An approach to the growth of YBa2Cu3O7-x single crystals by the flux method. II

An approach to the growth of YBa2Cu3O7-x single crystals by the flux method. II

Journal of Crystal Growth 114 (1991) 269 278 North Holland ~ An approach to the growth of YBa2Cu3O7 by the flux method. II ~o, CRYSTAL GROWTH sin...

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Journal of Crystal Growth 114 (1991) 269 278 North Holland


An approach to the growth of YBa2Cu3O7 by the flux method. II



single crystals

Koichi Watanabe Department of Material Chemistry, Faculty of Technology, Gunma Unu ersity, Kiryu, Gunma 376, Japan Received 17 May 1991; manuscript received in final form 22 July 1991

The growth of YBa2Cu~O7 single crystals was performed by the slow cooling technique with a wide variety of fluxes such as Li , Na , K carbonate, fluoride, PbO PbF2, L120 MoO3, BaCO-1, 0 in and size BaCl2 and 0.2ortowith 0.3 mm the BaO in thickness CuO self grew flux from system. the self Theflux desired with acrystals small amount offrom BaF the self flux. Plate-like crystals S X 5 mm only grew 2 as a dopant. The optimum growth condition is as follows: atomic ratio of Y : Ba: Cu, 1:4:12; BaF2, 1 wt%; soaking temperature, 10000 C; soaking time, 100 h; cooling rate, S C/h; cooling range, 1000 700 0 C.

1. Introduction Single crystals of the high temperature superconductor YBa2Cu-~O7~ YBCO, have been prepared by numerous investigators in order to study the superconductivity mechanism and some physical properties, using mixtures in the BaO—CuO system as a flux [1 13]. YBCO crystals were also grown from a mixed flux of KCI NaCl though these were only sheet-like crystals [14] and from K2C03 [151 or PbO B203 [16] solvent together with other crystal phases such as Y2BaCuO5, CuO, and BaCu2O. At present, there is no fixed rule for choosing the best flux for the growth of a given material. However, it is usual to select as a flux a chemical compound containing at least one ion in common with a solute and/or to examine the application of fluxes that have been successfully used for the growth of similar compounds. The superconducting temperature in the YBCO compound is enhanced slightly by the substitution of fluorine for oxygen [17—20].It is a well-known fact, moreover, that F remarkably ions can play an important role in improving crystal growth through its effect on nucleation [21]. The present paper has two aims; one is to search for other fluxes than the self-flux of the 0022 0248/91/$03.50 c 1991

BaO—CuO system for the growth of YBCO single crystals, the other is to decide the optimum conditions for growing large YBCO crystals of good quality by adding a small quantity of BaF2 to the BaO—CuO self-flux.

2. Experimental procedure 2.1. Fluxes other than the BaO—CuO system Sintered YBa2Cu3O7 (YBCO) which was prepared from the chemical reagents of Y203, BaCO3, and CuO, or chemical reagent mixture of Y: Ba : Cu 1: 2: 3 in atomic ratio was used as a solute. A wide variety of chemical reagents such as Li, Na, and K-carbonate, -fluoride, -chloride, -hydroxide, -borate, PbO—PbF2, Li20 MoO3, Ba-chloride, -carbonate, -peroxide, -hydroxide and mixed compounds were used as a flux [22,23]. An outline of the experimental procedure is as follows: A total 20 g of the solute and flux 3in Pta 20: 80 wt% in was a 20embedded mm crucible withratio cover.was Thepacked crucible in alumina powder inside a lidded porcelain pot to decrease the temperature fluctuation of the growth solution, and the sample was melted at a —

Elsevier Science Publishers B.V. All rights reserved


K, l4alanabe

,4pp,oach to çoowth of YBa ,C u ~O


single crystals hs fluv method II

selected temperature in the range of 1200 850 C for 30 h in an electric furnace. The growth solution was lowered to the lower temperature, 1000 550 C, about 200 300 C lower than the soaking temperature, with a cooling rate of 5 C/h. At the end of a run, the growth solution in the crucible was poured out by inverting the hot crucible. Temperature regulation of the furnace was performed by a PID controller with a Pt/Pt

3. Results and discussion

I3YRh thermocouple. Further details of the cx perimental conditions are displayed in table 1.

and well-developed faces, grew from alkali carbonate fluxes at the bottom and the wall of the crucible. They contained a high concentration of metallic platinum with metallic lustre and had the dimensions of about I x 2 x 7 mm. Fig. 1 shows YBa4Cu,Pt,O~ single crystals. The growth of numerous needle-like CuO crystals is observed in the halide compound fluxes of alkali or alkaline earth system. Fig. 2 shows needle-CuO crystals growing across the crucible. The external morphology and metallic color of YBa4Cu~Pt~Ok crystals resemble closely those of CuO crystals, but the former crystals tend to be a little thicker than the latter. Addition of a small amount of boron oxide so as to depress evaporation of the alkali system fluxes or the use of borate fluxes stimulates the growth of barium borate single crystals of hexagonal-plate form with the size of 1 to 2 mm in width and 0.5 mm in thickness. As can be seen in table 1, fluoride fluxes produce fine barium fluoride crystals. This fact may be cxplained if YBa,Cu307 in the solute was de composed by fluoride compounds and converted to barium fluoride. When heavier metallic oxides such as PbO or Bi 203 were used as a flux, the very interesting crystal phases, a Ba(Pb05,~ ~ 33)03 compound of perovskite-type and of superconducting Ba(Pb063 Bi1~35)03 were ohserved. Besides the solute with the atomic mole ratio of Y: Ba: Cu 1: 2: 3, other solute cornpounds with excess or deficient Y~O3,BaO or CuO were also used for the growth of YBCO crystals. Fig. 3 shows the phase diagram in the ternary system of Y~O3BaO CuO reproduced from ref. [24]. Triangles in fig. 3 indicate the chemical compounds which were examined in de tail by the use of the KF or K2C03 flux. A wide variety of fluxes other than the BaO—CuO selfflux were used in attempts to grow the single

2.2. the BaO CuO system It is well known that the quality, size, habit, etc. of crystals grown by slow cooling are noticeably influenced by the growth conditions such as the chemical compositions of solute and flux, the kind of dopant, the soaking temperature and time, the cooling rate of growth solution and the crucible size. Sintered YBCO and a mixture of BaO and CuO were used as solute and flux, resepectivity. The chemical composition of the growth solution was 1:4 :8, 1:4: 10, and 1:4: 12 in atomic ratio of Y : Ba : Cu. BaF-, of 0.5, 1.0, and 3.0 wt°~ replacing BaO was used as a dopant in an attempt to improve the size and quality of YBCO single crystals. The soaking temperature was 1000 C and soaking time 0, 30, and 100 h, respectively. The temperature range of slow cooling was 1000 700°C and the cooling rate3 with was 5°C/h. An Al,03 crucible A of desired 30 or 150 cm lid was used as container. quantity of solute, flux and dopant was weighed, and mixed in an agate mortar. A mixture of 30 or 180 g was charged in the crucible, and was then melted at a selected soaking temperature for a selected soaking time in an electric furance in an ambient atmosphere. Identification and chemical analysis of the grown crystals were achieved by means of the X-ray diffraction (XRD) method and the inductively coupled plasma (ICP) or electron probe micro-analysis (EPMA) method, respectively, Surface observation of as-grown crystals was carned out using an interference contrast microscope of Nomarski type.

3.1. Fluxes other than the .self-,uiux oft/ic BaO CuO sy,stern Variation of the precipitated crystal phase depending on the chemical composition of flux is shown in table 1. Prismatic hexagonal crystals of YBa4Cu~Pt2Ok. which were bounded by large


K. Watanabe

Approach to growth of YBa ,Cu ~O

single crystals by flux method. 11


Table 1 Variation of the precipitated crystal phases depending on the flux used Run No.

Solute composition



Temperature range of slow cooling (°C)

Results and product

960 1150 1150 950 960 960 960 960 960 1130 1000 960 960 950

611) 900 900 650 610 610 610 610 610 900 700 610 610 650

Li2PtO3 Y03, YBCPO ~, other products YBCPO CuO,Cu,O,Cu2y~O3,YBCPO Glassy ~, other products Glassy, Y,03, other products Glassy, Y2O3. other products YBCPO CuO, Cu,Y,O~Ni crucible ~ Glassy CuO, ph BB ~‘, other products YBCPO. Y,03, other products YBCPO, Y,O3, other products CuO, Y03, Cu,Y,O other products; Ni crucible CuO, Y,03, Cu-,Y,03. other products; Ni crucible CuO. Y203, other products; Ni crucible CuO, Cu2YO3, other products; Ni crucible Other products Cu2Y,03, hydroxide compound n CuO n CuO, hp BB n CuO, YBCPO CuCl,{CuCI’COJ2H,O n CuO, other products Glassy Glassy Other products Other products CuO, hp BB BaO2, Ba4Y2O7 YBCPO. other products n CuO, BaF,, other products n CuO Cu2Y2O Cu2Y2O3 Other products Other products Other products Other products n CuO, BaF2 n-CuO, BaF, n CuO, BaF2 n-CuO, BaF2, hp BB

2 3 4 5 6 7 8 9 10 11 12 13 14


Li,CO 3 Na2CO3 K2C03 K2C03 K2C03 (40)+ K2B407 K2C03 (60)+K2B4O7 K,C03 (70)+ K2B407 K2C03(75)+K2B407 K,C03 (70)+K2B40 K2B40, K2B4O (30) K2C03 (75) + K, B407 K2C03(75)+K2B40 K2C03







YBCO+CuO (5)





YBCO+CuO (10)




18 19 ~ 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43


900 u0 1150 850 950 800 950 1150 1200 1200 1201) 1000 850 1200 960 1150 960 960 1200 1200 1000 900 1150 1150 950 950


44 45 46


KOFI KUH KCI KCI (40)+B2O3(1 g) KCI (75)+K2B40 KCI(75)+K2B4O KCI (75)+K2B407 B203 B203 (30) k) B203 (16) B2O3 (14) BaB2O4 (30) BaO2 BaF2 (16) BaF2 (45)+KF (35) BaCI2 BaCI2 BaCI, (40) BaCI2 (16) BaCl2 (29) BaCI2 (40)+BaO(20) BaCI2 (40)+BaO(20) KF KF KF KF(40)+NaF(35) + K2B4O KF+B203(1 g) KF+B2O3 (1 g) KF+B2O3(1 g)

950 950 950

900 550 550 500 550 900 1000 1000 1000 700 550 100(1 610 900 610 610 1000 1000 700 600 900 900 650 550 650 650(0) ~° 650(30) ‘~

n-CuO, BaF2 n CuO, BaF2, hp BB n CuO, BaF2, hp-BB



K. Watanabe

Approach to growth of YBa

2Cu ~O7

single crystals by flux method. II

Table 1 continued Run No.

Solute composition


47 48 49 50 51 52 53


950 950 880 950 950 900 950

650(100) 650 (30) 550 (30) 650 (30) 650 (30) 550(30) 650

n CuO, BaF2, hp BB n CuO, BaF n CuO, BaF2, hp BB n CuO, BaF2, hp BB n-CuO, BaF,, hp BB n CuO, BaF2, hp BB n CuO, BaTiO295F~53

54 55

YBCO+CuO (10) YBCO+CuO(5) +BaO(5) YBCO (10) YBCO+CuO ( 5) YBCO + CuO ( 10) YBCO (30) YBCO

KF+B2O3(1 g) K.F KF+ B2O3 (1 g) KF+B2O3 (I g) KF (40)+ B203 (1 g) KF(40)+B2O3(1 g) KF+B2O3 (1 g) +TiO, (1 g) KE+B,O3 (1 g)



n CuO. BaF2

950 650 950 650 950 650 950 650 900 600 900 600

68 69 70


KF+B2O3 (1 g) KF (90) KF+B2O3 (1 g) KF + B2O3 (1 g) KF (20) + BaO (50) KF (20)+ BaO (50) +B2O3 (1 g) KF+B2O3(l g) KF+B2O3 (I g) KF+B2O3 (1 g) KF+B203(1 g) PbO PbO PbF, PbO(40)+PbF2 (40) PbO (20)+PbF2 (60) PhO (30)

950 950 950 950 1100 960 9n0 960 960 1100

71 72


Bi2O3 Bi2O3(30)

950 650 1100 800

73 74 75 76 77


MoO3 MoO3 (30) Li2MoO4 Li2MoO4 LiMoO4

950 900 1150 950 950

56 570)

58 59 60 61 62 63 64 65 66


Temperature range of slow cooling C)

650 650 650 650 800 610 olO 610 610 800

650 600 900 650 650

Results and product

n-CuO, BaF n-CuO, BaF2 n CuO, BaR, n-CuO, BaF, BaF2, other products; BaF,, other products n-CuO, BaF, n-CuO, BaF2 n CuO, BaF2, hp BB n CuO, BaF2, hp-BB Ba2Y2O3, other product Other products BaF2, other products Other products Other products CuO, Cu2Y2O3, Ba(Pb53 s5~Y15 33)03 fl Bi2O3, other products CuO, Cu2Y2O5, Ba(Pb1155 Bi55 35)03 Other products CuO, BaMoO4 n-CuO, BaMoO4 CuO, BaMoO4 CuO. BaMoO4

° YBCPO; prismatic hexagonal YBa4Cu2Pt2O0 crystal (fig. 1). ~ Other products; unknown crystals. ~ Glassy; amorphous phase. d) Ni crucible; Ni crucible was used in Run Nos. 9, 14, 15, 16, and 17 o) hp BB; hexagonal plate crystals of barium borate. ~ YBCO+BaO(4); sintered YBCO(16 wt%)+BaO(4 wt%). g) YBa3Cu2O5 ~ sintered solute. ~ Run No. 19; Fe crucible was used. n CuO; needle like CuO crystal (fig. 2). ~ KCI (40)+B203 (1 g); run No. 21 YBCO-solute (60 wt%)+KC1 flux (40 wt%)+B2O3 (1 g) YBCO (12 g)+KCI (8 g) +B2O3 (1 g). k) B2O3 (30); B2O (30 wt%). I) YBCO(p); powder mixture of chemical reagents. °~ Run Nos. 45 and 46; 950 650 (0) and 950 650 (30) mean zero and 30 h of the soaking time, respectively. o) YBCO (60); YBCO (60 wt%). °~ In run Nos. 57 and 58, non sintered mixtures of Y : Ba: Cu 1:2:2.6, 1:2:2.2 were used, respectively.

K. Watanabe


/ Approach

to growth of YBa

2Cu )07

single crystals by flux method. II




Fig. 1. YBa4Cu,Pt2O,~single crystals grown from the fluxes of alkali carbonate system.

crystals of YBa2Cu3O7 however, we could not find a promising flux for the growth of YBa2Cu3 07 crystals.





________ ______


3.2. The self-flux of the BaG CuO system As described before, growth of YBCO single crystals with the self-flux of BaO—CuO has been achieved by numerous investigators [1—13].As can be seen in fig. 3, a portion of the primary phase field of YBCO crystal in the ternary system of Y203—BaO—CuO exists in the region of partial melting and the chemical composition in this portion corresponds nearly to the respective atomic mole ratios of Y: Ba : Cu 1:4: 8, 1: 4: 10 and 1: 4: 12, which are indicated by circles in fig. 3. The growth conditions of YBCO single crystals with the BaO—CuO self-flux and the experimental results are summarized in table 2. The experi—

_____ _____

4 miii Fig. 2. Needle CuO crsstals growing across the crucible.



K Watanabe

Approach to growth of YBa ,Cu 50

single crystals byflux method. II








BA ‘1 0

Pr nary piase field of yBa2cu3o7


BAY2 01

1:1:8 1:1:10



1:1:12 —



40 Y20u205



Regi n of partial melting



Fig. 3. Phase diagram in the ternary system of Y2O3 BaO CuO (reproduced from reference [24]). Triangles indicate the solute compositions used in the flux of potassium fluoride or potassium carbonate system. Circles indicate experimental compositions by the self flux of the BaO CuO system.

melts of run Nos. 1 3 were carried out to search for a growth solution of suitable chemical composition for the growth of YBCO crystals. In the experiment of run No. 3, several plate-like crys-

tals, approximately 2 mm square from the inside of cavities formed fied melt near the bottom of the sintered YBCO and the chemical

in size, grew in the solidicrucible. The reagent were

Table 2 Experimental conditions and results with BaO CuO self-flux Run No. 1 2 3 4 5 6 7 8 ‘° 9 10 11 12 13 140) 15 ~

Composition YBa:Cu

150 cm

1:4:8 1:4:10 1:4:12 1:4:10 1:4:10(p) 1:4:12 1:4:12(p) 1:4:12 1:4:12 1:4:12 1:4:12 1:4:12 1:4:12 1:4:12 1:4:12 3 crucible.

Dopant BaF2

Soaking time (h)

Cooling rate (°C/h) 5 5 5 5 5 5 5 5

0.5 1.0 3.0 0.5 1.0

30 30 30 100 100 100 100 100 0 100 lOt) 100 100 100 100

5 5 5 5 5

Crystal size Maximum

2.0mm 1.2 mm 1.0mm 2.0mm 4.1 mm Sinter 3.0 mm 2.5mm 4.0mm 5.0mm

Average 200 ~xm 100 ~xm 1.0mm 0.2 mm 0.5mm 1.0mm 100 gem 1.0mm <1.0mm 1.0 mm 1.5mm 1.0mm 1.0mm 2.0mm

K. Watanabe

Approach to growth of YBa,Cu 507

single crystals by flux method. II


100 urn Fig. 4. YB.i

2Cu ~0— sin~lccrystals grov.n froni the self flux of B,iO CuO system.

used in run Nos. 4 7 to check its suitability as a solute for the growth of YBCO single crystals. The size of crystals grown depends on crucible used (run No. 8). In general, the precipitation of a crystal in the slow cooling method occurs during the cooling process of the solution, in the present study, however, it occurred when the solution was maintained at a constant temperature (run Nos. 9 and 10). It is already known that the size and quality of a grown crystal may be improved remarkably by substituting a portion of the solute by a fluoride compound containing the same cation [21]. Substituting BaF2 for a portion of BaO, we tried to improve the size and quality of the YBCO crystals. Comparatively large and good quality YBCO single crystals were obtained by the substitution of 0.5 or 1.0 wt% of BaF2 (run Nos. 11 13) and in3 run Nos. 14were andused. 15 larger in volume As a crucibles of 150 cm result, YBa 2 single crystals mm in maximum2Cu3O7 size and 0.2 to 0.3 mm of in 5.0 thckness were obtained in run No. 15. As described previously, YBCO plate-crystals develop from the inside of cavities in the solidified melt (fig. 4). There is no way to remove the crystals except manually. Fig. 5 shows the fragments of a YBCO

crystal (run No. 11). A BaCu3O4 crystal, observed rarely in the present study, is shown in fig. 6. The growth mechanism of a crystal from high temperature solution is commonly divided into two types: surface nucleation which occurs higher




)~ 4 j~ t VT



~ .~



~ I


&L1 ~1 L.







Fig. 5. Fragments of


B(’() crystal.



K. Watanahe

Approach to growth of YBa,Cu ~0

single crystals by flux method. 11


1.0mm Fig. 6. BaCu 30 crystal.

supersaturations and spiral growth which occurs at lower supersaturations. The former is grown by two-dimensional nucleation taking place on the flat surface of the crystal [25]. The latter, on the



other hand, is caused by screw dislocations and is characterized by polyhedral or circular spiral growth patterns being composed of the relatively shallow steps which are observed on the crystal



Fig. 7. Polyhedral growth pattern consisting of the bunching steps on the crystal surface.

K. Watanahe

Approach to growth of YBa ,Cu 507

surface [261. The growth solution being made up 1:4:8 or 1:4: 12 in the atomic ratio of Y : Ba : Cu is equivalent to a YBa~Cu200 solute concentration of about 48 wt% or 40 wt%. This is a high concentration of solute for crystal growth from high temperature solution. Consequently, it will be expected that the surface nucleation mechanism by the two-dimensional nucleation is dominant for the growth of YBCO crystals in the present study. However, we could not find characterization of 2D nucleation for a growth pattern evidence. Fig. 7 shows the polyhedral growth pattern which was observed on the surface of a YBCO crystal. Komatsu et al. [27] and Sun and Schmid [121 observed similar polyhedral growth patterns on the crystal surfaces of other oxide superconductors. This fact suggests that the growth of YBa2Cu3O7 crystals from the selfsolvent in the BaO CuO system is due to the spiral growth mechanism.

4. Conclusion The growth of YBa7Cu3O7 single crystals was attempted from the self-flux of the BaO CuO system and a wide variety of fluxes other than the BaO—CuO system, using the slow cooling technique. (1) YBCO single crystals were not obtained from fluxes other than the self-flux of the BaO—CuO system, although prismatic YBa4Cu2Pt2Oa single crystals with well-developed faces, needle-like CuO crystals, and many other crystals were grown. (2) YBCO crystals of mm size were obtained using the BaO—CuO self-flux and the use of BaF2 instead of a portion of BaO was recognized to be noticeably effective for the crystal size improvement. (3) The optimum conditions for the crystal growth of YBaCu3O7 are as follows: atomic mole ratio of Y : Ba : Cu, 1: 4: 12; amount of BaF2 as a dopant, 1.0 wt%; soaking temperature, 1000 C; soaking time, 100 h; cooling rate, 5 C/h; ternperature range of cooling, 1000 700 C. (4) It is conjectured that the growth mechanism of YBCO single crystals in the present study is 0

single crystals by flux method II


spiral growth which originates from a screw dislocation center.

Acknowledgements The author thanks Professor F. Takei of Tokyo University and Professor H. Komatsu of Tohoku University for their helpful discussions and suggestions. This work is partly supported by the Grand-in-Aid for Science Research on Priority Areas “Mechanism of Superconductivity” (No. 031) from the Ministry of Education, Science and Culture of Japan.

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nikov, O.K. Melnikov and SM. Stishov, J. Crystal Growth 91(1988) 302. [9] 1. Wolf. W. Goldacker, B. Obst, G. Roth and R. Flukiger, J. Crystal Growth 96 (1989) 1010. [10]l-1.J. Scheel, J. Less Common Metals 151 (1989) 199. [11]F. Holtzherg and C Feud, J. Crystal Growth 99 (1990) 915. [12] B.N. Sun and H. Schmid. J. Crystal Growth 100 (1990) 297.

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Approach to gronth of YBa ,Cu ~()

118] KM. Cirillo, J.C. Wright, J. Seuntjens, M. Daeumling and D.C. Larbalestier. Solid State Commun. 66 (1988) 1237. [19]BA. Tallerchik, A.0. Olesk, TN Egorova, ‘V V. Popou, R.V. Parfeniev ML. Shubnikos. DV. Mashovets and DV. Smirnov, J. Less Common Metals 150 (1989) 311. [20] ME. Yan. G.S. Grader. W.W. Rhodes and H.C. Ling, J. Mater. Sci. 24 (1989) 70). [21]E.A. Giess, CF. Guerci, J.D. Kuptsis. IF. Chang and D.J. Robbins, J. Crystal Growth 60 (1982) 219. 1221 K. Watanahe, J. Crystal Growth 100 (1990) 293. [71]G.F. Holland, R.L. Hoskins, MA. Dixon. PD. Ver Noooy. H C zur Lonve, G. Brimhall, D. Sullivan. R. Cormia, I-lW. Zandbergen. R. Gronsk~and AM. Stacy.

single crystals by flux method. II

in: Chemistry of High Temperature Superconductors. Lds DL. Nelson, MS. Whittingham and T.F. George (Am. Chern. Soc., Washington, DC. 1987). 124] R.S. Roth, K.L. Davis and JR. Dennis. in: Ceramic Superconductor’,. Ed. W.J. Smothers (Am. Ceramic Soc., Columbus. OH. 1987). [2S] D. Elwell and H.J. Schcel, Crystal Growth from High Tempciature Solutions (Academic Press, London. 197’s). [26]K. Watanahe and I. Sunagawa. J. Crystal Growth ‘s7 (198’) 367 [27]11. Komatsu, N. Hayashi and 1. lnoue. in: Chemistry of Oxide Superconductors, Eds. K. Fueki and K. Kitazawa (Kodansha, Tokyo, 1988) (in Japanese).