Phase diagram and new phases in the Y-Ba-Cu-O system at high oxygen pressure

PhysicaC 171 (1990) 187-193 North-Holland

Phase diagram and new phases in the Y - B a - C u - O system at high oxygen pressure M.R. C h a n d r a c h o o d , D.E. Morris a n d A.P.B. Sinha Morris Research, Inc., 1918 University Ave., Berkeley, CA 94704, USA Received 12 June 1990 Revised manuscript received 28 August 1990

The Y - B a - C u - O phase diagram has been determined at 980°C at high oxygen pressure 200 b a r = 2 0 mPa. It shows several differences from the phase diagram at 1 bar. Highly oxidized phases are stabilized: BaO2 instead of BaO and YBa2Cu408 instead of YBa2fu307_x. Two new phases have been observed: YBasCu20~o_x has a tetragonal unit cell with a = 5.88 A and c = 8.04 A, consistent with a four layered structure such as C u O 2 - B a O - ( ½Y+ ½Ba)O-BaO, while YBa:Cu206_x appears to be isomorphous with the superconductor LaLrSro.4CaCu2Or. In addition, we find a high-pressure BaCuO~* phase, its X-ray diffraction ( X R D ) pattern indicates a perovskite related layered structure which has a unit cell a--- 5.68 A, c = 14.40 A. The formation of compounds on the YOLs-BaO line: Y2Ba2Os, Y2BaO4 and Y2Ba407 is suppressed at high P(O2). Tie lines are observed to join C u O - 2 : 1 : 1, 2 : 0 : 2 - 2 : l : l , Y O i . 5 - 2 : l: 1, 2: l: 1-BaO2, C u O - l : 2 : 4 and 2 : l : l - 1 : 2 : 4 .

I. Introduction

2. Experimental techniques

After the discovery of 90 K superconductivity in by Wu et al. [ 1 ], systematic studies of the Y - B a - C u - O pseudoternary phase diagram at P ( O 2 ) = 1 bar were carried out independently by several workers [2-5]. Some compositions in this pseudoternary phase diagram were also investigated under high oxygen partial pressures at this laboratory, and changes in the relative stability of the phases were observed [ 6 ], e.g. YBa2Cu4Os was found to be more stable than YBa2Cu3OT_x at 980°C at oxygen partial pressures above 60 bar, while between 25 and 50 bar the stable phase is Y2Ba4Cu7Ols_x. The phase stability boundaries were found to be nearly independent of excess or deficit of CuO in the starting mixture. It was therefore thought interesting to examine the entire Y - B a - C u - O phase diagram at elevated oxygen partial pressure. In this paper we report the Y - B a - C u - O phase diagram determined by solid state reaction at 980°C at P ( O 2 ) = 2 0 0 bar.

The simple oxides Y203, CuO and BaO2 were weighed and thoroughly mixed in the desired stoichiometries by grinding in a mortar and pestle. The samples were pressed into pellets, wrapped in gold foil, and sintered at 9800C for 15 hrs in a commercially available high-pressure furnace [ 7 ] at oxygen pressure of 200 bar. The pressure was maintained during cooling. The cooling rate was 10 deg/min from 980 °C to below 700 ° C, and we do not anticipate any phase transformation during cooling. The samples were then reground and pressed into pellets and were retired under above mentioned conditions to ensure complete reaction. X-ray diffraction analysis ( X R D ) was carried out on a Siemens D500 diffractometer with Cu K a radiation filtered through a Ni filter. The detector slits width and 20 step size were both 0.05 °. Using the DIFFRAC-AT software, the Kot2 peaks were removed and the background was subtracted. The observed d values were assigned to different phases by comparison with known standard patterns and the fractions of different phases were estimated by corn-

YBa2Cu307_ x

0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

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M.R. Chandrachood et al. / Phase diagram and new phases in the Y-Ba-Cu-O system

paring the intensities of the strong non-overlapping lines.

3. Y - B a - C u - O pseudo-ternary phase diagram at P(O2)-- 200 bar The compositions investigated are given in table I and are marked by dots numbered 1 to 66 in fig. 1 (a). The phases formed, and their approximate fractions (as estimated by X-ray peak intensities) are listed in table I. Examination of these results clearly indicate the following single phase points; which are shown in fig. 1 (b): CuO, Y203, BaO2, Y2Cu205 ( 2 : 0 : 2 ) , Y2BaCuO5 (2: 1:1 ), YBazCu408 (1:2:4), YBa/Cu206_x ( 1 : 2 : 2 ) , YBasCu2Ox ( 1 : 5: 2), BaCuO* (0:1 : 1 ) and BazCuO3+x (0:2:1 ). Note that at high oxygen pressure, BaOz is stable [ 8 ] instead of BaO, so the end member in the phase diagram is BaO2. The BaO2 '--' B a O + ½02 boundary has been determined from the thermodynamical data, and follows approximately the relation log P(O2)=A-B/Twhere A=6.86 and B = 7 1 4 0 K in the temperature range between 700°C and 1000 ° C. At T = 9 8 0 ° C the BaO2 ,--, B a O + ½02 boundary lies at P(O2) = 15 bar. Tie lines were then determined by selecting compositions along the various possible lines, heating them under the conditions mentioned, and determining the phase fractions. The tie lines are marked on fig. l ( b ) ; they join C u O - 2 : l : l , 2 : 0 : 2 - 2 : 1 : 1 , YO~.5-2: 1:1, 2: 1: 1-BaOz, CUO-1:2:4 and 2: 1: 11 : 2: 4. Tie lines in region E are not established unambiguously, and are shown as dashed lines. For comparison, fig. 1 (c) shows the Y - B a - C u - O phase diagram at P ( O 2 ) = 1 bar [2]. At P(O2) = 1 bar, the tie lines join CuO-2: 1:0, 2: 1:1-2:0:2, YO~.52:1:1, 2 : 1 : 1 - 1 : 2 : 3 , CUO-1:2:3, 1 : 2 : 3 - 0 : 1 : 1 , 2:1:1-0:1:1, 0:1:1-2:4:0.

4. New phases at high P(O2) ( 1 ) A new phase is observed near the composition Y~BasCu2Os.5+x. The structure is tetragonal with a = 5 . 8 8 /k and c=8.04 A. (A similar unit cell was reported for Y3BaaCusO~8 by deLeeuw et al. [ 11 ] ). The composition and the unit cell size of

Y~BasCu208.5+zy suggests a layered structure of the type CuO ~.25+y-BaO- ( ½Ba, ~Y) O-BaO, containing three-layer rock-salt blocks alternating with single CuO~.25 +y defect perovskite layers. The central plane of the rock-salt blocks would be occupied by 50:50 by Y and Ba ions in an ordered arrangement giving rise to the doubling of the basal plane. At an oxygen content of 8.5 (i.e., y = 0 . 0 ) , all copper ions would be divalent, while for y=0.25 (all oxygen vacancies filled) the formal average copper valency (FACV) = 2.5, i.e, half of the Cu would be trivalent. The atomic positions in the proposed structure are given in table II. All the listed sites would be fully occupied except for the oxygen in the (4k) site. XRD peak intensities were calculated using POWD programme with the nominal site locations and occupancies given in table II. The calculated and the measured patterns are in good agreement. Rietveld refinement of the structure is in progress. (2) With the starting composition 1:2:2, a multiphase product was obtained which contained 2 : 1 : 1, 0:1 : 1 and in several experiments (but not all) a new phase was also observed. Experiments are in progress to obtain pure phase samples. The pattern of the new phase (by subtraction of2:1 : 1 and 0:1 : 1 lines) can be indexed on the basis of the tetragonal unit cell with a=4.01 A and c=20.43 A,. A preliminary investigation indicates that the structure is isomorphous with the layered compound La2CaCu206 [9], which has pairs of CuO2 planes separated by Ca, and which has recently been shown to become superconducting when doped with Sr and annealed in high P(O2) [ 10 ]. Our new 1 : 2: 2 compound would have Y in place of Ca, and BaO layers in place of LaO layers. At full oxygen occupancy of 8 the FACV = 2.5, so this compound, and also the previously discussed YBasCu208.5+x, are good candidates for high-temperature superconductivity. (3) At high oxygen pressures, the well-known BaCuO2 structure [ 12 ] which is formed at 1 bar is not observed at 0:1:1 composition, instead a new structure appears. In addition, in the low angle region, a line at d = 5.69/~ is also present. This structure was reported by Arjmond et al. [ 16] after heating BaO2+Cu(NO3)2 at 580°C in air and also by heating BaCuO2 at 600°C in P ( O 2 ) = 4 0 0 bar. The chemical composition, BaCuO2+x, would ordinarily suggest a two-layer perovskite structure, and the sim-

M.R. Chandrachood et al. / Phase diagram and new phases in the Y-Ba-Cu-O system

189

Table 1 Phase fractions in the YO~ 5 - B a O 2 - C u O ternary s y s t e m at oxygen pressure of 200 bar a n d t e m p e r a t u r e 980°C. Sample number

Starting s t o i c h i o m e t r i e s YOL5

Phases observed by X R D

CuO

BaO

Major

Minor

Trace

0.0 0.0 0.0 0.1 0.0 0.1 0.0 0.1 0.1 0.2 0.2

0.9 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3

CuO CuO CuO CuO 202 202 202 202 202,211 211 211

202 202 202 211 CuO CuO -

-

0.0 0.0 0.1 0.0 0.1 0.2 0.0 0.1

0.4 0.3 0.3 0.2 0.2 0.2 0.1 0.1

202 202 211 Y:03 Y203 211 Y203 Y203

0.1

0.2 0.2 0.2

0.8 0.7 0.6 0.5

CuO CuO 124 211

0.3 0.4 0.2 0.3 0.4 0.5 0.6 0.7 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

211 211 211 ,Y203 211 21 I,Y203BaO2 21 l,BaO2 211 ,BaO2 21 l,BaO2 Y203 Y203,BaO2 Y203,BaO2 YzO3,BaO2 Y2Os,BaO2 Y203,BaO2 Y203,BaO2 BaO2 BaO2

Region A ( C U O - 2 0 2 - 2 1 1 ) 2 4 7 8 11

12 16 17 23 24 31

0.1 0.2 0.3 0.2 0.4 0.3 0.5 0.4 0.5 0.4 0.5

CuO 202

124 211 21 l

-

Region B ( 2 0 2 - 2 1 1 - Y O I . s ) 22 29 30 37 38 39 46 47

0.6 0.7 0.6 0.8 0.7 0.6 0.9 0.8

YzO3 Y203

202 202 211

Y203

Y203

-

202 211

-

202

Region C ( C U O - 1 2 4 - 2 1 1 ) 5 9 13 18

0.1 0.1 0.2 0.3

124

211

124 CuO 124

211 CuO

BaOz Y203,BaO2 Y203 Y203 BaO2 -

BaO2 BaO2 -

Y203

-

Y203

-

Region D (YOI.5-21 l - B a O 2 ) 40 41 48 49 50 51 52 53 57 58 59 60 61 62 63 64 65

0.5 0.4 0.7 0.6 0.5 0.4 0.3 0.2 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

190

M.R. Chandrachood et al. / Phase diagram and new phases in the Y-Ba-Cu-O system

Table I (continued) Sample

Starting Stoichiometries

Phases observed by XRD

number YOI.5

CuO

BaO

Region E ( 124-123-2l 1-BaO2-021-011-122-152) Tie lines not established unambiguously in this region 3 0.0 0.1 0.9 6 0.0 0.2 0.8 10 0.0 0.3 0.7 15 0.0 0.4 0.6 21 0.0 0.5 0.5 28 0.0 0.6 0.4 36 0.0 0.7 0.3 45 0.0 0.8 0.2 55 0.0 0.9 0.1

19

Major

CuO,"U" CuO,"U" 011 a) Uninterpreted 011 a) 021 021 Melted Melted

Minor

0 1 1 a)

"U",CuO

0 1 1 a)

32

0.2 0.3 0.4

0.3 0.3 0.3

0.5 0.4 0.3

124 211 211

42 43 54

0.3 0.3 0.1

0.5 0.6 0.8

0.2 0.2 0.1

211 21 l,BaO2 BaO2

-

14 20

0.1 0.1

0.3 0.4

0.6 0.5

124 "'123T"

021

26 27

0.2 0.1

0.4 0.5

0.4 0.4

211,122,011 122

35 44

0.1 0.1

0.6 0.7

0.3 0.2

152 152

33 34

0.3 0.2

0.4 0.5

0.3 0.3

122 122

25

Trace

-

211

124 -

124

BaO2

021

152 152

" u " is an unidentified phase. Its strongest XRD peaks are at d values of 3.211, 1.937 and 1.609 A. a) This BaCuO2 +x phase has a layered structure, and is quite distinct from the BaCuO2 structure stable at low P(O2). "123T" is a tetragonal 123 phase with c=3a (pseudocubic) [ 19]. plest unit cell w h i c h a c c o u n t s for m o s t o f the diff r a c t i o n lines is a = b = 5 . 6 9 A a n d c = 4 . 8 0 ~,. H o w ever, this cell size has several difficulties: (i) it leaves s o m e w e a k lines u n a c c o u n t e d for, ( i i ) it d o e s not p r o v i d e any basis for d o u b l i n g the basal p l a n e a r e a o f the p e r o v s k i t e u n i t cell, a n d ( i i i ) it is difficult to a c c o u n t for a v a l u e o f c = 4 . 8 0 ~. We f a v o r a tent a t i v e i n t e r p r e t a t i o n o f the X R D p a t t e r n by t r i p l i n g the c axis (a = b = 5.69 A a n d c = 14.2 A ) ; t h e n all the w e a k lines can be i n d e x e d . T o be c o n s i s t e n t w i t h the c h e m i c a l c o m p o s i t i o n , this six-layer u n i t cell c o u l d h a v e layers o f BaO, CuO2 a n d (½Ba,½Cu)O. O r d e r ing o f the Ba a n d Cu in the m i x e d layer w o u l d ex-

plain the e n l a r g e m e n t o f the a a n d b axis by the factor o f x / 2 . T h e 'c' value is reasonable, w h e n c o m p a r e d w i t h that o f the Ba2CuO3+x phase, f r o m w h i c h it s e e m s to be d e r i v e d by r e p l a c e m e n t o f h a l f o f the Ba 2+ by Cu 2+ in one o f the two B a O layers. T h e layer s e q u e n c e m a y be A B C A B C or A B C C B A w h e r e B r e p r e s e n t s the C u O x layer. A n o t h e r s o l u t i o n w o u l d be 2 layers o f ( 3 B a , ~ C u ) O a l t e r n a t i n g with a single CuO2 layer. T h i s has the a d v a n t a g e o f d i s t r i b u t i n g the Cu ions on b o t h the Ba layers, a n d leads to the r e q u i r e d e n l a r g e m e n t o f a, w h e n the C u is o r d e r e d in the Ba layers. D e t e r m i n a t i o n o f the Cu p o s i t i o n s by c o m p a r i s o n with the synthetic p a t t e r n s and Riet-

M.R. Chandrachood et al. / Phase diagram and new phases in the Y-Ba-Cu-O system

191

CuO

CuO

02

08

06

04

2

:

0

~

0.6

~0:1:1 E

* O~k~O:

,

2:1

a

"

YOl.5

5e

~

5e

s~

60

~,

e~

e3

64

65 66B o O 2

YO 1.5~

02

D

04

° 06

BaO 2

CuO

Fig. 1. (a) Stoichiometries of 66 samples prepared for study of Y-Ba-Cu-O phase diagram at 200 bar, see table 1 and fig. 1(b) for results. (b) Y-Ba-Cu-O phase diagram for preparation at P(O2 ) = 200 bar at 980 °C indicating: ( 1 ) Three new phases (a) Y~BasCu20,o_x (c) a BaCuO2+x layered compound (b) YiBa2Cu206_x and (c) BaCuO~+x a layered compound. (2) BaO2 in place of BaO as an end member. (3) No single phase compounds on YOLs-BaO2 tie line. (4) 1:2:3 stability region replaced by 1 : 2 : 4 stability region. The stability of Ba2CuO3+xat 200 bar and 980°C is uncertain. Disproportionation to BaCuO+Ba is seen in some samples. (c) Y-Ba-Cu-O phase diagram at P(02 ) = l bar at 980°C, after Frase and Clarke [ 2 ].

~-- )-~0:2:1 - , , ; / /~0:3:1 C

_ i;V YO 1"5-

2:~.'0

2:4:0

veld r e f i n e m e n t is in progress. T h e stability o f this c o m p o u n d in place o f the w e l l - k n o w n low-pressure BaCuO2 c o m p o u n d m a y be a c o n s e q u e n c e o f the ease Table II Proposed atomic positions (nominal) in the tetragonal unit cell (P4/mmm) of YiBasCu2Os.s+2~,with a = 5.68 A and c= 8.04 A, and four metal layers. Atom

Position

x

y

z

Ba Y Ba Cu Cu 0 0 O O

(la) (lc) (4i) (ld) (lb) (2f) (2g) (2h) (4k)

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

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

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

with which oxygen can be a d m i t t e d in these K2NiF4 like layered c u p r a t e structures [ 13 ].

5. Comparison of the high-pressure and lowpressure phase diagrams ( 1 ) A l o n g the B a O 2 - C u O b i n a r y line, the Ba2CuO3+x phase [ 13 ] r e m a i n s stable u p t o elevated P ( O 2 ) a n d has i n c r e a s e d oxygen c o n t e n t [ 14] since the oxygen v a c a n c i e s are partially filled. Its stability at 9 8 0 ° C a n d 200 b a r is, however, u n c e r t a i n . Disp r o p o r t i o n a t i o n to BaCuO2+x a n d BaO2 is seen in s o m e samples. ( 2 ) T h e p a t t e r n f r o m c o m p o s i t i o n 15 is n o t interpreted. T h i s p a t t e r n a n d the extra lines in the patterns 3,6 a n d 10 are very possibly due to Ba3CusOs+x reported by d e L e e u w et al. [ 11 ], which is possibly

192

M.R. Chandrachood et al. / Phase diagram and new phases in the Y-Ba-Cu-O system

stabilized under high-pressure of oxygen. Further work on structure determination is in progress. The absence of BaCu202 in our phase diagram is easily explained as this is a reduced Cu ~+ c o m p o u n d and is most likely to be unstable at higher oxygen pressures. (3) Partial melting is observed for Ba rich compositions along the CuO-BaO2 line between 0.1CuO+0.9BaO2 and 0 . 2 C u O + 0 . 8 B a O 2 in high P(O2). This partial melting may result from a Ba2CuO3 5-BaO2 eutectic. This is different from the situation at low P(O2) (i.e., at 1 bar), where partial melting is observed in the Cu rich region [ 2,15 ] near 0 . 7 C u O + 0 . 3 B a O and seems to be due to a eutectic between CuO and BaCuO2. This is significant in locating a region of low melting point for growing single crystals. (4) Several compounds found at low P(O2) along the YOLs-BaO line, such as Y2Ba2Os, YzBaO4 and YzBa4O7 are absent from the phase diagram at P ( 0 2 ) = 2 0 0 bar, i.e., they are destabilised at high P(O2). This is probably related to the replacement of BaO by BaO2 as the end member; these changes lead to corresponding changes in the tie lines. However, in the light of our results at elevated P(O2) using oxides and the suggestion of deLeeuw et al. [ 11 ] that the reported c o m p o u n d on the YOI 5-BaO line, may in fact be partially carbonated, the earlier claims may need re-examination. (5) The left half of the phase diagram shows no apparent change in the relative stability of phases as compared to that at 1 bar. It is noteworthy that the single phase compounds namely, Y203, Y2Cu2O5and Y2BaCuO5 found in this region are also stable at elevated pressures, i.e., no highly oxidized phases appear in this region. Two other reported compounds at Y : C u = 1.1 namely YCuO2 and YCuO3 were not observed in our work. The former is a Cu ~+ compound, and is not expected in our phase diagram. YCuO3, on the other hand, is an oxidized phase but it was reported [16] to be formed at 800°C and is presumably not a stable phase at 980°C and P(O2) = 200 bar where we find Y2Cu205 is the stable phase at this composition. (6) As is well known, at high P ( 0 2 ) the 1 : 2:3 stability region is replaced by the 1 : 2 : 4 stability region [ 17 ] (however, composition #20 (0.1YOL5 + 0.4BaO2 + 0.5CuO ) showed tetragonal

1 : 2:3 as a minor phase; the reason for this is not clear). ( 7 ) As expected, higher P(O2 ) stabilized the more oxidized form wherever possible; 1 : 2 : 4 with higher formal average copper valence (FACV) replaces 1 : 2 : 3 (note that 1 : 2 : 4 (YIBazCu408) is more oxidized than 1:2:3, which has stoichiometry (YBazCu3O6.6 at P(02)=1 bar [15] at the 980°C synthesis temperature). For B a : C u = 1 : 1 stoichiometry, a K2NiF4 type 6 layered structure (with two metal atoms in each layer of the unit cell) is formed, to enable Cu to attain a higher oxidation state by filling vacant oxygen sites. The tie lines shown in the phase diagram at high P(O2) are useful in determining the conversion products during synthesis or conversion between phases. For example, it is apparent that if 1 : 2:3 is subjected to treatment in high oxygen pressure the products expected are 1 : 2 : 4 + 2 : 1 : l + B a C u O 2 + ~ . The latter two phases are nonsuperconducting and such conversion may be useful in providing flux pinning centers to increase the critical current [ 18 ].

Acknowledgements

The authors thank K. Takano, V.T. Shum, H. Bornemann, W. Vinje, P.K. Narwankar and T. Tomsia for valuable assistance and A. Marathe for calculating the BaO-BaO2 boundary from thermodynamic tables. DEM wishes to thank D. Fontaine, E. Haller, R.A. Muller, P.J. Oddone and N. Phillips for advice and support.

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M.R. Chandrachood et al. / Phase diagram and new phases in the Y-Ba-Cu-O system

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