Growth spirals on single crystals of YBa₂Cu₃O_7-x

Article

Reference

Growth spirals on single crystals of YBa

₂

Cu

₃

O

_7-x

SUN, Bing Nan, BOUTELLIER, Roland, SCHMID, Hans

Abstract

Polygonized octagonal spirals were observed on (001) surfaces of YBa₂Cu₃O_7-x crystals grown under very pure conditions. Square spirals were more frequent on crystals grown in impure media. This difference in behavior may be related to the degree of growth step roughness. The observed forms of growth spiral are consistent with predictions based on the periodic bond chain theory.

SUN, Bing Nan, BOUTELLIER, Roland, SCHMID, Hans. Growth spirals on single crystals of YBa₂Cu₃O_7-x. Physica C: Superconductivity and Its Applications, 1989, vol. 157, no. 1, p.

189-191

DOI : 10.1016/0921-4534(89)90486-3

Available at:

http://archive-ouverte.unige.ch/unige:32607

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PhysicaC 157 (1989) 189-191 North-Holland, Amsterdam

l

GROWTH SPIRALS ON SINGLE CRYSTALS OF YBa₂Cu₃07-x B.N. SUN, R. BOUTELLIER and H. SCHMID

Department of Mineral, Analytical and Applied Chemistry, University of Geneva, CH-1211 Gene1•a 4, Switzerland Received 27 November 1988

Polygonised octagonal spirals have been observed on ( 001) surfaces ofYBa₂Cu₃07 -x (YBCO) crystals grown under very pure conditions. Square spirals were more frequent on crystals grown in impure media. This difference in behaviour may be related to the degree of grmvth step roughness. The observed forms of growth spiral are consistent with predictions based on the periodic bond chain (PBC) theory.

1. Introduction

Since the discovery of high temperature supercon- ducting ·oxides [ 1,2], single crystals ofYBa₂Cu₃0₇ ^-x

(YBCO) have been grown from high temperature solutions by numerous laboratories [ 3]. Careful ex- amination of the surface morphology of as-grown crystals may be helpful for elucidating the growth mechanisms of YBCO and may lead to a better un- derstanding of its properties ( antisotropies, twin- '· ning structure, etc.). In this contribution, we report

the results of our recent observations on single crys- tals of YBCO. The occurrence of some unusual spi- rals is discussed.

2. Crystal growth experiments

The crystal growth experiments were performed in a muffle furnace which has been described elsewhere [ 4]. Preparation of YBCO single crystals was car- ried out as follows: YBCO powder was synthesized as a raw material by calcination of a stoichiometric mixture of Y ₂0_{3 ,} BaC0₃ and CuO for 18 hours at 960°C. The calcined YBCO powder was mixed with Cu0-BaC03 as flux in a YBCO to a flux weight ratio of 1:9. Three grams of this mixture were homoge- nized and pressed with 10 tons/cm² into a pellet of 8 mm diameter. With a view to avoiding any im- purity incorporation into the crystals the pellet was put into a self-made CuO crucible, positioned on a

Zr02 sheet in the furnace. The furnace was slowly heated from room temperature to 800°C over 6 hours, then to 1010oc at 40°C per hour, held at 1010oc for 12 hours, and slow-cooled to 900oC at 5°C per hour, then to 500°C at 35oC per hour, held at 500 o C for 7 hours, then furnace-cooled to room temperature. All the experimental steps were per- formed in air. In some of the experiments run under these conditions, many small square platelets of YBCO (up to dimensions 2X2XO.l mm³) had crys- tallized on the exterior surface of the lower part of the crucible. So far the growth process is not yet understood, but involves of course reaction of the starting mixture with the CuO of the crucible, which roughly maintains its initial form.

3. Morphology of growth spirals

We have carefully examined as-grown crystals by means of an optical microscope. Growth steps and growth spirals were observed on a large proportion of the crystals examined. The spirals observed show clearly octagonal forms (fig. 1 ). In order to under- stand the occurrence of these unusual octagonal spi- rals, let us firstly consider theoretically the morphology of growth spirals ofYBCO by means of the periodic bond chain (PBC) theory [5]. A de- tailed PBC analysis ofYBCO is beyond the scope of this paper and only the results which are helpful to explain our observations are given.

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190 B.N. Sun et al. I Growth spirals on single crystals ofYBa₁Cu₃07_x

Fig. 1. Photographs showing a simple octagonal spiral (a) and the cooperation of octagonal spirals originating from neigh bor- ing dislocations (b).

In general the morphology of a growth spiral is principally controlled by the roughness of the spiral step [ 6]. If the step is rough, which means that the kink density is high, the step can advance indepen- dently of the crystallographic directions and the spi- ral will become circular. If the spiral step is smooth, which occurs when the kink density is low, the crys- tallographic directions become pronounced and the spiral will be more polygonized. During the growth process of a crystal, growth steps along stronger bonds contain a lower kink density than those along weaker bonds. Therefore, when a growth spiral is polygon- ized, the morphology is defined by the stronger bonds in the face. These stronger bonds should be consis- tent with the PBC directions in the face. A PBC is an uninterrupted sequence of periodically repeated strong bonds in a certain crystallographic direction [ 5]. By a strong bond is meant here a bond which is in the first coordination sphere of an ion, atom or molecule. Only the bonds formed during the crystal

growth process are considered. A PBC must have a stoichiometric composition.

The crystal growth of YBCO occurs at a temper- ature above 900 o C, a temperature at which the te- tragonal phase is the thermodynamically stable one.

The crystal structure of tetragonal YBCO is shown in fig. 2 [7-10]. For this relatively complicated structure we consider simply the "primitive PBC"

which is defined as an uninterrupted periodic chain ( of strong bonds, disregarding the conditions of stoi- , . chiometry and electroneutrality [ 11]. We simplify by admitting that growth units ofYBCO are simply single atoms. Therefore, all the bonds are taken into consideration for PBC analysis. For the (001) face of YBCO, two different "primitive PBCs" can be distinguished: (I) parallel

0

00), consisting of · · ·- Cu,-03-Cu2-03-· · · chains and (II) parallel

<

110) consisting of .. ·-Ba-0,-Ba-02-· ··chains. Evi- dently, two PBCs can be obtained from these two primitive PBCs, respectively. The

<

100) PBC is by far stronger than the

<

110) PBC. Thus, if the spiral steps are smooth, the spiral on a ( 001) face ofYBCO will be a square bounded by

<

1 00) directions, the strongest PBC directions in the face. Under the con- dition corresponding to a higher roughness of spiral steps, the square spiral will be truncated by

<

¹¹⁰⁾

directions, the next-strongest PBC directions in the

Fig. 2. Crystal structure of tetragonal YBa2Cu₃07_x with indica- tion of the two "primitive PBC" directions.

B.N. Sun et al. I Growth spirals on single crystals ofYBa₂Cu₃0₇_x 191

face. In this case, we obtain an octagonal spiral. While for still higher roughness of spiral steps, circular spi- rals would be expected.

As shown in fig. I, the formation of the theoreti- cally deduced octagonal spirals on ( 00 I ) face of YBCO are confirmed by our observations. Square growth spirals and growth steps have been observed on ( 001) faces of YBCO crystals grown in Ah0₃ crucibles both with Ga203 as dopant (fig. 3) [ 12]

and without dopant [ 13]. The octagonal and square spirals obtained with CuO and Al203 crucibles, re- spectively, are probably due to a difference in spiral step roughness and related to the respective absence and presence of Al ( Ga) impurities in the growth medium.

In conclusion, the observed morphologies of growth spirals are in good agreement with the result of our simplified PBC analysis and show that the presence of Ga and AI in the growth medium may

Fig. 3. Square growth spiral observed on (001) face of a YBCO crystal grown in Al₂0₃ crucible with Ga₂0₃ as dopant.

decrease roughness of growth steps on ( 00 1 ) faces of YBCO.

Acknowledgements

The authors would like to thank E. Burkhardt and R. Cros for technical assistance. This work was sup- ported by the Swiss National Science Foundation.

References

[ 1] J.G. Bednorz and K.A. Muller, 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, Q. Wang and C.W. Chu, Phys. Rev.

Lett. 58 (1987) 908.

[ 3] The special issues on High Temperature Superconductors (Materials Aspects I and II), J. Crystal Growth 85 ( 1987) 569-614 and 91 (1988) 255-339.

[ 4] R. Boutellier, B.N. Sun, H.J. Scheel and H. Schmid, J.

Crystal Growth, submitted ( 1988 ).

[5] P. Hartman andi\V.G. Perdok, Acta Cryst. 8 ( 1955) 49, 521.

[ 6] I. Sunagawa and l?. Bennema, in: Preparation and Properties of Solid State Materials, Vol. 7, ed. W.R. Wilcox, (Marce1 Dekkerlnc., New York, Base!, 1982) pp. 1-129.

(7] S. Katano et al., Japn. J. Appl. Phys. 26 ( 1987) L1049.

[8] G. Van Tende1oo, H.W. Zandbergen and S. Amerlinckx, SolidStateCommun. 63 (1987) 389.

[9] M. Hervieu, B. Domenges, C. Miche1 and B. Raveau, Europhys. Lett. 4 (1987) 205.

[ 10] B. Domenges, M. Hervieu, C. Michel and B. Raveau, Europhys.Lett.4 (1987) 211.

[ 11] J. Hart, Thesis, Leiden University, The Netherlands ( 1976).

[ 12] B.N. Sun, R. Boutellier and H. Schmid, unpublished.

[ 13] H. Rabe, E. Burkhardt, J.-P. Rivera, H. Schmid, E. Walker, W. Sadowski, M. G. Karkut, L. Antognazza, J.M. Triscone and 0. Fischer, Ferroelectrics, t~ be published.