 0.04dhigh-Tc superconductivity also appears in the tetragonal phase.
Journal of Superconductivity, Vol. 7, No. I, 1994
Superconductivity in the Tetragonal Phase of La .gBio. CuO4
Annealed under High Oxygen Pressure
Masatsune Kato, I Takaaki Aoki, t Takashi Noji, 1 Yasuhiro Ono, i Yoji Koike, ~
Tomoyuki Hikita, 1 and Yoshitami Saito ~
Received 31 July 1993.
We have investigated the relation between the crystal structure and superconductivity in
La~.9Bi0.1CuO4+~, in which the phase separation observed in La2CuO4+8 is suppressed. A
phase diagram in the T-6 plane is given for Lal.9Bio.~CuO4+~ with excess oxygen. For very
small 6 values, the crystal structure is orthorhombic, and an orthorhombic tetragonal phase
transition occurs markedly at 6~0.03 in the measured temperature range between 13 and
293 K. Superconductivity is observed in the range of 0.04<6<0.11, This is clear evidence
that high-Tc superconductivity also appears in the tetragonaI phase.
KEY WORDS: High-T, superconductivity; La~ 9Bio.iCuO4+e; excess oxygen; crystal structure; super-
conducting transition temperature.
It is known that the parent compound La2CuO4
exhibits superconductivity with T~ ~ 40 K by incorpo-
rating excess oxygen . However, the superconduct-
ing volume fraction is poor. Jorgensen et al. 
showed by powder neutron diffraction that the oxy-
genated La2CuO4+6 is a mixture of two distinct
orthorhombic phases in the temperature range below
about 320 K: one is an insulating stoichiometric phase
(6 ~0) and the other a superconducting oxygenated
one (~0.1). Thus, this system is not suitable for
studies on the relation between the superconductivity
and the other physical properties. Hiroi et al. 
showed by the electrical resistivity and dc magnetic
susceptibility measurements that the phase separation
observed in the oxygenated La2CuO4+s is suppressed
through the partial substitution of Bi for La.
Herein, we prepare Lal.gBi0.1CuO.~+6 with vari-
ous c~ values by controlling oxygen pressure in the
annealing process, and determine the crystal structure
1Department of Applied Physics, Faculty of Engineering, Tohoku
University, Aramaki Aoba, Aoba-ku, Sendai 980, Japan.
between 13 and 293 K and T~. A phase diagram in
the T-8 plane is presented. The relation between the
crystal structure and superconductivity is discussed.
Polycrystalline samples of Lal.9Bi0.1CuO4+ 6 were
prepared by solid-state reaction and then were
annealed under 100 1000 atm 02 at 600~ for 6-60 h
in order to incorporate excess oxygen, or in Ar flow
at 800-850~ for 48 h in order to remove excess oxy-
gen. The excess oxygen content, c~, of the non-
annealed samples were chemically determined to be
c~ = 0.020 + 0.005 by iodometric titration. Those of the
annealed samples were estimated from the weight
change during the annealing process.
3. RESULTS AND DISCUSSION
Variations of the (110)tet diffraction peak in the
notation of the tetragonal phase with temperature for
the samples with ~ = 0.020, 0.032, 0.052, and 0.100 are
shown in Fig. 1. For 3 = 0.020, the orthorhombicity
increases with decreasing temperature. For c~ > 0.032,
on the contrary, no significant broadening of the peak
0896-1107/94/0200-0037S07.00/0 (c) 1994 Plel~um Publishing, Corporation
38 Kato, Aoki, Noji, Ono, Koike, Hikita, and Saito
6 = 0.020
d *~ 293K
*" * 100K
j ~ 13K
. ii = 0.052
g "~ 293K
i i I
i i i i i I i
,. ~ = 0.032
6 = 0.100
i i I p i i i I i
( deg. )
Fig. 1. Temperature dependence of the (110)tet diffraction peak in
the notation of the tetragonal phase for LaLgBi0jCuO4+6 with 6 =
0.020, 0.032, 0.052, and 0.100. The samples with 6 >0.032 remain
in the tetragonal phase at least down to 13 K.
indicative of the orthorhombic symmetry can be
detected in the temperature range between 13 and
293 K. Therefore, the samples with 0.032_< 6 <0.100
remain in the tetragonal phase at least down to 13 K.
The crystal structure of Lal.9Bi0jCuO4+a in the T-6
plane is summarized in Fig. 2. The tetragonal phase
is stable in a wide range of 6>0.03.
I I I r I i r I i I r
Fig. 2. Phase diagram of Lal 9Bi0jCuO4+a. The Tdi value is deter-
mined by the powder X-ray diffraction measurements. The Tc value
is determined by the electrical resistivity (O) and ac susceptibility
measurements (i). The superconductivity appears in the tetra-
The 6 dependence of Tc is also shown in Fig. 2.
The values of Tc determined from p and Z are in
approximate correspondence with each other. The
superconductivity sets in around 6=0.04. With
increasing 6, Tc increases at first rapidly and then
reaches to the maximum value, ~22 K, at 6 >0.07.
The maximum Tc value, ~22 K, in Lal.9Bi0.1CuO4+~,
is smaller than that in Laz-xSrxCuO4. This may be
due to the randomness caused by the excess oxygen
and/or the substitution of Bi 3+ whose ionic size is
slightly smaller than La 3+.
We will discuss the question whether the tetra-
gonal phase exhibits bulk superconductivity or not.
In most high-Tc cuprates, superconductivity appears
in the orthorhombic phase. Thus, Takagi et al. 
reexamined the superconducting
La2 xSrxCuO4 and found that the superconductivity
is suppressed upon traversing the orthorhombic-to-
tetragonal phase boundary at x ~ 0.22, which means
that the superconductivity appears not in the tetra-
gonal phase but only in the orthorhombic one. They
concluded that the orthorhombicity should be one of
the important factors for the appearance of the high-
Tc superconductivity. On the contrary, we have found
that the bulk superconductivity appears in a wide 6
range of Lal.9Bi0.]CuO4+~ with the tetragonal struc-
ture, as obviously shown in Fig. 2. Moreover, Yam-
ada and Ido  showed from the thermal expansion
and ac magnetic susceptibility measurements that the
bulk superconductivity is observed not only in the
orthorhombic phase but also in the pressure-stabilized
tetragonal one of La2-xSrxCuO4 with x<0.2, sup-
porting our results. Therefore, it is concluded that
orthorhombicity is not necessary for high-Tc super-
conductivity with respect to the overall symmetry
determined by X-ray diffraction, although local
orthorhombicity distortion in the overall tetragonal
symmetry may be effective for the occurrence of the
high- Tc superconductivity.
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