Structural investigation of superconductors with high transition temperature

Physica 148B (1987) 432-435 North-Holland, Amsterdam

STRUCTURAL INVESTIGATION OF SUPERCONDUCTORS WITH HIGH TRANSITION TEMPERATURE M. MATSUI, K. OHMORI, T. SHIMIZU and M. DOYAMA Department of Iron and Steel Engineering, Nagoya University, Nagoya 464, Japan Received 26 August 1987

Powder X-ray diffraction (14-1223K) and electrical resistivity measurements have been made for YBa2Cu3Oy superconductors. At low temperatures no distinct structural transformation is observed. At high temperatures the transformation of the orthorhombic phase to the tetragonal phase is started at 850 K and completed at 970 K. Then, the relation between the superconductive critical temperature and the ratios b/a and c/a of the orthorhombic phase is obtained for samples prepared by cooling down from 773, 973 and 1173 K with varying cooling rates. T °n and T °" have maximum values of 94 and 93 K, respectively, at b/a = 0.9805 and c/a = 3.0007. Both of these T~'s decrease as b/a approaches 1.0 and as c/a increases above 3.0007.

1. Introduction

Since the discovery of high-T c superconductivity of L a - B a - C u - O by Bednorz and Miiller [1], intense study has been made about the oxide superconductors. Wu et al. [2] have reported on the oxide Y - B a - C u - O with a critical temperature of about 93 K, and a number of studies on the crystal structure of YBa2Cu3Oy have been conducted using X-ray and neutron diffraction methods [3-7]. A significant discrepancy among proposed structures is on the occupancy of oxygen atoms sited at O(1) and O(2), adopting nomenclature of ref. [4] (which has been followed throughout this paper), which results in the essential difference of the space group. Neutron diffraction experiments for the orthorhombic phase, however, almost coincide with each other [3, 6, 7], which results from high resolution for oxygen atom sites. Izumi et al. [3] have refined two types of structure, the orthorhombic form (Pmmm) and the tetragonal form (P4/mmm), by neutron diffraction. The distinct difference between the two phases is the occupancy of oxygen at O(1) and 0(2) sites, which are 0.69 and 0.06 for the orthorhombic form and 0.16 and 0.16 for the tetragonal form. The oxygen deficiency is related 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation

to the length of the crystallographic axis. On the other hand, the temperature dependence of unit cell parameters a, b and c of the orthorhombic phase have revealed a structural transformation of orthorhombic to tetragonal symmetry at high temperatures [8-10]. The oxygen content y in the formula of YBa2Cu3Oy decreases with increasing temperature and transforms to tetragonal at about y = 6.0. The single tetragonal phase obtained by rapid quench into liquid nitrogen from 1173 K has been non-superconducting [8]. These previous results suggest that the lattice parameters and critical temperatures, T~'° and Tc°ff, depend on the oxygen deficiency. In this paper, we describe the relation between lattice parameters and Tc'S by means of the measurements of X-ray diffraction and resistivity for YBa2Cu3Oy oxides.

2. Experimental procedure

First, a sample Y B a 2 C u 3 O y w a s prepared by BaCO 3 and CuO. The wellmixed powder was fired for 15 h at 1173 K in air and cooled down to room temperature (R.T.) in the furnace. The fired sample was pressed into a pellet under 550 kg/cm 2 pressure. Then the pel-

firing 4N-Y203,

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M. Matsui et al. / Structural research o f high-T c superconductors

let was sintered for 15h at 1173K and also cooled down in the furnace (sample S). Electrical resistivity was measured for the pellet sample and X-ray diffraction was made for reground powder sample. Second, in order to clarify the relation between lattice parameters and Tc's nine samples were prepared for various cooling rates using the sample S. The starting temperatures of cooling were 773 (A), 973 (B) and 1173 K (C) and the cooling methods were (1) cooling in the furnace, (2) cooling in air and (3) quenching in liquid nitrogen. In this paper, the samples are named by the combination of A, B, C and 1, 2, 3. The X-ray diffraction experiments were made at low temperatures (14-300 K) utilizing an Oxford CF100 cryostat and at high temperatures (295-1223K) using a Rigaku CN2311R1 furnace. The diffraction angle was calibrated by a 4N-Si powder sample. The lattice parameters were determined by the least-mean-square method for 32 diffraction peaks. Electrical resistivity was measured by the conventional four-probe method. Susceptibility was also measured by VSM.

3. High- and low-temperature X-ray diffraction The phase of the sample S at room temperature was single orthorhombic and no extra phase was observed. The lattice parameters of the sample S were a = 3 . 8 8 9 A , b = 3 . 8 2 5 A and c = 11.672 A. The critical temperatures were T °n= 93.3 K and T °" -- 90.0 K. The volume fraction of diamagnetism was estimated to be 0.311 at 77.3 K by a susceptibility of 55 Oe. The temperature dependence of lattice parameters measured in air is shown in fig. 1 together with the unit cell volume V. At low temperatures below R.T., no distinct structural transformation was observed down to 14 K. At high temperatures, the orthorhombic phase transformed to the tetragonal phase. The transformation is started at 850 K and completed at 970 K. The transition temperatures are consistent with the experiments reported by previous authors [8-10]. The expansion of the unit cell

. . . . .

~8ot F ~76

x;1

f/ /

.-h2.o

~7.9

V~/"

172

71.6

f

0

i

t

400

i

|

800 T (K)

i

1200

Fig. 1. Temperature dependence of lattice parameters and unit cell volume of YBa2Cu3Oy.

volume by the transformation is estimated to be 1.1%.

4. Relation between lattice parameters and critical temperatures In fig. 1, the lattice parameters are classified into three temperature regions, single orthorhombic region (14-850K), transient region (850-970 K) and single tetragonal region (9701223 K). To obtain the different lattice parameters at room temperature, we prepared nine samples cooled from the three regions in the three cooling conditions as mentioned above. In fig. 2 the temperature dependences of resistivity of several samples are shown. Superconductivity above 4.2 K was obtained for samples A1, A3, B1, B2, C1 and C2. Since sample A2 showed diamagnetism of 19% for full Meissner effect at 77.3 K, it seems that superconducting grains also existed in A2. It should be noted that a slow cooling rate made the samples superconducting, but no clear dependence on the starting temperature of the cooling was obtained. On the other hand, the phase of these samples was perfect orthorhombic except for C3, the structure of which was perfect tetragonal. The

M. Matsui et al. / Structural research o f high-T~ superconductors

434

xlO 3

To(K)

16

xlo ~ u

6 u i

I

8--=

B-1

4

Q.

:::::L 4

o_ 2 0 0

4O

80

i 120

J

%c_~f

o98

~982

c/a

T (K)

Fig. 2. Temperature dependence of electrical resistivity of samples cooled down from 773,973 and 1173 K with varying cooling rates (see text). only sample with an o r t h o r h o m b i c phase and non-superconductivity was B3, but the ratio of lattice p a r a m e t e r s b/a was closed to 1.0 and c/a was large. The structural analysis suggests that the ratios of lattice p a r a m e t e r s b/a and c/a are related to the superconductivity of the present oxide. That is, the larger the ratios of lattice p a r a m e t e r s , b/a and c/a, are, the lower the on off critical t e m p e r a t u r e s T c and T c . The width of the transition AT( = T~°n - T off c ) b e c a m e wider for larger b/a and c/a values. Accordingly we obtained a three-dimensional plot for b/a, c/a and T c for the data of samples A1, A3, B1, B2 and C1 which showed rather large Meissner effect at 4.2 K. An approximation using the parabolic equation was made for the plot. According to the simulation of the equation, the rough relation between b/a, c/a and Tc is drawn for T °" and T °ff together with the experimental data in fig. 3. M a x i m u m T °n and T °ff were estimated to be about 94 and 93 K, respectively for b/a = 0.9805 and c/a = 3.0007. Despite of the low accuracy of the simulation, the tendency of dependence of T c on b/a and c/a is well described in fig. 3. T h a t is, the Tc's were lowered with b/a approaching 1.0 and with increasing c/a. Muromachi et al. [8] reported the t e m p e r a t u r e dependence of the oxygen content y in the formula of YBazCu3Oy. The content y is about 6.5 for the o r t h o r h o m b i c phase at R.T.

Fig. 3. Relation between Tc, b/a and c/a. Solid lines were estimated by the parabolic equation. (×) and (O) are the experimental data of T °" and T °", respectively. and the phase transforms to tetragonal at about y = 6.0. This implies that the oxygen site O(1) along the a-axis becomes vacant in the tegragonal phase, while 0 ( 2 ) is already vacant. Accordingly, the increase of b/a results in the increase of oxygen deficiency at O(1) site. On the other hand, the increase of c/a results from the increase of c or the decrease of a. However, the p a r a m e t e r c of rapidly quenched samples was larger than that of slowly cooled samples. The larger the c is, the lower the T c. The increase of c is associated with that oxygen atoms occupy the vacant site in the z = ½ plane for higher cooling rate samples. F u r t h e r m o r e , the abrupt increase of p a r a m e t e r c above 850 K in fig. 1 also suggests that oxygen atoms occupy the vacant sites in the z = ½ plane at high temperatures above 850 K. In conclusion, the present result suggests that Tc's are lowered due to two reasons: (1) the increase of oxygen deficiency along the a-axis in the z = 0 plane, while the oxygen atoms are already deficient along the b-axis, which means approaching to tetragonal symmetry (b/a = 1.0) and (2) the increase of possibility of occupation of oxygen atoms in the z = ½ plane by the rapid quenching from high temperatures, which corresponds to an increase of the p a r a m e t e r c and the increase of c/a above 3.0007.

M. Matsui et al. / Structural research o f high-T c superconductors

References [1] J.G. Bednorz and K.A. M/iller, Z. Phys. B 64 (1986) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 911. [3] T. Siegrist, S. Sunshine, D.W. Murphy, R.J. Cava and S.N. Zahurak, Phys. Rev. B 35 (1987) 7137. [41 F. Izumi, H. Asano, T. Ishigaki, E.T. Muromachi, Y. Uchida, N. Watanabe and T. Nishikawa, Jpn. J. Appl. Phys. 26 (1987) L649. [5] R.M. Hazen, L.W. Finger, R.J. Angel, C.T. Prewitt,

[6] [7] [8] [9]

[10]

435

N.L. Ross, H.K. Mao, C.G. Hadidiacos, P.H. Hor, R.L. Meng and C.W. Chu, Phys. Rev. B 35 (1987) 7238. J.E. Greedan, A.H. O'Reilly and C.V. Stager, Phys. Rev. B 35 (1987) 8770. F. Beech, S. Miraglia, A. Santoro and R.S. Roth, Phys. Rev. B 35 (1987) 8778. E.T. Muromachi, Y. Uchida, K. Yukino, T. Tanaka and K. Kato, Jpn. J. Appl. Phys. 26 (1987) L665. K. Nakamura, T. Hatano, A. Matsushita, T. Oguchi, T. Matsumoto and K. Ogawa, Jpn. J. Appl. Phys. 26 (1987) L791. K. Yukino, T. Sato, S. Ooba, M. Ohta, F.E Okamura and A. Ono, Jpn. J. Appl. Phys. 26 (1987) L869.