Stability of YBa2Cu4O8 under high pressure

PHYSICA ELSEVIER

PhysicaC 225 (1994) 181-184

Stability of YBa2Cu408under high pressure A. Ono National lnstitute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan

Received 31 January 1994;revised manuscript received 14 March 1994

Abstract The stability of the superconducting 124 phase of YBa2CuAOshas been studied under high pressure in the temperature range 790-920 °C. It is found that the stability of the 124 phase depends on pressure as well as on oxygenpressure. Above 1.5 GPa, the assemblage of YBa2CuaO7_a plus CuO is stable, while YBa2Cu408 is unstable under relatively low oxygen pressure. On the contrary, the 124 phase is stable at 2.2 GPa under oxygenpressure close to 2.2 GPa.

I. Introduction Three superconducting phases, YBa2Cu307, YBa2Cu408 and Y2Ba4Cu7Ols, are known in the BaY - C u - O system. These compounds are usually abbreviated as 123, 124 and 247 on the basis of the numbers of cations. The well-known 123 superconductor is the first superconductor exhibiting superconductivity above 77 K [ 1,2 ]. The pure 124 phase is a 80 K superconductor prepared under high oxygen pressure [ 3 ]. The structure of the 247 phase with Tc~20-93 K is considered to be an intergrowth of 123 and 124 phases [4,5 ] which contains both single and double CuO chains in the unit cell. Good-quality 124 and 247 crystals can be prepared under high oxygen pressure. In this context, it is interesting to study a phase diagram in the B a - Y - C u O system under high pressure. Kaldis et al. have proposed a phase diagram below 0.1 GPa [6 ]. According to them, the 124 phase is stable only under high oxygen pressures above 900 oC, while the assemblage of 123 and CuO is stable at low oxygen pressure. The 124 phase appears to be a high-pressure phase in comparison with the assemblage of 123 plus CuO. On the other hand, the molar volume of 124 is larger than

that of the 123 phase plus CuO [ 7 ]. Consequently, the assemblage of 123 and CuO appears to be stable at high pressure. The apparent inconsistency may be resolved by the study of the phase stability above ~ 1 GPa which has not been studied yet. In this paper, we report the crystallization conditions of the 124 phase under high pressure ( ~ 1-2 GPa).

2. Experimental Starting materials with nominal compositions of YBaECusO9, YBa2Cu4Os and (Cao.l Gdo.9) BaECusO9 were used for high-pressure experiments. Stoichiometric mixtures of Y203, BaCO3, CuO, CaCO3 and Gd203 were heated at 920 ° C in air, and then fired at 980°C in oxygen. The calcined materials were pressed into pellets and fired at 980°C for 12 h in oxygen with intermediate regrinds. Subsequently, samples were furnace-cooled to ~ 300°C under oxygen gas. After pulverization of the sintered samples, the samples were used for high-pressure syntheses. High-pressure syntheses were carried out using a piston-cylinder apparatus, as in our previous study [8 ]. An assembly of the pressure medium is sche-

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182

A. Ono / Physica (" 225 (1994) 181-184

i.e. and

thermocouple

sample lass ( rite

Fig. 1. Schematic cross section of the piston-cylinder assembly. Diameter of piston is 12.70 m m

matically shown in Fig. 1. A sample of ~0.2 g was packed into a 13 mm long, 3.8 mm diameter gold capsule with or without KCIO3 and then the gold capsule was sealed. The sealed starting sample was heated at 790-950°C for 5 h, and subsequently annealed under high pressure at 550-500°C for ~ 4 min before quenching to room temperature. Weight changes of gold capsules containing the samples were measured before and after high-pressure experiments in order to confirm the absence of leakage. The experimental pressure is a nominal value uncorrected for the effect of frictions between the cylinder and the pressure medium. The experimental temperature was monitored with an alumel-chromel thermocouple. Crystalline phases were identified by powder X-ray diffraction and DC magnetic susceptibility measurements. X-ray diffraction patterns in the range 20 = 271 ° were registered at room temperature on a Philips diffractometer with Cu Kct radiation. Lattice parameters were calculated by a least-squares refinement. DC susceptibility Z ( = M / H ) was measured on cooling for sintered samples in an applied field of 20 Oe using a Quantum Design SQUID susceptometer.

Y B a 2 C u 4 O s = Y B a 2 C u 3 O-, , ~ + C u O + ½(~02 ( l ) Y B a 2 C u 4 0 8 + ½(~O2 = Y B a 2 C u 3 0 7 t,~+ C u ( )

((fi> 0) (II). Taking into consideration the fact that the oxygen number of the 123 compound in the starting material is slightly below 7.0, experiments without KCIO3 are useful for the determination of a P - I boundary of reaction 1, if the attainment of equilibrium is assumed. However. reaction II takes place under very high oxygen pressure. Since large amounts of KCIO3 were sealed in gold capsules in several experiments, the oxygen pressure is nearly equal to the total pressure for these cases. X-ray diffraction patterns for representative sampies prepared at high pressure without KC10, arc shown in Fig. 2. The nominal composition of the starting material is 125 for A and B, while it is 124 for C and D. The X-ray diffraction patterns B and C

A 123+124+Cu()!

B (123+CUO) o

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oo

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The stability of YBa2Cu408 depends on oxygen pressure since loss or uptake of oxygen takes place in the reactions determining the stability of YBa2Cu4Os,

o

i~

C (123+CUO)

•

3. Results and discussion

+

(Cu

K+r)

Fig. 2. X-ray diffraction patterns tbr high-pressure samples. (A) 0.8 GPa, 860°C, (B) 1.6 GPa, 850°C, (C) 2.0 GPa, 900°C, (D) 1.4 GPa, 890°C. ( • ) 124 phase, ( O ) 123 phase, ( × ) CuO.

A. Ono / Physica C 225 (1994) 181-184

are similar to those for the starting materials, although sample B was prepared at 850°C, 1.6 GPa and sample C at 900°C, 2 GPa. In fact, the lattice parameters of the 123 phase for sample C are similar to those for the starting sample (Table 1 ). This fact suggests that the oxygen content of the 123 phase is close to 6.9. Samples A and D prepared at pressures lower than 1.5 GPa show several reflections being attributable to the 124 phase. Fig. 3 shows the crystallization conditions of the 123, 124 and 247 superconducting phases. Cross symbols are P-T conditions for high-pressure samples prepared under the presence of a significant amount of KCIO3. Other symbols are P-Tconditions for samples obtained without addition of KC103. The crystallization of the superconducting phases must be described in a T-P-P(02) diagram. The oxygen pressure, however, is unknown for our experiments. Fig. 3 is a diagram projected from the supposed P(02)-axis. Judging from X-ray diffraction patterns and magnetization data, the compound YBa2Cu4Os did not crystallize above ~ 1.5 GPa in the temperature range Table 1 Lattice parameters of orthorhombic 123 phases. (1) Starting sample, (2) preparation without KC103, (3) preparation under oxygenpressure of ~ 2.2 GPa

a (nm) b (nm) c (nm)

3.0 12_

~

1

2

3

0.3822 0.3888 1.1678

0.3827 0.3894 1.1674

0.3823 0.3884 1.1667

I

'

'

'

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.

.

.

800-900°C when KC103 was not added to the starting samples (solid circles in Fig. 3). However, YBa2Cu408 crystallized below ~ 1.5 GPa. Coexistence of YBa2Cu4Os, YBa2Cu3Oz and CuO occurred below 900°C (open squares). On the contrary, Y2Ba4Cu70]5, YBa2Cu408 and CuO crystallized at 910°C under 1.2 GPa (open circles). A high-pressure experiment was performed using high-pressure samples containing 124 and 247 phases as starting materials. The new starting materials which do not contain KC103 gave an X-ray diffraction pattern similar to that shown in Fig. 2(A). The experiment was performed at 870 ° C + 10 ° C under 2.2 GPa. The X-ray diffraction pattern for the yield is similar to that shown in Fig. 2(B). X-ray reflections of the 124 phase were not seen in the diffraction pattern. Clearly, the 124 phase is unstable at 2.2 GPa under the oxygen pressure compatible with Ba/YCu3Oz (z~6.9). When a considerable amount of KC103 was added to the starting sample, 124 and 123 phases were detected in all high-pressure samples prepared at 1.22.2 GPa. The 124 phase crystallized under high oxygen pressure. The amount of the 123 phase, however, increases with increasing pressure. This fact suggests that the 124 phase becomes unstable above a certain pressure slightly higher than 2.2 GPa under high oxygen pressure. Fig. 4 shows magnetization curves for high-pressure samples prepared without using KCIO3. Three types of magnetization curves are observed, depending on assemblages of the superconducting phases, i.e.

I

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2.0 • 123

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,

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800

,

,

i

i

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850

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900

Temperature (°C) Fig. 3. Phase relations among the superconducting 123, 124 and 247 phases. ( O ) 123+CUO, (rq) 124+123+CUO, ( © ) 124+247+CUO, ( + ) 123+ 124+CUO (addition of KClO3).

.= F.C. 2c Oe

124+247

20

. . . .

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i i

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i J i

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i ,

i i * J t

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80

Temperature (K) Fig. 4. Magnetization curves for high-pressure samples prepared without KCIO3.Sampleswithout 124 phase wereprepared at 795, 840 and 900°C (Fig. 3). The value of Tcincreases with increasing synthesistemperature.

184

A. O n o / P h y s i c a C 225 (1994) 1 8 1 - 1 8 4

0.0

~

, t , , , ,

I ....

.

.

i ....

[ ....

.

J ....

.

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r '

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~ ~ I

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~ 2.22 GPa

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80

Temperature (K) Fig. 5. Magnetization curves for high-pressure samples prepared by the addition of KC103. Synthesis temperature is (840-850 _+7 ) oC. Pressure is given in the figure.

123, 123 plus 124 and 124 plus 247. The superconducting transition temperature (Tc) is 75-82 K for samples containing the 123 phase and CuO. Diamagnetic signals at 75-82 K are weak for these samples. On the contrary, samples containing the 124 phase show sharp diamagnetic signals at 75-82 K. Particularly, diamagnetic signals are strong for samples prepared above ~ 900 ° C. For the sample containing both 124 and 247 phases, a sharp increase of the diamagnetic signal occurs at 82 and ~ 47 K. This fact suggest that the Tc of the 124 phase is 82 K and that of the 247 phase is ~ 47 K for this sample. The transition temperatures of the 123 phase are 75-82 K for samples prepared above ~ 1.5 GPa, which are considerably lower than 91 K. This does not imply that the oxygen contents of the 123 phase are significantly lower than 6.9 _+0.1 for these highpressure samples. Actually, the lattice parameters of the 123 phase are similar for starling and high-pressure samples (Table I ). The depression of Tc is considered to be due to the disordering of oxygen atoms in CuO planes which occurred through the crystallization under high temperature. Fig. 5 shows magnetization curves for high-pressure samples prepared by adding KC103 to the starting materials. The diamagnetic signal appears below 91 K for all samples studied. Moreover, the c-lat-

tice parameter of the 123 phase in a highly oxidized sample is shorter than that in the starting sample (Table 1). Taking into consideration the previous high-pressure study on the 123 phase [9], Tc and lattice parameters suggest that the oxygen numbers of our highly oxidized 123 phases are slightly above 7.0 Another characteristic feature of Fig. 5 is the presence of an inflection at ~ 80 K for two magnetization curves. This corresponds to the Tc of the 124 phase which can be identified by X-ray diffraction. The absence of an inflection for one magnetization curve is due to a small amount of the 124 phase in the sample. Finally, the possible existence of the 125 phase. YBa2CusO9 [10] or (Ca0.1Gdo.9)Ba2CusO9 was studied under high pressure. The compound YBa2Cu509 was not detected in a sample prepared at 860°C under 0.8 GPa using a starting sample with nominal composition of YBa2CusO9 (Fig. 2 (A)). For (Ca, Gd)Ba2Cu5Og, two samples with different K C 1 0 3 content were synthesized at 850°C under 1.2 GPa. High-pressure samples obtained consist of (Ca,Gd)Ba2Cu3Oz, CuO and minor impurity phases. The 124 and 125 compounds did not crystallize under high pressure.

References [ I ] C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang and Y.Q. Wang, Phys. Rev. Left. 58 ( 1987 ) 405. [2] R.J. Cava, B. Batlogg, K.M. Rabe, E.A. Rietman, P.K. Gallagher and L.W. Rupp Jr., Physica C 156 ( 1988 ) 523. [ 3 ] J. Karpinsky, E. Kaldis, E. Jitek, S. Rusiecki and B. Bucher. Nature 336 (1988) 660. [4] P. Bordet, C. Chaillout, J. Chenavas, J.L. Hodeau, M Marezio, J. Karpinsky and E. Kaldis, Nature 334 t 1988J 596. [ 5 ] H. Schwer, E. Kaldis, J. Karpinsky and C. Rossel, Physica C (1993) 165. [6] E. Kaldis, J. Karpinsky, S. Rusiecki, B. Bucher, K. Conder and E. Jilek, Physica C 185-189 ( 1991 ) 190. [7] B. Okai, Jpn. J. Appt. Phys. 30 (1990) L2180. [ 8 ] A. Ono, Jpn. J. Appl. Phys. 32 ( 1993 ) L 1599. [9] B. Okai and M. Ohta, Jpn. J. Appl. Phys. 30 ( 1991 ) L1378. [ 10] R. Ramesh, S. Jin and P. Marsh, Nature 346 (1990) 420.