The crystal structure and property of ternary compounds and phase relations in the system Yb2O3BaOCuO at 950°C

Solid State Communications, Vol. 74, No. 6, pp. 509-516, 1990. Printed in Great Britain.

0038-1098/90 $3.00 + .00 Pergamon Press plc

T H E CRYSTAL S T R U C T U R E AND P R O P E R T Y OF T E R N A R Y C O M P O U N D S AND PHASE R E L A T I O N S IN T H E SYSTEM Yb203-BaO-CuO AT 950°C Jingkui Liang, Xiaolong Chen, Shanlu Wu, Jin Zhao, Yuling Zhang and Sishen Xie Institute of Physics, Academia Sinica, Beijing 100080, P.R. China

(Received 19 December 1989 by W.Y. Kuan) The subsolidus phase relations of the Yb203-BaO-CuO ternary system sintered in the air at 950°C has been investigated by X-ray powder diffraction and thermal analysis. In this system there exist four ternary compounds: Yb2BaCuOs, YbBa2Cu307, (Yb, Ba, Cu)YbBa4Cu20867 and (Yb, Ba)Ba2CuO4.2s and 12 ternary compatabilities. The compounds YbzBaCuOs, an insulator with space group Pbnm, is isostructural with YzBaCuOs. Its lattice parameters are a = 7.059, b = 12.097, c = 5.617 A. YbBa2Cu307 possesses a critical transition temperature 92 K and a space group of Pmmm, isomorphous with the structure of YBa2Cu307. The other two compounds (Yb, Ba, Cu)YbBa4Cu2Os67 and (Yb, Ba)Ba2CuO425 are both semiconductors. The former has a space grouop of P4/mmm with lattice parameters as a = 5.793, c = 8.002 A. Its positions of the ions in a unit cell are as follows: l(Yb, Ba, Cu) randomly occupies l(c) equivalent point position, 1 Yb cation occupies I(a), 4Ba cations occupy 4(i) with z = 0.255, 2Cu cations occupy l(b) and l(d) respectively, 8.67 Oxygen anions occupy 2(f) with occupancy 1/3, 2(g) with z = 0.24, 2(h) with z = 0.26 and 4(k) with x -- 0.25 respectively. The compound (Yb, Ba)Ba2CuO42s occurs a deoxygen phase transition at 812°C with a change in lattice parameters, with a space group of P4/mmm, The lattice parameters of high temperature phase are a = 4.054/~, e = 2a, while the lower has a =4.106, c = 8.033 A,. The positions of their ionic sites are the same as the following: l(Yb, Ba) randomly occupies l(a) equivalent point position, 2 Ba cations occupy 2(h) with z = 0.255, lCu cation occupies l(b) equivalent point position and 4.25 Oxygens occupy 1(c) equivalent point position (occupancy 1/4), 2(1) and 2(g) with z = 0.255, respectively.

But others [4, 14] regarded the crystal structure of superconducting phase in Y b - B a - C u - O system as SINCE the high T~ superconductors m the ternary isomorphic to that of YBazCu307. Recently Kogure systems R203-BaO-CuO (R = rare-earth elements) et al. [15, 17] discovered a series of new structures were discovered [1-4], many authors have been paying of superconducting phase with a general formula much attention to their phase diagrams in searching Yb,,Ba2,,Cu3,,+lO65,,+l (n = 1, 2, 3 . . . . . ) in Yb203for the optimum synthetical conditions of high T~ BaO-CuO system by oxidation of Y b - B a - C u - A g materials. The phase relations in ternary systems metallic precursor. These new structures were generated Yb203-BaO-CuO [5-7], La20~-BaO-CuO [8, 9], by the replacement of the copper oxide double layers Nd203-BaO-CuO [10], Gd203-BaO CuO [11] and in each unit cells of YbBa2Cu307 structure. H%O3-BaO-CuO [12] have been studied in certain In this paper we investigated the structure and degree, but too little work has been done on the phase properties of ternary compounds and phase relations diagrams of systems with heavy rare earth oxide. in the ternary system Yb~O~-BaO-CuO sintered in the The superconductor with T~ about 90K in air at 950 ° C. Y b - B a - C u - O system was observed by some authors [4, 13, 14], for instance Kitazawa et al. [13] considered 2. E X P E R I M E N T S that the superconducting phase is attributed to (Yb~ ,Ba,)3Cu207 ,~, which has a similar structure to 2. I. Preparation of samples Yb203 (99%), BaCO3 or BaO2(A.R) and CuO that of Sr~Ti207 with a large oxygen deficiency. 1. I N T R O D U C T I O N

509

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THE STRUCTURE OF TERNARY COMPOUNDS

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YbO~5

(A.R) were used as starting materials. Y b - B a - C u oxide samples were prepared by solid state reaction. The raw powders with proper compositions were thoroughly mixed, ground and pressed into pellets, which were sintered at about 950°C in the air for about 24 h, and then slowly cooled in the furnace to room temperature or quenched in the air. However, we found that the samples with rich barium oxide were unstable and easily deliquesced in the air.

j°

.@~'/

Trr

!

2.2. X-ray powder diffraction analysis The phase identification was made with a Guinierde Wolffmonochromatic focusing camera using CuK~ or CoK~ radiation. For the measurement of lattice parameters of the compounds pure Si was added to the specimens as an internal standard. 2.3. Differential thermal analysis measurements The DTA measurements were made by a homemade CR-G type DTA apparatus. Because a Pt crucible reacts with samples at high temperature, A]203 crucibles were used as vessels and ~-AI203 powder was used as a reference. Pt-PtRh (10%) thermocouples were employed to measure and control the temperature and indicate the differential temperature. The heating rate usually was 10°Cmin J. The temperature of extrapolated onset or peak value in the DTA curve was adopted as the phase transition point. Because the molten samples strongly react with the A1203 crucible, the cooling measurement was not carried out after melting. 2.4. Electrical measurements Measurements of resistance with the temperature were carried out with the standard four-probe method. The a.c. magnetic susceptibility was measured by mutual inductance. 3. RESULTS

3.1. Subsolidus phase relations According to the results of X-ray diffraction analysis and superconducting measurements of 45 samples with different compositions, the subsolidus phase relations of Yb203-BaO-CuO system, sintered at 950 ° C in the air, are shown in Fig. 1. Because in the BaO-rich region of this system the samples easily deliquesce and absorb carbon dioxide from the air, so the results are still uncertain, we think that further study of phase relation is needed, really.

3. I. 1. BaO-CuO system. When the samples were sintered in the air at 950 ° C, and then cooled down to room temperature in the furnace, only one binary compound BaCuO2 exists in BaO-CuO system.

,L

\ 152~ BoO

0 BQCu02

CuO

Fig. 1. Solidus phase relations in Yb203-BaO-CuO system sintered in the air at 950 ° C, ® single phase, • binary phases, o trinary phases.

BaCuO2 possess cubic cell with space group •432 and lattice constant a = 18.285A. The preliminary experimental results of the melting point of BaCuO2 reported by [7, 8] were different, we obtained that BaCuO2 congruently melted at about 1000 ° C. Under the above mentioned synthetic conditions we have not observed formation of other binary compounds, such as BaCu202 [7, 18], Ba3Cu50 ~ [7] and Ba2CuO 3 [7, 19], which were obtained with specific synthetic conditions. 3.1.2. Yb203-CuO system. The only one compound Yb2Cu205 was found in Yb203-CuO system. It crystallizes in an orthorhombic unit cell with space group Pna2, and is isomorphous to Y2Cu2Os structure. The lattice parameters are: a = 10.724, b = 3.433 and c = 12.349A. It is worth noting that the compound Yb2Cu205 formed in Yb203-CuO binary system is different from the binary compound in the respective light or middle rare-earth oxide R203-CuO systems, such as La203-CuO and Gd203-CuO systems, in which compounds R2CuO 4 and KzNiF 4 structure type were formed, and similar to that of Y203-CuO and other heavy rare-earth oxide-CuO systems, such as H%O3-CuO system. 3.1.3. Yb203-BaO system. A preliminary phase equilibrium diagram of the.Yb203-BaO system had been studied by Lopato et al. [20]. In this system only one compound Ba3Yb409 was formed, and it congruently melts at 2200°C. Yb3Ba409 crystallizes in a hexagonal system. The pseudo-binary system Ba3Yb40,~-BaO and Ba3Yb409-YbzO) are eutectic

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with the different eutectic temperature 1750°C and 2110 ° C, and their eutectic points are 79 mol % BaO and 48 mol % BaO, respectively. In addition, we did not synthesize the other compound BaYb204 in this system, reported by [21]. Our investigation verified the existence of the compound Ba3Yb4Og. The X-ray powder diffraction lines of Ba3Yb409 were indexed with hexagonal lattice parameters are: a = 6.038, c = 24.86,&,. 3.1.4. Yb203-BaO-CuO system. From Fig.1 we can see that there are four ternary compounds Yb2BaCuOs, YbBa2Cu~O7, (Yb, Ba, Cu) YbBa4Cu20, 67 and YbBasCu2085 occur in Yb203-BaO-CuO system. The subsolidus phase relations in Yb203-BaO-CuO system can be divided into twelve ternary compatibilities: (I) Yb203 + Yb2BaCuOs (211) + Yb,Cu2Os, (II) Yb2Cu205 --F (211) + CuO, (III)(211) + YbBa2 Cu307(123) + CuO, (IV) (123) + BaCuO, + CuO, (V) (211) + (123) + BaCuO2, (VI) (211) + BaCuO2 + (Yb, Ba, Cu)YbBa4Cu20~.67 (1142), (VII) (211) + Yb203 + Ba3Yb4Og, (VIII) (211) + Ba3Yb409 + (I142), (IX) YbBasCu208 5(152) + (1142) + Ba3Yb4Og, (X)BaCuO 2 + (152) + (1142), (XI) (I52) + Ba3Yb409 + BaO, (XII) (152) + BaCuO2 + BaO. Two phase region is a line between respective phases.

3.2. The crystal structure and properties of ternary compounds' 3.2.1. Yb2BaCuO~. The orthorhombic compound Yb2BaCuOs, an insulator has lattice parameters a = 7.059, b = 12.097, c = 5.167/k. The result indicates that its space groups is Pbnm and quite coincide with [22]. 3.2.2. YbBa2Cu307. The compound YbBa,Cu3Ov is a superconductor. Its resistance and a.c. susceptibility with temperature are shown in Fig. 2, with zero

511

J(

@ g

t <]

I

700

t

900

q

I000

A__ I

1200

T (°C)

Fig. 3. DTA curve of YDBa2Cu307. resistance superconducting transition temperature To(0) 92 K. It belongs to orthorhombic cell with lattice constants a = 3.871, b = 3.807, c = 11.658A. Its space group is Pmmm, isomorphous with YBa2Cu307. In our experimental conditions, Yb,, Ba2nCu3,,+iO65,,+1 type compounds failed to be synthesized. The DTA measurement shown in Fig. 3 indicates that the compound YbBa2Cu307 starts to melt at 1190°C and decomposes from Cu 2+ to Cu + at 1000°C. The small endo-thermal effect at 912°C maybe belongs to eutectic reaction, because minor second phase exists in the sample with a nominal composition of YbBa:Cu307. In the YbzO3-BaO-CuO system YbBa2Cu307 is only one superconducting phase under our synthetic conditions employed. The multiphase samples in the regions involving YbBaECU307 are superconducting, such as in the regions (III, IV, V). The deviation of nominal composition from YbBa2Cu.~O7 is larger and the T~.(0) is lower. The superconducting transition temperature is related to sintering and heat treatment conditions, sometimes the non-equilibrium samples in the region (ll) may be superconductive. 3.2.3. (Yb, Ba, Cu)YbBa4Cu20~67 and YbBasCu2Os s

3.2.3.1. Structure of (Yb, Ba, Cu)YbBa4Cu20~67. ~5

g

2

~3

0

I 250

L 200

I 150

I 92K I00

I 50

T(K)

Fig. 2. Resistivity (a) and a.c. magnetic susceptibility (b) of the compound YbBa2Cu307 versus temperature.

The diffraction data used for the structure analysis of (Yb, Ba, Cu)YbBa4Cu20~67 were obtained from the samples with tile composition Yb3BagCusO~.5 which was nearly single phase and only contained a minor second phase " 2 1 1 " . The diffraction lines were indexed using Werner's T R E O R program [23], and listed in Table 1. There is no systematic extinction of diffraction lines. The unit cell is a primitive tetragonal system with lattice constants: a = 5.793, c = 8.002/~. The space group is P4/mmm. The calculated planar distances dc,~c coincide with the observed ones quite well.

512

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Table 1. Calculated and observed intensities I and planar distanees d of (Y b, Ba, Cu)YbBa4Cu2Os67. (P4/mmm, B = 2.0, a = 5.793 X, e = 8.002A)

hk l

4~,,< (A)

d,,b~ (A)

G,~

1,,b,

001 100 101 110 002 lll 102 200 112 201 003 211 202 113 22O 0O4 221 203 311 312 204 223 313 115 4OO 224 401 2O5 331 420 332

8.002 5.793 4.692 4.096 4.00 I 3.646 3.292 2.897

8.005 5.832 4.685 4.104 3.989 3.639 3.285 2.905 2.859 2.720 2.664 2.473 2.346 2.241 2.048 2.000

15 8 13 2 6 66 4 643 1000 60 6 5 33 7 289 133

w m f f f f w-m vf s vs w f vf w m vf m-s m

1.961 1.787 1.664 1.644

34 19 417 231 7 5 II 101 194 2 3 5 143 127

2.862 2.724 2,667 2.465 2.346

2.235 2.048 2.000 .984 .962 .786 .666 .646 .625 .510 .49 I .448 .431 .425 .401 1.346 1.295

1.292

4

1.511 1.490 1.444 1.425 1.400 1.347 1.293

w f s m-s vf f w m m vf vf m-s

vs-very strong, s-strong, m-middle, w-weak, f-faint, vf-very faint

According to the volume of unit cell V = 268.5 A ~ and space group, each unit cell should contains 8 cations and corresponding contents of oxygen. Its chemical formula is (Yb, Ba, Cu)YbBa4Cu2Os67. Using LAZY program we tried to put rough positions and atomic parameters of cations and fit calculated intensities of diffraction lines with that of observed ones. Based on ionic radii of cations, cation-oxygen coordination polyhedra and Pauling's bond-valence theory [24], the rough positions and occupancy of axygen were determined. Based on the primary positions of ions the atomic ~arameters were adjusted and refined. A set of atomic

(~ Yb,Ba,Cu

@ cu @ Yb 0

Bo

Oo Fig. 4. The crystal structure of (Yb, Ba, Cu)YbBa4 Cu:O~ <>:. parameters and occupancy factors which induces a good coincidence with the experimental restilts are also shown in Table 2. Table 1 gives experimental data with Bragg's diffraction angles less than 44 ° by using CoK~ radiation and their corresponding calculated intensities greater than 2/1000 of maximum wllue. Total 60 possible lines exist in this angle region of which 29 lines have calculated intensity less than 2/1000 of the maximum and no detected by the Guinier-de Wolff" monochromatic focusing camera fire not shown in Table 1. In this Table the observed intensities are visual results of Guinier-de Wolff photograph. In this Table it is also clear that the calculated intensities of diffraction lines are in agreement with experimental restllts without any abnormality. At the same time to the bond valence theory [24] the calculated atomic valence of cations (Yb, Ba, Cu), Yb, Ba and Cu in the crystal structure are 2.2, 2.8, 2.3 and 2.0 2.4 respectively. They are close to the normal values of atoms. It seems that the obtained atomic parameters are reasonable find basically correct. To refine the structure a single phase specimen and precise composition of a compound are needed, The crystal structure of compound (Yb, Ba, Cu) YbBa4Cu2Os ~,7is shown in Fig. 4. The coordination of Cu, Yb and (Yb, Ba, Cu) to oxygens are all octahedra with six coordinations. The coordination polyhedron of Ba to oxygen is thirteen-hedron with nine coordinates. The ionic distances are listed in Table 3.

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THE STRUCTURE OF T E R N A R Y COMPOUNDS

513

Table 2. The atomic parameters and occupation factors of the crystal structure of (Yb, Ba, Cu) YbBa4Cu208.67 Ion

Yb(l) Ba(l) Cu(1) Ba(2) Cu(2) Cu(3) Yb(2) O(1) 0(2) 0(3) 0(4)

Equivalent point positions

Atomic parameters x

y

z

l(c) l(c) l(c) 4(i) l(d) l(b) l(a) 2(f) 4(k) 2(g) 2(h)

!/2 1/2 1/2 1/2 i12 0 0 1/2 1/4 0 1/2

1/2 1/2 1/2 0 1/2 0 0 0 1/4 0 1/2

0 0 0 0.255 1/2 112 0 0 0.5 0.24 0.26

3.2.3.2. The crystal structure of YbBasCu208 5. The compound YbBasCu2Os.s under different temperature has different crystal structures. The X-ray diffraction patterns obtained from specimens quenched at temperature higher than 950 ° C, 800 ° C and slowly cooled to room temperature are different. After indexing the diffraction lines we found that both the specimen quenched at 950°C and that quenched at 800°C to P4/mmm. Their lattice constants are: a = 4.054/k, c = 8.108,~ and a = 4.106, c = 8.033~ respectively. The formula unit per cell z is 1/2, i.e. each unit cell contains 0.5Yb, 2Ba, 1Cu and 4.25 Oxygens. The atomic parameters of crystal structure of (Yb, Ba)Ba2CuO425 are listed in Table 4. Table 5 shows a comparison of calculated intensities with that of observed via different planar distances d and also demonstrates that they are coincided well without obvious abnormality. So the obtained crystal structures and their atomic parameters are basically correct. The change of lattice constants at different temperatures may be caused by different contents of oxygens which may have different influence on lattice

Occupancy

1/3 1/3 1/3 ! 1 1 1 !/3 1 1 1

parameters a and c. For the specimen quenched at 950 ° C the splitted two lines combine into a single one that implies c = 2a. Figure 5 shows the crystal structure of (Yb, Ba) Ba2CuO4.25 and the cation-oxygen coordination polyhedron. The coordination polyhedron and ionic distances of Cu, (Yb, Ba) and Ba to oxygens are all the same as the situation in the crystal structure of (Yb, Ba, Cu)YbBanCu208.67. Sometimes the (1 10), (1 02) double diffraction lines and (112) single line of samples with composition YbBasCu2Os.5, obtained by slowly cooling down to room temperature, are split to three and double lines respectively, shown in Fig. 6. It means that the crystal structure of (Yb, Ba)Ba2CuO4.25 is not distorted to orthogonol system, but to monoclinic system with a space group P112/m. The lattice parameters are close to that of tetragonal system: a -b = 4.100, c = 8.020 A and 7 is close to 89 ° according to distances of splitted lines.

3.2.3.3. The thermal and electrical properties. The resistances of (Yb, Ba, Cu)YbBa4Cu20867 and

Table 3. Ionic distances in the structure of ( Yb. Ba, Cu) YbBa4Cu208.67 Cation-oxygen

Distance (~)

Cation-oxygen

Distance (~)

Cu(2)-40(k) 20(h)

2.048 1.920

Cu(3)-40(k) 20(g)

2.048 2.081

Ba(2)-40(k) 20(h) 20(g) 10(f)

2.835 2.897 2.899 2.041

(Yb, Ba, C u ) - 4 0 ( f ) 20(f)

2.897 2.081

Yb(2)-40(f) 20(g)

2.897 1.920

THE STRUCTURE

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OF TERNARY

COMPOUNDS

Vol. 74, N o . 6

Table 4. The atomic parameters and occupation factors q[ crystal structure of ( Yb, Ba) Ba2Cu04.25 Ion

E q u i v a l e n t point positions

Yb Ba( 1) Ba(2) Cu O(1) 0(2) 0(3)

l(a) 1(a) 2(h) 1(b) 2(e) 2(g) I(c)

A t o m i c parameters

Occupancy

x

)'

z

0 0 1/2 0 1/2 0 1/2

0 0 1/2 0 0 0 1/2

0 0 0.255 1/2 1/2 0.255 0

YbBa5 CueOs,5 increases with decrease of temperature. Both of them possess s e m i c o n d u c t i n g character. The D T A curves of the samples with compositions a r o u n d (Yb, Ba, Cu)YbBa4CueOs.67 and (Yb, Ba) BaeCuO425 are shown in Fig, 7. The composition,

1/2 1/2 1 I 1 1 0.25

phase identification a n d thermal effect of samples are listed in Table 6. The e n d o t h e r m a l effect at 2 9 0 ° C in the # D T A curves of four samples all appears which was produced by the desorption of air h u m i d i t y and c a r b o n dioxide. The weight of samples lost 1%

Table 5. CalcMated and observed intensities I and planar distances d of ( Yb, Ba)Ba2Cu04 25 at 950 ° C and 800:' C quenching. B = 2.0 hk l

001 100 002 101

1 10 1 02 111 003 112 103 200 004 201 113 210 202 104 211 2 12 I 14 203 )05 3.13 [05 ~.20 ~.04 ~.21 15

I~

a = 4.106, c = 8.033A

16

I 5 71 615 1000 58 7 30 9 274 135 4 35 2 20 406 230 8 6 11 97 191 2 3

a = 4.054, c = 8.108A

dcalc ( A )

dob s (A)

/Fobs

dcalc ( A )

dob s ( A )

lob s

8.033 4.106 4.017 3.656 2.903 2.871 2.730 2.678 2.353 2.243 2.053 2.008 1.989 ~ 1.968 J" .836 .828 .804 .790 .670 .651 .629 .607 .514 .496 .452 .436 .429 .406

8.046 4. l 16 4.012 3.644 2.904 2.868 2.648 2.355 2.244 2.053 2.013 1.995

w f f w s vs f f-w f m w-m f

8.199 4.065

w f

3.690 2.866

w vs

2.336 2.242 2.027

w f m

1.670 1.649 1.452 1.434 -

m m w m -

8.108 4.054 4.054 3.626 2.867 2.867 2.703 2.703 2.341 2.249 2.027 2.027 1.967 1.967 1.813 1.813 1.813 1.769 1.655 1.655 1.622 1.622 !.506 1.506 1.433 1.433 1.411 1.411

1.652

m

}

m

m

--

m

_

L

1.427

m

T H E S T R U C T U R E OF T E R N A R Y C O M P O U N D S

Vol. 74, No. 6

515

o

LX

U~

k/eTY

".;2/11

£

Z

"U"

© ox,0eo

Fig. 5. The crystal structure and coordination polyhedra of (Yb, Ba)Ba2CuO4.25. and showed no change of crystal structures. The endothermal effect at 812 ° C is related to the existance of (Yb, Ba)Ba2CuO4.25. The loss of weight of sample was about 1%. It means that at this temperature deoxygen phase transition takes place and the crystal structure is transformed from with lattice parameters c < 2a to c = 2a. The thermal effects at 290°C and 812°C are irreversible in/~DTA experiments, but the sample YbBasSu2Os.s can absorb the corresponding gases reversibly, if it is exposed in the air for some time. The endothermal effect at about 985 _+ 5°C appears in the multiphase samples. So we consider that the thermal effect belongs to eutectic reaction. The compound (Yb, Ba)Ba2CuO425 melts at 1140 ° C, but (Yb, Ba, Cu)YbBa4Cu208.67 melts at above 1t50°C and has not phase transition in solid state. There is a close relationship between structures of

17!5

,

18

o

I

1815

I - ~

19 22

I

225

23

o

Fig. 6. The comparison of partial diffraction lines of (Yb, Ba)Ba2CuO425 at different temperatures with CoK~ radiation. (a) P4/mmm, c = 2a, 950°C quenching; (b) P4/mmm, c < 2a, 800°C quenching (c) Pll2/m. a = b, c < 2a, r 4= 90 ° , room temperature. (a)(b)(c) is shown from top to bottom. (Yb, Ba)Ba2CuO4.25 and (Yb, Ba, Cu)YbBa4Cu208.67 . When the content of BaO increases, in the structure of (Yb, Ba, Cu)YbBa4Cu~O8.67 the cations of Ba and Yb at the corners and centre of a-b tetragonal plane turn to disorder which causes the disappearance of diffraction lines with h + k ¢ 2n and the lattice spacings of a and b become 1/.,/2 of the original ones. Though (Yb, Ba)Ba2CuO42s has the same phase site as (Yb, Ba, Cu)YbBa4Cu208.67 they have different properties for different compositions. For example, they display different phenomena in desorbing oxygen and phase transition when increasing or decreasing temperature. So we reckon preferably them as two independent phase instead of solid solution.

Table 6. The t~DTA results of some samples Curve in Fig. 7

Composition formula

Phase identification

a b

YbBasCu2Os. 5 YbBa4Cu3085

c

Yb3 BagCusOis.5

dI

Yb 2BagCU4Ol6

(Yb, (Yb, + (Yb, + (Yb, +

Ba)Ba2CuO4.25 Ba, Cu)YbBa4Cu2Os.~7 Yb2BaCuO5 + BaCuO2 Ba, Cu)YbBa4Cu2Os.67 Yb2 BaCuO5 Ba)Ba2CuO4.25 BaCuO2

Endothermal effect (° C) Onset

Onset

Onset

Peak top

290 290

812 -

990

1140 1150

290

-

980

1150

290

812

980

1123

~The sample may also contains (Yb, Ba, Cu)YbBa4Cu2Os.67, but is was not observed in Guinier-de Wolff photograph, because the diffraction lines of (Yb, Ba, Cu)YbBa4Cu2Q 67 were overlapped with that of (Yb, Ba) Ba2CuO4.25.

516

THE STRUCTURE OF TERNARY COMPOUNDS

d

.~o

g

t <~

c

7.

.2%

/k

b

/k

o

I%

8.

L, A I

A I

500

9.

PO00

p

t500

T (°C)

Fig. 7. DTA curves of samples with composition: (a) YbBasCu2085 (b) YbBa4Cu30~5 (c) Yb3BagCusOl~5 (d) Yb2 BagCuaO](,.

10. 11. 12.

Acknowledgement - T h e authors wish to appreciate the support of Chinese National Centre for Research and Development on Superconductivity and their measurements of electric properties of some samples. Also they would like to express their sincere appreciation of the analyses of chemical composition done by the chemistry laboratory of the Institute of Physics. REFERENCES 1. 2. 3.

4. 5. 6.

J.G. Bednorz & K.A. Mfiller, Z. Phys. BCondensed Matter 64, 189 (1986). M.K. Wu, J.R. Ashburn, C.J. Torng, P,H. Hor, R.L. Meng, L. Gao, Z.J. Huang Y.Q. Wang, & C.W. Chu, Phys. Rev. Lett. 58, 908 (1987). Z.X. Zhao, L.Q. Chen, Q.S. Yang, Y.Z. Hunag, G.H. Chen, R.M. Tang, G.R. Liu, C.G. Cui, L. Chen, L.Z. Wang, S.Q. Gao, S.L. Li & J.Q. Bi, Kexue Tongbao 32, 661 (1987). P.H. Hor, R.L. Meng, Y.Q. Wang, L. Gao, Y.J. Huang, J. Bechtold, K. Forster & C.W. Chu, Phys. Rev. Lett. 58, 1891 (1987). G.C. Che, J.K. Liang, W. Chen, Q.S. Yang, G.H. Chen& Y.M. Ni, J. Less-Common Metals 138, 137 (1988). D.G. Hinks, L. Soderholm, D.W. Capone II, J.D. Jorgensen, I.K. Schuller C.U. Segre, K.

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