Crystallographic Axis Transition of Sm1+xBa2-xCu3O7-δ FilmPrepared by Liquid Phase Epitaxy (LPE)

Chen Y. Tang,† Yan Q. Cai,† Wei Li,‡ Li J. Sun,† Xin Yao,*,† and Milos Jirsa§

Department of Physics, State Key Laboratory for Metal Matrix Composites, Shanghai Jiao TongUniVersity, 800 Dongchuan Road, Shanghai 200240, P. R. China, Instrumental Analysis Center,Shanghai Jiao Tong UniVersity, 1954 Huashan Road, Shanghai 200030, P. R. China, and Institute ofPhysics ASCR, Na SloVance 2, CZ-182 21 Praha 8, Czech Republic

ReceiVed April 30, 2008; ReVised Manuscript ReceiVed December 21, 2008

ABSTRACT: The effect of two significant factors, flux composition and growth atmosphere, on the crystallographic axis orientationof the Sm1+xBa2-xCu3O7-δ “SmBCO” film was studied. We succeeded in achieving a pure a-axis SmBCO LPE film by applyingCu-rich Ba-Cu-O flux (Ba/Cu ratio 3:7) and 1 atm oxygen atmosphere. Compared to YBCO, it is more difficult to grow SmBCOalong a-axis. Several possible reasons were considered, especially the mutual substitution of Sm3+ and Ba2+, which may preferc-axis growth. Significant features of the LPE process, like nucleation, supersaturation, and growth rate, manifest tendency to preventa-axis growth.

Introduction

Because of the large anisotropy of REBa2Cu3O7-δ “REBCO”(RE ) rare earth) crystals,1 it is highly desirable to preparehigh-quality films in both principal orientations for bothfundamental study and applications. For instance, an a-axisYBCO film is an indispensable physical basis for sandwich-type Josephson junction devices,2-5 whereas a c-axis orientedfilm is required for coated conductors.6 Substitution of otherRE elements for Yttrium can bring some advantages, e.g.,SmBCO and NdBCO have higher typical superconductingtransition temperatures (Tc) than YBCO. Besides, it has beenfound that SmBCO has a high current density in high magneticfields.7 Although the preparation of c-axis REBCO film has beenextensively studied, the a-axis REBCO film growth and the a-ctransition mechanism remain still insufficiently understood. Mostof the work yet done, including NdBCO,8 ErBCO,9 PrBCO,10

etc., was performed on samples prepared by conventional vaporphase deposition, such as PLD. Only a few works have usedliquid phase epitaxy.11-13 Compared to vapor phase deposition,LPE method can easily achieve high quality of the superconduc-tor film because of the growth conditions close to the thermo-dynamic equilibrium.14 In the study of a-c transition in YBCOfilms NdGaO3, (110) substrates have been commonly used.15-17

NdGaO3 (NGO) is also suitable as a buffer layer for coatedconductors.18 Recently, our group reported that oxygen atmo-sphere could strongly encourage the growth of a-axis YBCOfilms by LPE.19,20 Also, a higher growth rate of SmBCO andNdBCO can be reached, due to the higher solubility of Sm andNd in BCO flux in comparison with yttrium. However, growthof a-axis SmBCO films by LPE has not yet been studied at all.In this paper, we report on LPE growth of SmBCO films onNGO substrates. For the first time, pure a-axis orientatedSmBaCuO films were grown by combination of flux composi-tion control and 1 atm oxygen atmosphere. The influence onthe a-c coming from a difference in growth modes in YBCO

and Sm1+xBa2-xCu3O7-δ (“REBCOss”, materials with RE/Basolid solution) was studied.

Experimental Section

The experiments were performed by means of vertical dippingmethod.21 A Sm2O3 crucible was used to provide the Sm solute andminimize contamination from the crucible. The solvent with a molarratio of Ba:Cu ) 3:5 or Ba:Cu ) 3:7 was prepared by calcining rawpowders of BaCO3 and CuO in air at 900 °C for 48 h. Depending onthe growth conditions, the growth temperature was set between 1040and 1060 °C. The oxygen atmosphere was provided by an airtightsystem with gas circulation. The oxygen partial pressure (pO2) waschecked in situ by a TORAY LC-750 Oxygen Analyzer. (110) NdGaO3

single-crystalline substrate was used in the dipping. The rotation rateof the substrate was 80 rpm. The dipping time was about 10 s. Afterdipping, the film was annealed in flowing oxygen at 300 °C for 72 h.

The film morphology was observed by optical interference micro-scope (Olympus BX-51M). X-ray diffraction (Rigaku D/MAX255ovl/84) was performed to identify the out-of-plane orientation of the film.

Results and Discussion

By varying the Ba/Cu ratio in the melt and the growthatmosphere, differently oriented SmBCO LPE films wereobtained. Table 1 briefly summarizes our experimental results.The optical images of the top morphology of the films arepresented in Figure 1. In the case of dipping in Ba:Cu ) 3:5flux, as shown in Figure 1a, square spiral and c-axis twinningclearly manifest c-axis oriented normal to the film surface.Figure 1b shows an a-c mixed growth taken from an edge areaof the film. The green dashed line indicates the boundarybetween a- and c-axis growth regions. In both parts, typicaltwinning lines and spirals can be clearly identified. Figure 1cgives a magnified picture of the film surface dipped into a Cu-rich Ba-Cu-O flux (3:7 Ba:Cu) under 1 atm oxygen atmo-

* Corresponding author. Tel.: 86-21-54745772. Fax: 86-21-54741040. E-mail:xyao@sjtu.edu.cn.

† Department of Physics, Shanghai Jiao Tong University.‡ Instrumental Analysis Center, Shanghai Jiao Tong University.§ Institute of Physics ASCR.

Table 1. Experimental Results of SmBCO LPE Film Using DifferentFlux Composition and Atmosphere

Ba/Cu ratio inthe flux 3:5 3:7

atmosphere air O2 air O2

growth temperature(°C)

1050 1075 1042 1060

growth result dominant c-axis a-c mixed a-c mixed a-axis

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sphere. Typical a-axis twinning can be seen under polarizedlight. Most area of the film is rather flat and no spiral wasobserved. A SEM cross-section view was present in Figure 2.The white dashed line indicates the boundary of the film and

the substrate. The average thickness of the film is about 6 µm.Some attached flux can be seen on the surface of the film.

Figure 3 shows the θ-2θ XRD profiles of the film fromFigure 1. Only (h00) and (0k0) series peaks appeared in Figure

Figure 1. Top view of SmBCO LPE films with different surface morphology. (a) Typical c-axis square spiral and twinning; (b) a-c mixed regionat the film edge. The green dashed line indicates the boundary between a- and c-axis growth area. (c) Optical polarization micrograph of the a-axisSmBCO film. Typical a-axis twinning is seen.

Figure 2. Cross-section view of SEM pattern of a-axis SmBCO LPE film. The dashed line indicates the boundary of the film and the substrate.

1340 Crystal Growth & Design, Vol. 9, No. 3, 2009 Tang et al.

3(a), which corresponds to the film in Figure 1c. No (00L) peak,indicating c-axis growth, was detected: a pure a/b-axis growthwas reached. Although we observed a-axis area in Figure 1b,only (00L) peaks present in Figure 3b. It is possibly due to thevery low fraction of the a-axis area. Besides, the a-axis areaswere located on the edge of the film, which made it harder tobe detected by XRD.

Compared to YBCO, the a-c transition in SmBCO occursonly under very special conditions. In YBCO system a-axis filmgrowth can be accomplished either by using Cu-rich Ba-Cu-Oflux in air19 or by applying 1 atm PO2

.20 In contrast, in theSmBCO system all our attempts to get a-axis film applying samemethods have failed. Therefore, barriers preventing the out-of-plane growth of SmBCO films require a serious discussion. First,from the point of view of SmBCO solid solution, the reversiblesubstitution between Sm3+ and Ba2+ may facilitate the prefer-ential c-axis growth of SmBCO. According to the surfacemigration concept,22 in the case of REBCO system, c-axisgrowth requires a longer distance for the atoms to move to anappropriate site than a-axis growth. The kinetic limit plays acritical role in the a-c transition mechanism. In YBCO, the Yatom must travel at least 1 unit cell length along the c-axis from

one yttrium plane to another one. However, in SmBCO, becauseof the opportunity of Sm3+ ions to occupy Ba2+ sites, the abovetravel distance can be divided into a few short jumps. In Fig-ure 4a, possible diffusion paths of Sm3+ are presented. Forinstance, Sm3+ can jump to Ba site (Ba site 1) above and thenreach the terminal Sm site at the growth edge at a lower level.The same mechanism, in opposite direction, can undergo Ba2+

ions. In Figure 4b), the effect of Sm-Ba substitution on thegrowth mode is illustrated. When the kinetic energy was low,a long distance jump (marked as the red arrow) was forbidden,but a short distance jump (marked as the green arrow) waspossible. As a result, in the SmBCO system, the growth of c-axisoriented film was still possible in low kinetic energy conditions.However, the a-axis oriented growth was not strongly affectedby the substitution since the minimum diffusion distanceremained the same from one up layer to the lower layer. It wasreported that with the increase of x in the Sm1+xBa2-xCu3O7-δ

solid solution, the lattice parameter of c-axis rapidly reduces,whereas that of a-axis shows only a little change.23 Thus, theminimum jump length for both Sm3+ and Ba2+ required forc-axis growth of SmBCO is reduced. Because of the suppressedkinetic barrier, the growth of the c-phase is preferred. This might

Figure 3. XRD profile of (a) the a-axis SmBCO film grown at 1060 °C in a pure oxygen atmosphere, (b) the a-c mixed SmBCO film grown at1042 °C in air.

Figure 4. (a) Schematic diagram of the influence of Sm3+ and Ba2+ substitution on the diffusion path of Sm3+ ion. (b) A comparison of growthmode change between a-axis and c-axis SmBCO film considering the Ba-Sm substitution.

Crystallographic Axis Transition of Sm1+xBa2-xCu3O7-δ Film Crystal Growth & Design, Vol. 9, No. 3, 2009 1341

explain why the flux with 3:7 Ba:Cu supports a-axis growth inYBCO but does not work in the SmBCO system, where it isknown to strengthen the mutual substitution of Sm and Ba.24

In SmBCO, the enhancement of a-axis growth by enriching fluxby Cu is neutralized by the solid solution phenomenon.Consequently, we need to find a growth mode, different fromYBCO that would work in fabrication of a-axis REBCOss films.Some experimental results by vapor phase deposition have beenpublished that supported our analysis. In vapor deposition, themigration driving force strongly depends on the substratetemperature. As we discussed above, the substitution of RE andBa leads to a suppression of required jump length. The reductionof diffusion length indicates the reduction of driving forcerequired for the migration. Therefore, the c-axis growth can takeplace at a lower substrate temperature. The a-c transi-tion temperature reduced with the increase of x inNd1+xBa2-xCu3O7-δ

25 and Sm1+xBa2-xCu3O7-δ.26

Second, the crystal growth mode of SmBCO in LPE deviatesfrom that of YBCO. This also introduces barriers toward a-axisfilm growth. A schematic illustration (Figure 5) helps tounderstand the causality. Because SmBCO and YBCO haveidentical crystal structure and very close crystal constants, wesuppose the nucleation mode of SmBCO is similar to YBCO,which we described before.19 One can deduce that the a-phaseSmBCO nucleates prior the c-phase and that the critical

supersaturation difference between the a-phase and the c-phasemay be close to that of YBCO. However, it was reported thatthe interfacial energy between SmBCO crystal and the melt isabout 20% lower than that between YBCO crystal and themelt.27 This means nucleation of SmBCO might occur at a smallsupersaturation. Accordingly, the curves of growth rate depend-ing on supersaturation should move left along the σ-axis, aswe illustrated in Figure 5. At the same time, the “growthwindow” of a-axis SmBCO film could be compressed by thesubstitution of Sm3+ and Ba2+ as we discussed above. In otherwords, compared to YBCO, the “a-axis dominated growthregion” of SmBCO should move toward lower supersaturationarea. In addition, the upper limit of the region, indicated by thecrossover of c-axis growth and a-axis growth, should be furthershifted to the low supersaturation side.

On the other hand, it is well-known that the liquid slope ofsamarium solubility in the solution is considerably smaller thanthat of yttrium.28 A small undercooling in the SmBCO systemcan produce a large supersaturation. In the LPE process, thesupersaturation in melt can easily exceed the a-axis growth limitsof SmBCO. The large supersaturation also leads to a high growthrate of Sm123 grains. According to the experimental data, thegrowth rate of SmBCO is several times higher than that ofYBCO.28 The high growth rate can cause that both a-phase andc-phase nuclei grow into large grains in a short period of time.This makes it difficult to isolate pure a-phase growth from thea-c mixed one.

Conclusion

In summary, solvent composition and growth atmosphereplay a critical role in the growth orientation control ofSmBCO LPE film. Pure a-axis oriented SmBCO films weresuccessfully obtained under special growth conditions.Compared to YBCO, several obstacles inhibit Sm123 filmfrom growing along a-axis. The substitution of Sm3+ for Ba2+

ions suppresses the a-c transition in SmBCO by facilitatingc-axis growth via reduction of the minimum diffusion length.The high supersaturation in the melt, high growth rate, andlow interface energy may also restrain the a-phase growth.

Acknowledgment. The authors appreciate the kind help ofDr. Gu for acquiring the XRD data. The authors are gratefulfor financial support from National Science Foundation of China(Grants 50572065, 50872082), the MOST of China (973 Project2006CB601003, 863 Project 2007AA03Z206), Shanghai Sci-ence and Technology Committee (Grant 075207028), andSpecialized Research Fund for the Doctoral Program of HigherEducation (Grant 20070248033). M.J. acknowledges supportof the Grant GACR 202/08/0722.

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