Crystal growth and characterization of the superconducting phase in the BiSrCuO system

Physica C 156 (1988) 434-440 North-Holland, Amsterdam

CRYSTAL G R O W T H AND C H A R A C T E R I Z A T I O N OF T H E S U P E R C O N D U C T I N G P H A S E IN T H E Bi-Sr-Cu-O SYSTEM P. STROBEL *, K. KELLEHER, F. H O L T Z B E R G and T. W O R T H I N G T O N 1BM Research Division, Thomas ,L Watson Research Center, P.O. Box 218, Yorktown Heights, N Y 10598, USA Received 8 August 1988

Crystal growth was investigated in the system Bi-Sr-Cu-O. Layered bismuth cuprates with formulas close to (Bi, Sr)4CuO • or (Bi, Sr)5Cu20, and a typical micaceous, lath-like habit were grown by slow cooling from a wide range of compositions partially or totally melted in the temperature range 890-1000 °C. Only (Bi, Sr) 4CuO, crystals with 0.8 < Sr/Bi < 0.9 were superconducting, with Tc 9-10 K. In Sr-rich melts, platy crystals of orthorhombic SrCuO2 and SrCu~ 7Ox were obtained. Bi-Sr-Cu-O melts react with platinum below ca. 1000 ° C, leading to the formation of Sr4PtO6 crystals.

1. Introduction

The discovery of superconductivity in the Bi-SrC u - O system [ 1-2 ] has given rise to a new surge of research in cuprate ceramics, leading to new high critical temperatures in the B i - S r - C a - C u - O system [ 3 ]. The superconducting phases belong to a structural family with ideal formula Bi2SrzCa~n_l)CUnO(4+2n), containing double Bi layers and variable number n of Cu layers [2]. Compounds belonging to this structural family will be referred to as "layered bismuth cuprate" (LBC) throughout this paper. In the Bi system, the preparation of single-phase LBC's with n ~ 2 has eluded all attempts so far. Many questions remain unanswered even in the simplest quaternary B i - S r - C u - O system, where superconductivity was reported in the 5-22 K range [ 3-5, 25 ]. The composition first published by Michel et al. [ 3 ] (Bi: Sr: Cu = 1 : 1 : 1 ) does not fit the LBC formula for n = 1. Numerous authors did not find any superconductivity in this system [6-10]. Working on multiphase ceramic samples, Xiao et al. [ 5 ] and Franck et al. [25 ] reported conflicting results about the influence of composition on superconductivity. Oxygen annealing does not seem to be a major factor in

the occurrence of superconductivity [ 6 ]. Physicochemical data are scarce in the ternary BiCu-O, Bi-Sr-O and S r - C u - O systems. Only the BiC u - O phase diagram has been investigated [ 11 ]. It includes one ternary compound, CuBizO4, which melts incongruently a t 840-845 °C. A tetragonal Icentered "Sr0.9Bil.tO2.55" compound has been reported (JCPDS file ~31-1341 ). In the S r - C u - O system, Teske and Mueller-Buschbaum [12] have characterized SrzCuO3 (isostructural with C a z C u O 3 ) and SrCuO2. No Cu-rich compound was reported, in spite of the occurrence of C a C u 2 0 3 . Two recent reviews of the L a - S r - C u - O system [ 13,14 ] gave hints of the occurrence of another S r - C u - O compound with Cu/Sr ratio equal to 2 [13] or 3/2 [14]. In this paper, we investigate the conditions of crystal growth of LBC's in the B i - S r - C u - O phase diagram. This includes an analysis of melting temperatures and stability range of the LBC phase, and the chemical characterization and magnetic susceptibility tests for the crystals formed. Crystals obtained include both superconducting and semiconducting LBC with n = 1, non-superconducting LBC with n = 2 , and the ternary compounds SrCu02 and SrCul.vOx.

* On leave from Laboratoire de Cristallographie CNRS, Grenoble, France.

0921-4534/88/$03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

P. Strobel et aL /Superconducting crystals in the Bi-Sr-Cu-O system

2. Experimental Starting mixtures for crystal growth were prepared using "Puratronic" bismuth oxide, strontium carbonate and copper(II) oxide (Johnson-Matthey, 99.999% purity). The reagents were mixed and ground in an agate mortar and pressed into pellets at ca. 2 T cm 2. Heat treatments were carried out in gold capsules, with the exception of one run at 1000°C where platinum was used (run E). Subsequent analysis showed no evidence of gold contamination of the samples. Firings were carried out in tubular furnases with programmable temperatures. They included two preliminary treatments at 820°C then 850-880°C for 15-20 hours prior to melting and crystal growth runs. Differential thermal analysis (DTA) was performed on various compositions in the range 251100°C using a Perkin-Elmer 1700 system. The samples (40-60 mg) were contained in platinum crucibles and heated in flowing oxygen. The structural characterization of melts was carried out using an automated X-ray powder diffractometer with Guinier-focusing geometry (Cu-Kc~ radiation). Crystals were investigated using an X-ray precession camera ( M o - K a radiation). Selected flat crystal planes were analyzed by the electron microprobe technique on a JEOL 733 Superprobe, using the BiMo~, Sr-La, Cu-Ko~ and O - K a lines. An accelerating voltage of 12 keV was chosen to maximize the excitation potential of these lines. The apparatus was calibrated using standards of elemental Bi and Cu, and a SrZiO3 crystal. ZAF corrections were applied. Tests for superconductivity were carried out using a simple a.c. susceptibility technique down to 4.2 K, using a 1 mm ID soil [ 15].

435

( 1 ) Low melting temperatures are found in the Birich region. However, compositions such as M in fig. 1 (melting incongruently at 820°C) are out of the stability range of the LBC phase. Crystals obtained from M were Bi203 plates. (2) The range of crystallization of the LBC phase with n = 1 (ideal composition A in fig. 1 ) is fairly large, as shown by the circles in fig. 1. All these compositions give a DTA peak at 907+ 5°C, probably corresponding to the incongruent melting of LBC. As will be seen later, the n = 1 LBC phase itself encompasses a wide range of compositions located on the Bi-side of point A (in the XYZ triangle, fig. 1 ). Slight variations in melting temperatures may reflect a solid solution range in the crystallizing phase (this point was not studied in detail here). (3) Strontium-rich mixtures were investigated, in search of conditions to grow crystals with a large Sr/ Bi ratio [5]. However, melting temperatures increase rapidly on the Sr-rich side (see the temperatures used on points H, I and J in fig. 1 ), and the LBC with n = 1 no longer forms. Instead, crystals with a similar lamellar texture, but with compositions close to those expected for an n = 2 LBC were obtained, together with large crystals of S r - C u - O ternary oxides. A strontium-only LBC with n = 2 would Cu

3. Results and discussion

3.1. Phase diagram and melting temperatures It was early observed that compositions Bi2Sr2CuO6 (i.e. the stoichiometric composition for a LBC with n = 1 ) and Cu-rich Bi2SrzCuxOy(x> 1 ) show partial melting when heated near 900°C. A broad DTA survey of the Bi-Sr-Cu diagram yielded the following results:

Sr

Fig. 1. Bi-Sr-Cu-O diagram, showingthe compositionsused for melts A-M. Crystals grown: ( • ) superconducting LBC; ( O ) non-superconductingLBC; (/x ) other phases. The ideal compositions of LBC's with n= 1 and n---2 are in points A and X, respectively.

436

P. Strobel et al. / Superconducting crystals in the Bi-Sr-Cu-O system

indeed be shifted towards the Sr-rich side of the diagram (see point X in fig. 1 ). (4) The S r - C u - O phase diagram is very different from Ba-Cu-O. The latter includes a low eutectic in the copper-rich region [ 16,17 ], which has been used to grow crystals of the superconducting yttrium barium cuprate [ 15 ]. In contrast, a compound is formed in the S r - C u - O system in this composition region. DTA on the composition SrCu203 shows evidence of incongruent melting for this composition at 995 ° C.

study are SrCuO2 and a nonstoichiometric phase with a composition close to SrCu~ 7Ox. Both crystallize with a lath-like morphology (see figs. 3 and 4), and can be mistaken for LBC. However, SrCuO2 crystals do not exhibit the typical lamellar features of LBC, and distinctly appear copper-colored when thin. SrCu~ rOy crystal surfaces exhibit typical slip lines perpendicular to the lath long direction (fig. 4). These compounds crystallized simultaneously with LBC in runs I, J and L.

3.2. Crystal growth

3.3. Characterization o f crystals

The maximum temperatures used are given in fig. 1. They corresponded to partial melting only for runs H, I, J and L. Melts were cooled at 5°C/hour. Crystals were recovered mechanically from the melt. This recovery was made easier by the tendency of crystal seeds to appear at the melt-container interface, and by the softness of the thin gold sheets used as crucibles, which could easily be flexed to free the crystals. The results are summarized in table I. A comparison of runs D and E shows the effect of higher temperatures on the crystallization of LBC. Crystals obtained in run E (at 1000°C) were not s~perconducting and very strontium-deficient, and compositions closer to a n = 2 LBC were also observed (table I). In addition, strong attack of platinum occurred, yielding crystals of a Sr-Pt oxide, with a composition close to Sr4PtOr, a rhombohedral compound known to crystallize as platelets [ 18,19 ]. LBC crystals appeared as elongated plates ("laths"), usually showing evidence of a lamellar structure (see fig. 2a). Some plates were heavily fractured and actually consisted of clusters of much smaller, lath-like crystals (fig. 2b). The latter characteristics were repeatedly obtained for crystals grown from the ideal composition A. Larger and more perfect crystals were obtained along the A-CuO line (points B-F, fig. 1 ). In any case, they are extremely fragile and delaminate often on handling with tweezes. Small undamaged single crystals (1 m m in length) could be isolated and were used for X-ray studies. They are elongated, with a typical length/ width ratio of about 5, and thicknesses in the range 5-20 Ixm. The dimensions of the laths correspond in reverse order to the crystallographic axes. The other phases obtained as single crystals in this

3.3. I. The layered bismuth cuprate phase The analytical results given in table I show a considerable spread in composition for the LBC phase. Considering first the ( B i + S r ) / C u ratio, the experimental data fit two distinct groups where this ratio is either <2.75 or >3.59. The ideal LCB formula can be written as BizM(I+~)Cu~O(4+2,), where M stands for the total alkaline-earth content and n for the number of copper layers. It follows that the ideal ( B i + M ) / C u ratio is equal to 4 for n = 1 and to 2.5 for n = 2. This ratio is probably a better structural parameter than the individual Bi/Cu or Sr/Cu ratios, because the proximity of the ionic radii of Bi 3+ and Sr 2+ allows easy mutual substitution between these two cations. The four compositions having (Bi + Sr) / Cu < 2.75 (from runs E, I and J ) are likely to belong to the n = 2 family, which is well-known for M = S r + C a (an 85 K superconductor). Individual crystals showing clear evidence of the typical c=30.6 ~ unit cell for n = 2 could not be isolated, but single crystals from runs E and J have complex superstructures along c, probably due to n = 2 / n = l intergrowth [20]. The 30.6 /~ cell was detected by electron diffraction. Moreover, we observed Raman spectra exhibiting modes clearly incompatible with an n = 1 structure, but strongly reminiscent of the n = 2 structure [ 21 ]. It can be concluded that the n = 2 LBC with alkalineearth sites occupied by strontium alone can be stabilized. A remarkable result is the absence of superconductivity in any of these samples, in contrast with the calcium-containing n = 2 LBC. The second group of compositions has (Bi + Sr) / Cu ratios in the range 3.6-4.8, to be compared with the stoichiometric value 4 for n = 1. Values < 4 may

437

P. Strobel et al. / Superconducting crystals in the Bi-Sr-Cu-O system

Table 1 Characterization of crystals. Melt composition *

Morphology

Structure

Crystal composition ** Bi

Sr

Bi + Sr

Tc

A

plates/laths 4>(0.5 mm

LBC ***

A1 A2

1.95 2.33

1.81 2.18

3.76 4.51

none

B

plates 5 × 5 mm

LBC

B1 B2

2.03 2.35

1.66 1.98

3.69 4.33

IOK

C

plates/laths up to 2 mm long

LBC

C1 C2

2.06 2.31

1.75 1.93

3.81 4.24

IOK

D

plates 5 ×2 mm

LBC

D1 D2

1.98 2.32

1.61 1.87

3.59 4.19

lOK

E

laths 2x0.4 mm

LBC

El E2

1.70 2.47

1.05 1.49

2.75 3.96

none

plates

CuO, Sr3PtO4

F

plates 3×2 mm

LBC(+CuO)

2.09

1.81

3.90

5K

G

laths 2)<0.4 mm

LBC

2.19

1.47

3.66

none

H, I

black laths < 1 mm

LBC

0.85 1.02

1.30 1.38

2.15 2.40

none

reddish laths, 2 mm

SrCuO2

1.02

laths 4× 1 mm

SrCu,Oy

0.57-0.62

J

I1 I2

LBC(minor phase)

Jl J2

2.12 1.05

1.99 1.60

4.11 2.65

none

Kl K2

2.26 2.60

1.90 2.19

4.16 4.79

10K

K

plates 2× l mm

LBC

L

laths< 1 mm

LBC+SrCu~Ov

M

yellowish plates

Bi203

none

" See fig. 1. "" Referred to 1 Cu. *'"LBC = layered bismuth cuprate. be d u e to partial i n t e r c a l a t i o n o f n = 2 p h a s e i n t o t h e d o m i n a n t n = 1 m a t e r i a l . Values o f ( B i + S r ) / C u > 4 m a y reflect c o p p e r vacancies. It has b e e n argued [22 ] t h a t Bi c o u l d s u b s t i t u t e n o t o n l y Sr in L B C ' s , but Cu as well. C o m p o s i t i o n s o f crystals b e l o n g i n g to this n = 1 g r o u p are s h o w n in an e n l a r g e d p o r t i o n o f the B i - S r C u - O d i a g r a m in fig. 5. As s h o w n by the d a s h e d lines in this figure, c o m p o s i t i o n s w e r e f o u n d to v a r y sign i f i c a n t l y w i t h i n a g i v e n sample. H o w e v e r , this varia t i o n is essentially a c h a n g e in c o p p e r c o n t e n t . N o n e o f the s a m p l e s a n a l y z e d m a t c h e d t h e ideal Bi: Sr: C u = 2 : 2: 1 c a t i o n ratio, a n d all w e r e B i - r i c h e r

a n d S r - p o o r e r t h a n this c o m p o s i t i o n . T h e experim e n t a l c o m p o s i t i o n s can be d i v i d e d into three groups as a f u n c t i o n o f t h e i r S r / B i ratios. Bi-rich L B C ' s ( g r o u p A ) w e r e f o r m e d in runs E (at 1 0 0 0 ° C ) a n d G (a Bi-rich m e l t ) , Crystals w i t h c o m p o s i t i o n s close to the ideal ratio S r / B i = 1 ( g r o u p C ) w e r e g r o w n f r o m r u n s A a n d J, b o t h m e l t s w i t h t h e ideal Sr fract i o n ( 2 / 5 ) . Finally, m o s t runs in C u O - r i c h m e l t s p r o d u c e d crystals w i t h i n t e r m e d i a t e c o m p o s i t i o n s ( g r o u p B ) . C o n t r a r y to p r e v i o u s reports [ 5 ] , m e a s u r e m e n t s o n single crystals s h o w e d t h a t a high Sr c o n t e n t is n o t t h e key to s u p e r c o n d u c t i v i t y . G r o u p C crystals, which h a v e S r / B i ratios close to ( b u t lower

438

P. Strobe[ et al. / Superconducting crystals in the B i - S r - C u - O system

Fig. 4. SrCu~.70~crystal (from melt J ).

t h a n ) 1, are n o t s u p e r c o n d u c t i n g . T h e synthesis o f crystals with higher Sr contents, which w o u l d have m a t c h e d the c o m p o s i t i o n s reported by Xiao et al. [ 5 ] for s u p e r c o n d u c t i n g LBC's, was n o t possible, due to the increase i n m e l t i n g t e m p e r a t u r e s a n d the emergence o f other Sr-rich solid phases i n this region o f the phase d i a g r a m (see r u n s H a n d I, fig. 1 ).

0.40.3 z

Fig. 2. SEM photographs of LBC's showing (a) typical lamellar growth features (from melt B), (b) the actual lath-like morphology (from melt A).

¢\ \ Od

x00.3 .

Fig. 3. SrCuO4crystal (from melt H).

6

~0.2 \

/\

V 0.4

~,

\0.1 y 0.5

Sr~

Fig. 5. The XYZ section of the Bi-Sr-Cu-O diagram (cf. fig. 1 ), showing compositions of LBC crystals. (O) superconductors; (O) non-superconductors; (m) ideal, stoichiometric composition for n= 1 (point A in fig. 1). The Sr/Bi ratio scale refers to the 0.2 Cu line.

P. Strobel et al, / Superconducting crystals in the B i - S r - C u - O system 1

I

Table II Powder X-ray diffraction pattern of SrCu~ 7Ox.

I

Ill

~)0

0

0

f

>-0,02

0 EL LJ (D - 0 . 0 4 (.© D bO G

oo

o o

o

-o.oe

-0.08

I 8

I

112

116

439

20

TEMPERATURE (K) Fig. 6, a.c. susceptibility o f a LBC crystal from melt B.

As shown in fig. 5, superconductivity was reproducibly obtained in crystals of group B, i.e. with Sr/ Bi ranging from 0.8 to 0.9. Typical Tc values were in the range 9-10K (see fig. 6). A correlation with chemical composition in terms of average copper valence was not attempted, due to the inaccuracy of the oxygen contents given by the microprobe analysis. Apparent Cu fraction variations (along nearly vertical lines in fig. 5 ) actually reflect the scattering in (Bi + Sr)/Cu ratio. Oddly enough, this variation does not seem to affect Tc in group B samples. Preliminary single-crystal X-ray work has shown the existence of a 5-fold supercell along b, which may be due to ordered cationic substitutions or vacancies. Structural results on the n = 1 LBC will be published elsewhere [20 ].

dobs (ilk)

dcalc (A)

I/Io

h

k

l

3,578 3.335 2.882 2.854 2.681 2.170 1.963 1.834 1.808 1.6963 1.6688 1.6228 1.5894 1.5649 1.4724 1.4570 1.4481 1.4302 1.3688 1.3409 1.3145 1.3049 1.2717 1.2484 1.2337 1.2056

3.580 3.338 2.883 2.852 2.681 2.1685 1,9643 1.8344 1.8075 1.6969 1.6690 1.6234 1.5895 1.5649 1.4727 1.4570 1.4484 1.4308 1.3686 1.3407 1.3151 1.3043 1.2719 1.2488 1.2333 1.2051

ll 14 100 50 51 19 21 18 15 3 14 11 5 8 7 3 4 7 4 3 3 3 5 5 5 5

1 0 2 1 3 1 0 6 5 1 0 2 5 3 0 4 6 8 6 6 8 3 0 1 3 9

1 4 4 3 1 5 0 2 3 7 8 4 5 7 6 4 6 0 0 2 4 9 8 3 1 1

1 0 0 1 1 1 2 0 1 1 0 2 1 I 2 2 0 0 2 2 0 1 2 3 3 1

ascribed to a Cao.6Sro.4CUl.750 3 compound. More recently, crystals of (Sr, X)~4Cu2404~ ( X = Y , Ca) with a similar structure have been described [ 24,26 ]. Our results are in good agreement with the 24/14 cation ratio and the layered structure (with layers perpendicular to b) found by Siegrist et al. [24] and LePage [26].

3. 3.2. Strontium cuprates

Crystals of SrCuO2 fit the orthorhombic Cmcm cell described previously by Teske and Mueller-Buschbaum [ 12], with lath dimensions c < b < a . Srful.7Ox possesses an F-centered orthorhombic cell with possible space groups F222, Fmm2 or Fmmm. Its lattice parameters, obtained by powder diffraction, are a = 1 1 . 4 4 7 ( 2 ) A, b=13.352(3) A, c = 3 . 9 8 7 ( 7 ) A. The crystal form a plate-like structure parallel to the a - c plane. The powder diffraction pattern, which has not been previously reported, is given in table II. This phase is frequently found in multiphase B i - S r - C u O ceramic samples. A very similar cell has been found [23] in multiphase B i - C a - S r - C u - O samples and

4. Conclusion Single crystals of various phases in the quaternary Bi-Sr-Cu-O system were grown. The layered bismuth cuprates crystallize in a wide range of melt compositions, yielding varying cation ratios in the crystals. All LBC crystals analyzed were strontiumdeficient. Superconducting crystals were obtained only for melt compositions below ca. 920°C. They have a narrow range of Sr/Bi ratio (between 0.8 and 0.9). Evidence of calcium-free crystals with the two

440

P. Strobel et al. / Superconducting crystals in the Bi-Sr-Cu-O system

Cu-layer LBC structure was found. Sr-rich LBC's c o u l d n o t b e o b t a i n e d . H o w e v e r , t h e large c r y s t a l s o f SrCuO2 and of the new layered strontium cuprate SrCul.7Ox w e r e o b t a i n e d i n S r - r i c h m e l t s .

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[ 11 ] M. Hrovat and D. Kolar, J. Mater. Sci. Lett. 3 (1984) 659. [12] C.L. Teske and H. Mueller-Buschbaum, Z. Anorg. Allg. Chem. 371 (1969) 325; 379 (1970) 234. [ 13] J.B. Torrance, Y. Nokura and A. Nazzal, Chemtronics 2 (1987) 120. [ 14] J. Hahn, T.O. Mason, S.J. Hwu and K.R. Poeppelmeier, Chemtronics 2 (1987) 126. [ 15 ] D.L. Kaiser, F. Holtzberg, M.F. Chisholm and T.K. Worthington, J. Crystal Growth 85 (1987) 593. [ 16 ] K.G. Frase and D.R. Clarke, Adv. Ceram. Mater. 2 ( 1987 ) 295. [ 17 ] R.S. Roth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1987) 303. [ 18 ] J.J. Randal and L. Katz, Acta Crystallogr. 12 ( 1959 ) 519. [ 19 ] L. Ben-Dorl J.T. Suss and S. Cohen, J. Crystal Growth 64 (1983) 395. [20] R. Boehme, S. La Placa, P. Strobel and F. Holtzberg, to be published. [21] G. Burns, F. Dacol, M. Shafer, G.V. Chandrashekhar and P. Strobel, Solid State Commun. 67 (1988), 603. [22]T. Kajitani, K. Kusaba, M. Kikuchi, N. Kobayashi, Y. Syono, T.B.Williams and M. Hirabayashi, Jpn. J. Appl. Phys. 27 (1988) L587. [23] R.M. Hazen et al., Phys. Rev. Lett. 60 (1988) 1174. [24] T. Siegrist, L.F. Schneemeyer, S.A. Sunshine, JN. Waszczak and R.S. Roth, preprint. [25] J.P. Franck, J. Jung, W.A. Miner and M.A.-K. Mohamed, Phys. Rev. B 38 (1988) 754. [26] Y. LePage, Amer. Crystallogr. Assoc., Philadelphia Meeting (1988), PK12.