Review Growth Techniques for Layered Superconductors

Masanori Nagao

Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 400-8511, Japan; [email protected]

Academic Editors: Yoshikazu Mizuguchi and Antonio Bianconi Received: 23 August 2017; Accepted: 16 October 2017; Published: 16 October 2017

Abstract: Layered superconductors are attractive because some of them show high critical temperatures. While their crystal structures are similar, these compounds are composed of many elements. Compounds with many elements tend to be incongruent melting compounds, thus, their single cannot be grown via the melt-solidification process. Hence, these single crystals have to be grown below the decomposition temperature, and then the flux method, a very powerful tool for the growth of these single crystals with incongruent melting compounds, is used. This review shows the flux method for single- technique by self-flux, chloride-based flux, and HPHT (high-pressure and high-temperature) flux method for many-layered superconductors: high-Tc cuprate, Fe-based and BiS2-based compounds.

Keywords: flux method; incongruent melting compounds; solution growth; high-Tc cuprate superconductor; Fe-based superconductor; BiS2-based superconductor

1. Introduction

This review presents the single-crystal growth of high-Tc cuprate, Fe-based and BiS2-based superconductors as typical layered superconductors. These layered superconductors form many analogous compounds and show other interesting properties. Additionally, layered superconductors have high anisotropy. In order to reveal such intrinsic properties, their single crystals are necessary. However, layered superconductors are composed of multiple elements and they are incongruent melting compounds. This suggests that the growth of single crystals cannot use the melt-solidification process and, thus, the solution-growth process is necessary. During the single-crystal growth process, the temperature must be lower than the decomposition temperature of the layered superconductors. Hence, the flux method is useful for single-crystal growth of layered superconductors. In this review, the single-crystal growth technique for high-Tc cuprate, Fe-based and BiS2-based compounds using various fluxes are introduced.

2. Single-Crystal Growth Techniques for Incongruent Melting Compounds In order to grow single crystals, their raw materials of layered superconductors have to become solution (liquid phase) below the decomposition temperatures of the products. The solvent of that solution (liquid phase) is called “flux”. Flux plays a role as a solvent in the solution-growth process. Therefore, flux should be composed of low-melting compounds, such as metals, alkali chlorides, and other compounds with a eutectic composition. The conventional flux method, the traveling solvent floating-zone (TSFZ) method, the top-seeded solution growth (TSSG) method, and the high-pressure and high-temperature (HPHT) flux method are briefly explained in this chapter. The conventional flux method does not have to use a special apparatus, but obtained crystals are small. On the other hand, TSFZ and TSSG methods use special apparatuses which are expensive. However, these methods

Condens. Matter 2017, 2, 32; doi:10.3390/condmat2040032 www.mdpi.com/journal/condensedmatter Condens. Matter 2017, 2, 32 2 of 13 can grow large-sized crystals, and the HPHT flux method is necessary for single-crystal growth under high pressure.

2.1. (Conventional) Flux Method There are two kinds of flux methods. The components of the flux are the elements in the products; this is called as “self-flux method”. The other is the use of flux which does not include the same element in the products. The flux and raw materials for target compounds are mixed using a mortar, and put in the crucible. The mixed powder is heated for the solution-growth process, and then the single crystals are obtained. Generally, oxide materials (e.g., high-Tc cuprate compounds) are heated in open atmosphere, and non-oxide materials (e.g., Fe-based and BiS2-based compounds) are performed in a closed atmosphere, such as sealed in a quartz tube in vacuum. Finally, the important point of flux selecti is the following: flux should not react with the target materials, and should be easily separated from the grown single crystals. Examples of single-crystal growth of a layered superconductor using the flux method are shown below.

2.1.1. High-Tc Cuprate Superconductor

Many compounds are reported as high-Tc cuprate superconductors. This review shows high-Tc cuprate of (R,Ae)2CuO4 ((R,Ae)-214 phase) (R: rare earth element, Ae: Ba,Sr,Ce) [1], RBa2Cu3Ox (R-123 phase) [2], Bi2Sr2Can−1CunOx (n = 1: Bi-2201, n = 2: Bi-2212, n = 3: Bi-2223 phase) [3] single crystals. CuO-based (self-flux) or alkali chloride-based compounds are generally employed for flux in the single-crystal growth of high-Tc cuprate. Single crystals several millimeters (mm) in size are obtained by the flux method. When nominal compositions of precursors are (La,Sr)2CuO4:CuO = 1:4 (molar ratio) and YBa2Cu3Ox:CuO = 3:2 (molar ratio), (La,Sr)-214 and Y-123 single crystals are grown from CuO flux (self-flux) [4]. Another example is the growth of Y-123 single crystals using 7BaCuO211CuO as a flux. Y-123 single crystals are grown from the precursor of Y-123:7BaCuO211CuO = 16:9 (molar ratio) [5]. In contrast, Bi-2212 single crystals are obtained from the precursor of the cation ratio Bi:Sr:Ca:Cu = 2:2:1:2 which are weighed from Bi2O3, SrCO3, CaCO3, and CuO. This precursor is put in a crucible with a weight on the cover (cap), and then the precursor is melted for Bi-2212 single-crystal growth. The cation ratio of this precursor is the stoichiometric composition for Bi-2212, and their melt acts as flux (self-flux) [6]. Alkali chloride-based compounds are also useful for flux. Y-123 and Bi-2212 single crystals are grown using KCl-based flux. The melting point of KCl is 776 ◦C although eutectic composition exists between KCl and NaCl. Its composition is KCl:NaCl = 1:1 (weight ratio) and eutectic temperature is 657 ◦C. This eutectic composition (KCl/NaCl) has the advantage for the growth of single crystals by lowering the melting points. Y-123 single crystals with sizes of several millimeters (mm) are grown from 2 wt % KCl/NaCl added Y-123 raw materials [7]. In Bi-2212 single crystals, some reports show using only KCl for flux [8–10]. In one instance, much KCl flux is added in Bi-2212 raw materials for single-crystal growth, which nominal composition is Bi-2212:KCl = 1:99 (weight ratio) [9]. Finally, I introduce the growth of single-crystalline whiskers. Whisker is a special morphology of the . They have high aspect ratio (needle shape) with perfect crystallinity. However, the growth mechanism is not completely clear. Growth of whiskers of high-Tc cuprate layered superconductors is reported [11]. Those growth methods use glassy precursor and/or special flux. High-Tc cuprate superconductors are composed of many elements. Therefore, growth of high crystallinity single crystals is difficult. However, the whiskers have perfect crystallinity, and are free of dislocation. Figure1 shows a typical scanning electron microscope (SEM) image of a Y-123 single-crystal whisker. The growth method of high-Tc cuprate single crystal whiskers has two routes, which use a glassy precursor and a TeO2/Sb2O3-doped precursor. When using the glassy precursor method, Al2O3-doped Bi-2212 raw materials are melted and quenched, thus, the glassy precursor Condens. Matter 2017, 2, 32 3 of 13

Condens. Matter 2017, 2, 32 3 of 13 (Bi–Sr–Ca–Cu–Al–O) is obtained. Bi–Sr–Ca–Cu–Al–O glassy precursor is heat-treated and then Bi-2212 whiskers are in highgrown-Tc oncuprate the precursor superconductors. [11]. However, Whiskers this method of Bi-2212, can Bigrow-2201, only Bi- Bi-22122223, and whiskers R-123 inphases high- canTc cuprate be grown superconductors. by TeO2/Sb2O3 Whiskers doping method. of Bi-2212, For Bi-2201, Bi-2212 Bi-2223, and Bi and-2201, R-123 TeO phases2-doping can and be grownCa-rich by precursors TeO2/Sb2O 3 aredoping used method. for whisker For Bi-2212 growth and Bi-2201, of which TeO2-doping nominal and compositions Ca-rich precursors are areBi2Sr used2Can forCu whiskernTe0.5Ox ( growthn = 1 for of Bi which-2201, nominal n = 2 for compositions Bi-2212). Those are precursors Bi2Sr2CanCu arenTe pressed0.5Ox ( ninto= 1 pellets for Bi-2201, and nheated.= 2 for Whiskers Bi-2212). Thoseare obtained precursors on the are pressedpellets [12,13]. into pellets Bi1.6Pb and0.4Sr heated.2.0Ca2.0Cu Whiskers3.0Ox single areobtained phase powder on the pellets(1.0 mol) [12 and,13]. TeO Bi1.62Pb (0.50.4 Srmol)2.0Ca are2.0 Cumixed3.0Ox andsingle pelletized. phase powder This pellet (1.0 mol) is heated, and TeO and2 (0.5 then mol) (Bi,Pb) are mixed-2223 andwhiskers pelletized. are obtained This pellet [14]. is On heated, the other and thenhand, (Bi,Pb)-2223 R-123 whiskers whiskers can also are obtainedbe grown [ 14from]. On TeO the2/Sb other2O3 hand,doped R-123precursor whiskers pellets. can TeO also2 or be Sb grown2O3 doping from TeOand2 /SbR and2O 3Badoped-rich precursors precursor pellets.(R1.5–2.0Ba TeO3Cu23Teor0.5 SbO2xO or3 dopingR1.5–2.0Ba and2.75Cu R3 andSb0.5O Ba-richx) are used precursors and the (R same1.5–2.0 Batreatment3Cu3Te0.5 isO applied.x or R1.5–2.0 R-123Ba2.75 withCu R3Sb = 0.5Y,O Sm,x) are Eu, used Gd, andDy, theHo, same Er, Tm, treatment and Yb is applied. whiskers R-123 can with be grown R = Y, Sm, from Eu, the Gd, precursor Dy, Ho, Er, pellet Tm, and [15– Yb17]. whiskers Additionally, can be grownCa-substituted from the R precursor-123 with pellet R = Y, [15 La,–17 Nd,]. Additionally, Sm, Eu, Gd, Ca-substituted Dy, Ho, Er, Tm, R-123 Yb, with and R Lu = Y, whiskers La, Nd, Sm, are Eu,grown Gd, from Dy, Ho,RBa Er,2-3Cu Tm,3Ca Yb,1.00–1.75 andTe Lu0.5O whiskersx precursor are pellets grown [18 from–20]. RBa We2-3 assumeCu3Ca that1.00–1.75 TeOTe2 0.5andOx Sbprecursor2O3 play pelletsa role as [18 flux.–20 ]. We assume that TeO2 and Sb2O3 play a role as flux.

FigFigureure 1. TypicalTypical SEM image of a Y Y-123-123 single crystal whisker.

2.1.2. Fe Fe-Based-Based Superconductors

Fe-based superconductors include R(O,F)FeAs (1111) (R: rare earth elements) [21], AeFe2As2 Fe-based superconductors include R(O,F)FeAs (1111) (R: rare earth elements) [21], AeFe2As2 (122) (122) (Ae: alkaline earth metal) [22], AFeAs (111) (A: alkali metals) [23], β-FeSe (11) [24], and (Ae: alkaline earth metal) [22], AFeAs (111) (A: alkali metals) [23], β-FeSe (11) [24], and (A,Ae)2Fe4Se5 (245)(A,Ae [)252Fe],4Se Fe-based5 (245) [25], with Fe perovskite-based with oxide perovskite layer compounds oxide layer (so-calledcompounds 42622) (so-called [26], and 42622) so on.[26], Some and typesso on. of Some Fe-based types superconductors of Fe-based superconductors are reported as are single reported crystals as whichsingle arecrystals grown which by the are flux grown method. by Typicalthe flux fluxes method. are Typical As-based fluxes self-flux, are As Sn- fluxbased and self chloride-based-flux, Sn flux flux.and chloride Especially,-based growth flux. of Especially, 122 single crystalsgrowth of have 122 been single intensively crystals have reported. been intensively reported. (Ae,K)Fe2As2 (Ae = Ba, Sr) single crystals can be grown using FeAs as a self-flux [27]. One of the (Ae,K)Fe2As2 (Ae = Ba, Sr) single crystals can be grown using FeAs as a self-flux [27]. One of the growth methodsmethods isis as as follows. follows. The The starting starting materials materials of Baof orBa Sr,or K,Sr, and K, FeAsand FeAs in a molar in a molar ratio of ratio 0.5:1:4 of are0.5:1:4 put are into put an into alumina an alumina crucible crucible and sealed and in sealed a welded in a Ta welded crucible Ta under crucible 1.6 under atm of 1.6 Ar. atm This of product Ar. This is sealedproduct in is an sealed evacuated in an quartzevacuated ampoule quartz and ampoule is heated and for isthe heated growth for the of single growth crystals of single [28]. crystals In addition [28]. In addition to the growth of KFe2As2 single crystals [29], Ba0.6K0.4Fe2As2 single crystals are obtained to the growth of KFe2As2 single crystals [29], Ba0.6K0.4Fe2As2 single crystals are obtained from a stoichiometricfrom a stoichiometric compound, compound, which playswhich a plays role of a self-fluxrole of self [30-].flux [30]. Growth of 111 single crystals can also be applied to the growth of LiFeAs and NaFe1−xCoxAs (x = Growth of 111 single crystals can also be applied to the growth of LiFeAs and NaFe1−xCoxAs 0–0.3). In LiFeAs, Li metal added Fe-As mixture is placed into an Al2O3 crucible. The crucible is (x = 0–0.3). In LiFeAs, Li metal added Fe-As mixture is placed into an Al2O3 crucible. The crucible isinserted inserted into into the the Nb Nb container, container, covered covered by by a aNb Nb cap cap with with outlets outlets for for the the flux flux (for (for a sieve). The Nb container is welded in an Ar atmosphere with a base pressure of 1.5 atm, and the sealed Nb container is sealed in a quartz tube under 0.25 atm of Ar to prevent any oxidation. This product is heated for single-crystal growth. Figure 2 shows optical and SEM images of obtained LiFeAs single

Condens. Matter 2017, 2, 32 4 of 13

container is welded in an Ar atmosphere with a base pressure of 1.5 atm, and the sealed Nb container

Condens.isCondens. sealed Matter Matter in a2017 2017 quartz, ,2 2, ,32 32 tube under 0.25 atm of Ar to prevent any oxidation. This product is heated44 of of for13 13 single-crystal growth. Figure2 shows optical and SEM images of obtained LiFeAs single crystals [ 31]. crystalsOncrystals the other [31]. [31]. hand, On On the the NaFe other other1−x Co hand, hand,xAs NaFe( NaFex = 0–0.3)1−1−xCoxCox singleAsxAs ( (xx crystals== 0 0––0.3)0.3) are single single grown crystals crystals using NaAs are are grown grown self-flux. using using NaAs, NaAs NaAs Fe, selfandself--flux. Coflux. powders NaAs, NaAs, Fe, Fe, are and and weighed Co Co powders powders to the molar are are weighed weighed ratio of NaAs:Fe:Coto to the the molar molar = ratio ratio 4:(1 −of of xNaAs:Fe:Co ):NaAs:Fe:Cox, and ground. = = 4: 4:( The(11 − − x mixturesx)):x:x, ,and and ground.areground. put into TheThe Al mixturesmixtures2O3 crucibles, areare putput then intointo sealed AlAl2O2O in3 3 ironcrucibles,crucibles, crucibles thenthen under sealedsealed 1.5 in atmin ironiron of high-purity cruciblescrucibles underunder Ar gas. 1.51.5 Then atmatm the ofof highcruciblehigh--puritypurity is heated Ar Ar gas. gas. for Then Then single-crystal the the crucible crucible growth is is heated heated [32]. for for single single--crystalcrystal growth growth [32]. [32].

FigureFigure 2. 2. Optical Optical image image of of a a LiFeAs LiFeAs crystal crystal ( (aa)) )and andand a aa SEM SEMSEM image imageimage ( (b(bb))[ )[31]. [31].31]. Single Single-CrystalSingle-Crystal-Crystal Growth Growth and and CharacterizationCharacterization of of Superconducting Superconducting LiFeAs LiFeAs.LiFeAs. .

ThereThere areare reportsreports aboutabout thethe growthgrowth ofof SrSr4V4V2O2O6Fe6Fe2As2As2 2 singlesingle crystals,crystals, whichwhich isis aa FeAsFeAs selfself--fluxflux There are reports about the growth of Sr4V2O6Fe2As2 single crystals, which is a FeAs self-flux method.method. The The nominal nominal composition composition of of the the raw raw materials materials is is FeAs:Sr FeAs:Sr4V4V2O2O6Fe6Fe2As2As2 2= = 2:1 2:1 (molar (molar ratio). ratio). The The method. The nominal composition of the raw materials is FeAs:Sr4V2O6Fe2As2 = 2:1 (molar ratio). mixture is sealed in a quartz tube in vacuum, then heated [33]. Moreover, growth of nano-sized mixtureThe mixture is sealed is sealed in a in quartz a quartz tube tube in invacuum, vacuum, then then heated heated [33]. [33 ]. Moreover, Moreover, growth growth of of nano nano-sized-sized whiskerswhiskers is is reported. reported. The The Ca Ca1010(Pt(Pt4As4As8)(Fe8)(Fe1.81.8PtPt0.20.2AsAs2)2)5 5precursor precursor with with 10 10 wt wt % % CaAs CaAs self self--fluxflux is is put put into into whiskers is reported. The Ca10(Pt4As8)(Fe1.8Pt0.2As2)5 precursor with 10 wt % CaAs self-flux is a Ta capsule and sealed in a quartz tube in vacuum. This product is heated, and the aput Ta into capsule a Ta capsule and sealed and sealed in a quartzin a quartz tube tube in vacuum. in vacuum. This This product product is isheated, heated, and and the the CaCa1010(Pt(Pt4As4As8)(Fe8)(Fe1.81.8PtPt0.20.2AsAs2)2)5 5nano nano--sizedsized whiskers whiskers are are obtained. obtained. SEM SEM images images are are shown shown in in Figure Figure 3. 3. The The Ca10(Pt4As8)(Fe1.8Pt0.2As2)5 nano-sized whiskers are obtained. SEM images are shown in Figure3. whiskers have lengths of 0.1–2.0 mm, widths of 0.4–5.0 μm, and thicknesses of 0.2–1.0 μm [34]. whiskersThe whiskers have havelengths lengths of 0.1 of–2.0 0.1–2.0 mm, mm,widths widths of 0.4 of–5.0 0.4–5.0 μm, andµm, thicknesses and thicknesses of 0.2 of–1.0 0.2–1.0 μm [34].µm [34 ].

10 4 8 1.8 0.2 2 5 FigureFigure 3. 3. SEMSEMSEM images images of of of Ca Ca Ca1010(Pt(Pt(Pt44AsAsAs88)(Fe)(Fe1.8PtPt0.20.2AsAsAs2)25) ) 5whiskerswhiskerswhiskers [34]. [34].[34]. Growth Growth Growth of of Single Single-Crystal Single-Crystal-Crystal 10 4 8 1.8 0.2 2 5 CaCa1010(Pt(Pt(Pt4As4AsAs8)(Fe)(Fe8)(Fe1.8Pt1.8Pt0.2PtAsAs0.22As)5) Nanowhiskers 2Nanowhiskers)5 Nanowhiskers with with withSuperconductivity Superconductivity Superconductivity up up to to up33 33 toK. K. 33( a(a) ) K.W Wide (idea) field Widefield of of field view view of, , (view,b(b) )E Enlargednlarged (b) Enlarged image image. image. .

SnSn flux flux is is also also effective effective for for 122 122 single single--crystalcrystal growths, growths, since since Sn Sn has has a a low low melting melting point point (232 (232 °C). °C). Sn flux is also effective for 122 single-crystal growths, since Sn has a low melting point (232 ◦C). BaFeBaFe2As2As2 2and and Ba Ba0.550.55KK0.450.45FeFe2As2As2 2single single crystals crystals can can be be grown grown using using Sn Sn flux. flux. Ba, Ba, K, K, Fe, Fe, and and As As are are added added BaFe2As2 and Ba0.55K0.45Fe2As2 single crystals can be grown using Sn flux. Ba, K, Fe, and As are toto Sn Sn in in the the molar molar ratio ratio of of (Ba/K)Fe (Ba/K)Fe2As2As2:Sn2:Sn = = 1:48 1:48 and and put put into into MgO MgO crucible. crucible. The The crucible crucible containing containing added to Sn in the molar ratio of (Ba/K)Fe2As2:Sn = 1:48 and put into MgO crucible. The crucible silicasilica wool wool is is placed placed on on to topp of of the the growth growth crucible crucible and and these these are are sealed sealed in in a a silica silica tube tube under under containing silica wool is placed on top of the growth crucible and these are sealed in a silica tube approximatelyapproximately 0.330.33 atmatm ofof Ar.Ar. ThenThen thisthis silicasilica tubetube isis heatedheated forfor singlesingle--crystalcrystal growth.growth. SnSn fluxflux isis decanteddecanted from from the the obtained obtained single single crystals crystals at at 500 500 °C °C [35].[35]. Nevertheless, Nevertheless, the the remaining remaining SnSn at at the the singlesingle crystalcrystal surfacessurfaces isis subsequentlysubsequently dissolveddissolved atat roomroom temperaturetemperature forfor aa fewfew daysdays inin liquidliquid Hg.Hg. TheThe obtained obtained single single crystals crystals are are heated heated for for one one hour hour at at 190 190 °C °C in in vacuum vacuum to to evaporate evaporate t thehe remaining remaining tracestraces of of Hg Hg [36]. [36].

Condens. Matter 2017, 2, 32 5 of 13 under approximately 0.33 atm of Ar. Then this silica tube is heated for single-crystal growth. Sn flux is decanted from the obtained single crystals at 500 ◦C[35]. Nevertheless, the remaining Sn at the single crystal surfaces is subsequently dissolved at room temperature for a few days in liquid Hg. The obtained single crystals are heated for one hour at 190 ◦C in vacuum to evaporate the remaining traces of Hg [36]. Finally, growth of 11 (β-FeSe) single crystals is exhibited, for which chloride-based flux is effective. β-FeSe decomposes into Fe and a hexagonal phase (α-FeSe) at 457 ◦C. This suggests that single-crystal growth has to be perform at low temperature (less than 457 ◦C). Therefore, the eutectic chloride composition is useful for flux. Some reports are shown in the following. A eutectic temperature of LiCl:CsCl = 29:21 (molar ratio) composition is 326 ◦C, which can use the flux for 11 (β-FeSe) single-crystal growth. After the growth, this flux can be removed by water wash. 11 (β-FeSe) single crystals are grown by LiCl/CsCl flux [37]. The eutectic composition of KCl/AlCl3 is also used for 11 (β-FeSe) single-crystal growth [38]. AlCl3-based compositions have low eutectic temperature. However, their handling should be done carefully to prevent an explosion hazard. The eutectic temperature of NaCl:KCl = 1:1 (molar ratio) composition is higher than β-FeSe decomposition temperature, but β-FeSe single crystals are obtained by rapid cooling between the NaCl/KCl eutectic temperature and room temperature [39].

2.1.3. BiS2-Based Superconductors

BiS2-layered superconductors include Bi4O4S3 [40], Rn(O,F)BiS2 (Rn: La, Ce, Pr, Nd, Yb) [41–45], AbFBiS2 (Ab: Sr, Eu) [46–48], (La,M)OBiS2 (M: Ti, Zr, Hf, Th) [49], and Eu3F4Bi2S4 [50] and Bi(O,F)BiS2 [51,52]. Especially, Rn(O,F)BiS2 shows similarity to the of superconducting R(O,F)FeAs (1111) [21], and S-site can be substituted by Se, which also becomes superconductive [53]. In contrast, substitution of Sb in Bi sites depress superconductivity. Single crystals of those superconductors can be grown using alkali metal chloride flux. CsCl-based composition is extensively used as a flux for Rn(O,F)BiS2 single-crystal growth, since CsCl exhibits little reactivity to the quartz tube at high temperature. Figure4 shows the reactivity between each alkali chloride (LiCl, NaCl, KCl, RbCl, CsCl) and the quartz tube at high temperature (800 ◦C 10 h). The quartz tube is slightly reacted to CsCl, RbCl, and KCl. Furthermore, a melting temperature of CsCl (645 ◦C) is lower than those of RbCl and KCl, which is advantageous as a flux. This review focuses on growth of Rn(O,F)BiS2 single crystals using CsCl-based flux. A eutectic CsCl/KCl flux with CsCl:KCl = 5:3 molar ratio ◦ (eutectic temperature: 616 C) is mainly used for Rn(O,F)BiS2 single-crystal growth [54]. Concretely, raw materials of Rn(O,F)BiS2 with nominal composition (0.8 g) and the CsCl/KCl flux (5.0 g) are mixed using a mortar, and then sealed in a quartz tube under vacuum (~10 Pa). This mixed powder is heated for single-crystal growth, and then furnace-cooled to room temperature. The quartz tube is opened in air, and the flux is dissolved using distilled water. The remaining product is filtered. Therefore, Rn(O,F)BiS2 single crystals are obtained [55,56]. Figure5 shows a typical SEM image of R n(O,F)BiS2 (Rn = Nd) single crystals. For comparison, F-free RnOBiS2 single crystals are grown by CsCl flux [57,58]. Since the heat-treatment temperature for the growth of F-free single crystals is higher than that of Rn(O,F)BiS2, thus, the quartz tube corrosion from KCl is enhanced at high temperature. Review of Rn(O,F)BiS2 superconducting single crystals is available [59]. With another flux, a eutectic composition of KCl:LiCl = 3:2 (molar ratio) can use the flux for Rn(O,F)BiS2 single crystals. The molar ratio of KCl/LiCl flux and Nd(O,F)BiS2 raw materials is KCl/LiCl:Nd(O,F)BiS2 = 25:1. The heat-treatment for single-crystal growth is performed at a lower temperature, below 750–450 ◦C to prevent corrosion of a quartz tube [60]. La(O,F)BiSe2 is similar compound for La(O,F)BiS2; these single crystals are also obtained using CsCl flux. The weight ratio of raw materials and CsCl flux is same for La(O,F)BiS2 case [61]. Moreover, Rn(O,F)SbS2 are non-superconducting compounds, those single crystals can be grown by the same growth method used for Rn(O,F)BiS2 [62]. Condens. Matter 2017, 2, 32 6 of 13 Condens. Matter 2017,, 2,, 32 6 of 13

Figure 4.4. ReactivityReactivity betweenbetween each each alkali alkali chloride chloride (LiCl, (LiCl, NaCl, NaCl, KCl, KCl, RbCl, RbCl, CsCl) CsCl) and and quartz quartz tube tube at 800 at ◦ 800Cfor °C 10for h. 10 h.

FigureFigure 5.5. Typical SEMSEM imageimage ofof anan R(O,F)BiSR(O,F)BiS22 (R(R == Nd) single crystal.

2.2. Traveling SolventSolvent Floating-ZoneFloating-Zone (TSFZ)(TSFZ) MethodMethod The traveling traveling solvent solvent floating-zone floating-zone (TSFZ) (TSFZ) method method [63] is[63] based is based on the on floating-zone the floating (FZ)-zone method, (FZ) whichmethod, is one which of the is one single-crystal of the sing growthle-crystal methods growth for methods congruently for congruentlymelting compounds. melting This compounds. method doesThis method not use adoes crucible, not use which a crucible, means which that the means reaction that with the areaction crucible with does a notcrucible need does to be not considered. need to Thebe considered. schematic imageThe schematic of the FZ image method of the is shown FZ method in Figure is shown6a. The in Figure bottom 6a. of The the feedbottom rod of and the seed feed crystalrod and are seed partially crystal melted are partially by the melted local heating, by the andlocal the heating, feed rod and and the seed feedcrystal rod and are seed connected crystal viaare theconnected melt by via surface the melt tension. by surface Then, tension. the molten Then, zone the is molten formed zone between is formed the melt between of the the feed melt rod of and the thefeed seed rod crystal. and the The seed feed crystal. rod and The seed feed crystal rod move and seed down crystal for the move single-crystal down for growth. the single In the-crystal TSFZ method,growth. In the the solvent TSFZ ismethod, welded the to thesolvent bottom is welded of the feed to the rod bottom (Figure of6b). the The feed solvent rod (Figure plays a6b). role The as flux.solvent Figure plays7 shows a role theas flux. typical Figure phase 7 diagramshows the of typical incongruent phase melting. diagram The of incongruent target phase melting. ( α) becomes The target phase (α) becomes melts incongruently at Td. α cannot be grown by the simple melt growth, meltstarget incongruentlyphase (α) becomes at Td. meltsα cannot incongruently be grown by at the Td.simple α cannot melt be growth, grown whereasby the simple the composition melt growth, on 1 2 X-Ywhereas liquidus the composition line (β1 to β 2on) is X selected-Y liquidus for line the solvent,(β1 to β2) and is selected the liquid/melt for the solvent, of β is and slowly the cooled,liquid/melt just belowof β is theslowly X-Y cooled, liquidus just line, below it begins the X to-Y freezeliquidus into line, a crystal it begins of α to[64 fr].eeze In otherinto a words, crystal aof feed α [64]. rod In is dissolvedother words, in the a feedmolten rod zone is dissolved with the solvent, in the molten and then zone the withfeed rod the becomes solvent, aand liquid then solution the feed below rod thebecomes incongruent a liquid melting solution temperature. below the incongruent Subsequently, melting it is slowly temperature. cooled and Subsequently, the single crystal it is slowly of the incongruentcooled and the melting single compound crystal of the is obtained. incongruent melting compound is obtained.

Condens. Matter 2017, 2, 32 7 of 13 Condens. Matter 2017, 2, 32 7 of 13 Condens. Matter 2017, 2, 32 7 of 13

Condens. Matter 2017, 2, 32 7 of 13

FigureFigure 6. 6.Schematic Schematic image of of (a () afloating) floating zone zone (FZ) and (FZ) (b and) traveling (b) traveling solvent floating solvent zone floating (TSFZ) zone Figure 6. Schematic image of (a) floating zone (FZ) and (b) traveling solvent floating zone (TSFZ) (TSFZ)methods. methods. methods. Figure 6. Schematic image of (a) floating zone (FZ) and (b) traveling solvent floating zone (TSFZ) methods.

Figure 7. Typical phase diagram of an incongruently melting compound. FigureFigure 7. Typical 7. Typical phase phase diagram diagram ofof anan incongruentlyincongruently melting melting compound. compound.

The TSFZ method is very effectual for single-crystal growth of high-Tc cuprate The TSFZ Figure method 7. Typical is very phase effectualdiagram of an for incongruen single-crystaltly melting growth compound. of high-Tc cuprate Thesuperconductors. TSFZ method Large is very (centimeter effectual size) for single-crystal single crystals growth of high-T ofc cuprate high-Tc superconductorscuprate superconductors. were superconductors. Large (centimeter size) single crystals of high-Tc cuprate superconductors were Largesuccessfully (centimeter grown size) for single the first crystals time by of high-the TSFZT cuprate method superconductorsin [65]. For (R,Ae)-214 were single successfully crystals, the grown successfullyThe TSFZ grown method for the first is verytime byeffectual the TSFZ c for method single in-crystal [65]. For growth (R,Ae)- 214 of single high -crystals,Tc cuprate the solvent for the TSFZ method is the compounds with CuO-rich composition. For instance, a for thesuperconductors.solvent first time for theby the TSFZ Large TSFZ method (centimeter method is the in size) [65 compounds single]. For (R,Acrystals withe)-214 of CuO high single-rich-Tc crystals,cuprate composition. superconductors the solvent For instance, for were the a TSFZ (La,Sr)-214 single crystal is grown from solvent of CuO with a composition from 78 mol % [66], methodsuccessfully(La,Sr) is the-214 compounds singlegrown crystal for the with is first grown CuO-rich time from by the solventcomposition. TSFZ ofmethod CuO For within [65]. instance, a compositionFor (R,A a (La,Sr)-214e)-214 from single 78 singlecrystals, mol % crystal [66], the is growth of (Nd,Ce)-214 single crystal also uses CuO-rich solvent with 85 mol % CuO [67]. Figure 8 grownsolventgrowth from of forsolvent (Nd,Ce) the TSFZ of-214 CuO methodsingle with crystal a is composition the also compounds uses CuO from -rich with 78 solvent mol CuO %-rich with [66 composition.], 85 growth mol % ofCuO For(Nd,Ce)-214 [67]. instance, Figure singlea8 shows a (Nd,Ce)-214 single crystal grown by TSFZ method. Single crystals of Bi-based high-Tc (La,Sr)shows -214 a (Nd,Ce) single - crystal214 single is grown crystal from grown solvent by TSFZ of CuO method. with a Single composition crystals from of Bi 78-based mol % high [66],-Tc crystalcuprate also uses superconductors CuO-rich solvent are also with grown 85 molby the % TSFZCuO [method.67]. Figure For 8 Bi shows-2212 single a (Nd,Ce)-214 crystals, the single growthcuprate of superconductors (Nd,Ce)-214 single are crystal also grown also uses by CuO the TSFZ-rich solvent method. with For 85 Bi mol-2212 % singleCuO [67]. crystals, Figure the 8 crystalnominal grown composition by TSFZ method. of the Single feed rod crystals is Bi2.2 ofSr Bi-based1.8Ca1.0Cu2.0 high-O8, andTc cuprate that ofsuperconductors the solvent is are showsnominal a (Nd,Ce) composition-214 singleof the crystal feed grown rod is by Bi TSFZ2.2Sr 1.8 method.Ca1.0Cu2.0 SingleO8, and crystals that of of Bi the-based solvent high -T isc also grownBi2.4Sr1.5Ca by1.0 theCu1.8 TSFZOx, which method. is Bi-rich For and Bi-2212 Sr-poor single [68]. crystals,Bi-2223 has the the nominal highest T compositionc in Bi-based high of the-Tc feed cuprateBi2.4Sr1.5 Ca superconductors1.0Cu1.8Ox, which areis Bi also-rich grown and Sr - bypoor the [68]. TSFZ Bi- 2223 method. has the For highest Bi-2212 T c singlein Bi- based crystals, high the-Tc rod iscuprate Bi2.2Sr superconductors.1.8Ca1.0Cu2.0O8 ,A and Bi-2223 that single of the crystal solvent is grown is Bi2.4 bySr using1.5Ca a1.0 BiCu-rich1.8O anx,d whichSr-poor is feed Bi-rich rod and nominalcuprate superconductors. composition of A theBi- 2223 feed single rod crystal is Bi2.2 isSr grown1.8Ca1.0 Cuby2.0 usingO8, anda Bi -rich that an ofd Sr the-poor solvent feed rod is Sr-poorand [ 68a very]. Bi-2223 slow growth has the rate highest (0.05 mm/h).T in Bi-based Nominal high-compositionT cuprate of the superconductors. Bi-rich and Sr-poor A feed Bi-2223 rod is single Biand2.4Sr a 1.5veryCa1.0 slowCu1.8 growthOx, which rate is (0.05 Bi-rich mm/h).c and Sr Nominal-poor [68]. composition Bi-2223c has of the Bihighest-rich and Tc in Sr Bi-poor-based feed high rod-T isc crystalBi2.1 isSr grown1.9Ca2.0Cu by3.0O usingx which a Bi-richplays a androle as Sr-poor the solvent feed for rod the and TSFZ a verymethod slow [69]. growth rate (0.05 mm/h). cuprateBi2.1Sr1.9 Casuperconductors.2.0Cu3.0Ox which Aplays Bi-2223 a role single as the crystal solvent is forgrown the TSFZby using method a Bi- [69].rich an d Sr-poor feed rod Nominaland a composition very slow growth of the rate Bi-rich (0.05 mm/h). and Sr-poor Nominal feed composition rod is Bi2.1 ofSr the1.9 BiCa-rich2.0Cu and3.0 OSrx-poorwhich feed plays rod is a role as theBi solvent2.1Sr1.9Ca for2.0Cu the3.0O TSFZx which method plays a [role69]. as the solvent for the TSFZ method [69].

Figure 8. A (Nd,Ce)-214 single crystal grown by the TSFZ method. Figure 8. A (Nd,Ce)-214 single crystal grown by the TSFZ method.

FigureFigure 8. A8. (Nd,Ce)-214A (Nd,Ce)-214 single single crystalcrystal grown grown by by the the TSFZ TSFZ method. method.

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2.3. Top Top-Seeded-Seeded SolutionSolution GrowthGrowth (TSSG)(TSSG) MethodMethod The op-seeded op-seeded solution solution growth growth (TSSG) (TSSG) method method is like is a like combination a combination of the single-crystal of the single- growthcrystal techniquegrowth technique for flux methodfor flux method and the Czochralskiand the Czochralski (CZ) method. (CZ) method. A A seed is crystal placed is in placed the solution in the feedsolution with feed a flux, with and a flux, single and crystals single crystals of the target of the material target material are slowly are slowly pulled pulled from the from bottom the bottom of the seedof the crystal. seed crystal. The schematic The schematic image image of the of TSSG the methodTSSG method is shown is shown in Figure in Figure9. In the 9. modifiedIn the modified TSSG method,TSSG method, the temperature the temperature and/or and/or concentration concentration gradient gradient are given are given between between the top the and top bottom and bottom of the crucibleof the crucible [70]. [70].

Figure 9. Schematic image of the top-seeded solution growth (TSSG) method. Figure 9. Schematic image of the top-seeded solution growth (TSSG) method.

(La,Ba)-214 single crystal is grown from a CuO flux solution of which the nominal composition of the(La,Ba)-214 solution is single La(Ba) crystal-214:CuO is grown = 3:17 from (molar a CuO ratio) flux [71]. solution Actually, of which the TSSG the nominal method compositionmainly uses of the solution is La(Ba)-214:CuO = 3:17 (molar ratio) [71]. Actually, the TSSG method mainly uses R-123 for single-crystal growth. Y-123 single crystals are grown from Y-123:Ba3Cu7O10 = 17:83 (molar R-123ratio) for [72] single-crystal and Y:Ba:Cu growth. = 5:36:59 Y-123 (cation single ratio) crystals [70] are solution grown from by the Y-123:Ba TSSG 3 method.Cu7O10 = Additionally, 17:83 (molar ratio)BaO/CuO [72] flux and is Y:Ba:Cu employed = 5:36:59 for Nd (cation-123 single ratio)-crystal [70] solutiongrowth, of by which the TSSG the nominal method. composition Additionally, is BaO/CuO flux is employed for Nd-123 single-crystal growth, of which the nominal composition is Nd-123:Ba3Cu5O8 = 1:3 (molar ratio) [73]. Nd-123:Ba3Cu5O8 = 1:3 (molar ratio) [73]. 2.4. High-Temperature and High-Pressure (HPHT) Flux Method 2.4. High-Temperature and High-Pressure (HPHT) Flux Method This flux method (See Section 2.1) is performed in a cubic anvil high-pressure apparatus under This flux method (See Section 2.1) is performed in a cubic anvil high-pressure apparatus under high-temperature and high-pressure (HPHT), which is named as the HPHT flux method. The typical high-temperature and high-pressure (HPHT), which is named as the HPHT flux method. The typical schematic image of the sample cell assembly for the HPHT flux method is shown in Figure 10. This schematic image of the sample cell assembly for the HPHT flux method is shown in Figure 10. method is a powerful method for single-crystal growth. The HPHT flux method can avoid the This method is a powerful method for single-crystal growth. The HPHT flux method can avoid vaporization from raw materials. Therefore, this method enables difficult single-crystal growth the vaporization from raw materials. Therefore, this method enables difficult single-crystal growth under ambient pressure. It can grow single crystals, not only layered superconductors, but also other under ambient pressure. It can grow single crystals, not only layered superconductors, but also other superconductors. For instance, β-ZrNCl and β-HfNCl superconducting single crystals are grown superconductors. For instance, β-ZrNCl and β-HfNCl superconducting single crystals are grown using NH4Cl flux at 900–1200 °C under 3 GPa [74]. MgB2 single crystals are successfully grown by using NH Cl flux at 900–1200 ◦C under 3 GPa [74]. MgB single crystals are successfully grown by the the HPHT4 flux method [75,76]. 2 HPHT flux method [75,76]. Some reports exhibit the single-crystal growth of Fe-based superconductors by the HPHT flux Some reports exhibit the single-crystal growth of Fe-based superconductors by the HPHT flux method. Ln(O,F)FeAs (Ln = Pr, Nd, Sm) single crystals are grown using NaAs or KAs flux by the method. Ln(O,F)FeAs (Ln = Pr, Nd, Sm) single crystals are grown using NaAs or KAs flux by the HPHT HPHT flux method. The molar ratio of Ln(O,F)FeAs raw materials and NaAs or KAs flux are flux method. The molar ratio of L (O,F)FeAs raw materials and NaAs or KAs flux are weighed as weighed as between 1:1 and 1:10, nrespectively. That mixture puts in boron nitride (BN) container, between 1:1 and 1:10, respectively. That mixture puts in boron nitride (BN) container, and then pressure and then pressure is applied at 3 GPa. This anvil is heated for single-crystal growth. The remaining is applied at 3 GPa. This anvil is heated for single-crystal growth. The remaining NaAs or KAs fluxes NaAs or KAs fluxes are dissolved in water [77]. By the same method, NaCl/KCl flux is used for are dissolved in water [77]. By the same method, NaCl/KCl flux is used for Sm(O,F)FeAs single-crystal Sm(O,F)FeAs single-crystal growth, and a nominal composition of Sm(O,F)FeAs raw materials and growth, and a nominal composition of Sm(O,F)FeAs raw materials and NaCl/KCl flux are between NaCl/KCl flux are between 1:1 and 1:3, respectively [78]. O-deficient SmOFeAs (SmO0.85FeAs) single 1:1 and 1:3, respectively [78]. O-deficient SmOFeAs (SmO FeAs) single crystals are grown from crystals are grown from stoichiometric compound at 3.3 GPa,0.85 which is the HPHT self-flux method stoichiometric compound at 3.3 GPa, which is the HPHT self-flux method [79]. Furthermore, the [79]. Furthermore, the HPHT flux method can also grow Pr(O,F)FeAs single-crystalline whiskers. HPHT flux method can also grow Pr(O,F)FeAs single-crystalline whiskers. Flux is NaAs, and the Flux is NaAs, and the molar ratio of Pr(O,F)FeAs precursor to NaAs flux is 1:1, respectively. This molar ratio of Pr(O,F)FeAs precursor to NaAs flux is 1:1, respectively. This mixture is put into a BN mixture is put into a BN container and pressure is applied at 3 GPa. The product is heated and then whiskers with various lengths (400–1300 μm), widths (40–70 μm), and thicknesses (5–10 μm) are

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Condens. Matter 2017, 2, 32 9 of 13 container and pressure is applied at 3 GPa. The product is heated and then whiskers with various lengthsobtained (400–1300 [80]. A µ reviewm), widths of single (40–70-crystalµm), and growth thicknesses of Fe-based (5–10 superconductorsµm) are obtained by [80 the]. A HPHT review fl ofux single-crystalmethod is available growth in of [81]. Fe-based superconductors by the HPHT flux method is available in [81].

FigureFigure 10. 10.Schematic Schematic image image of of the the sample sample cell cell assembly assembly for for the the high-temperature high-temperature and and high-pressure high-pressure (HPHT)(HPHT) flux flux method. method.

3.3. Summary Summary FluxFlux is is necessary necessary for for the the growth growth of of single single crystals crystals of of layered layered superconductors. superconductors. Small Small (mm (mm size) size) singlesingle crystals crystals can can be be grown grown by by the the conventional conventional flux flux method. method. Low Low melting melting point point metals metals (e.g., (e.g., Sn), Sn), chloride-basedchloride-based compounds compounds (e.g., (e.g., KCl) KCl) and and self-flux self-flux are are used used for for the the flux. flux. Furthermore, Furthermore, the the particular particular fluxflux can can grow grow single-crystalline single-crystalline whiskers whiskers with with perfect perfect crystallinity. crystallinity. In In contrast, contrast, large large (cm (cm size) size) single single crystalscrystals are are obtained obtained by TSFZby TSFZ and and TSSG TSSG methods. methods. The HPHT The HPHT flux method flux method makes single-crystal makes single growth-crystal possible,growth possible, which is difficultwhich is under difficult ambient under pressure.ambient pressure.

Acknowledgments:AcknowledgmentsThe: The author author would would like like to to thank thank Akira Akira Miura Miura (Hokkaido (Hokkaido University) University) for for useful useful discussion discussion and critical reading, and Isao Tanaka (University of Yamanashi) for providing the (Nd,Ce)-214 single crystal. and critical reading, and Isao Tanaka (University of Yamanashi) for providing the (Nd,Ce)-214 single crystal. Conflicts of Interest: The author declares no conflict of interest. Conflicts of Interest: The author declares no conflict of interest. References References 1. Bednorz, J.G.; Müller, K.A. Possible high-Tc superconductivity in the Ba-La-Cu-O system. Z. Phys. B 1. Bednorz, J.G.; Müller, K.A. Possible high-Tc superconductivity in the Ba-La-Cu-O system. Z. Phys. B Condens. Matter 1986, 64, 189–193. [CrossRef] Condens. Matter 1986, 64, 189–193. 2. Wu, M.K.; Ashburn, J.R.; Thorng, C.J.; Hor, P.H.; Meng, R.L.; Gao, L.; Huang, Z.J.; Wang, Y.Q.; Chu, C.W. 2. Wu, M.K.; Ashburn, J.R.; Thorng, C.J.; Hor, P.H.; Meng, R.L.; Gao, L.; Huang, Z.J.; Wang, Y.Q.; Chu, C.W. Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure. Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure. Phys. Phys. Rev. Lett. 1987, 58, 908. [CrossRef][PubMed] Rev. Lett. 1987, 58, 908. 3. Maeda, H.; Tanaka, Y.; Fukutomi, M.; Asano, T. A New High-Tc Oxide Superconductor without a Rare Earth 3. Maeda, H.; Tanaka, Y.; Fukutomi, M.; Asano, T. A New High-Tc Oxide Superconductor without a Rare Element. Jpn. J. Appl. Phys. 1988, 27, L209. [CrossRef] Earth Element. Jpn. J. Appl. Phys. 1988, 27, L209. 4. Hidaka, Y.; Enomoto, Y.; Suzuki, M.; Oda, M.; Murakami, T. Single-crystal growth of (La1−xAx)2CuO4 4. Hidaka, Y.; Enomoto, Y.; Suzuki, M.; Oda, M.; Murakami, T. Single-crystal growth of (La1−xAx)2CuO4 (A = (A = Ba or Sr) and Ba2YCu3O7−y. J. Cryst. Growth 1987, 85, 581–584. [CrossRef] Ba or Sr) and Ba2YCu3O7−y. J. Cryst. Growth 1987, 85, 581–584. 5. Asaoka, H.; Takei, H.; Iye, Y.; Tamura, M.; Kinoshita, M.; Takeya, H. Growth of Large Isometric YBa2Cu3Ox 5. Asaoka, H.; Takei, H.; Iye, Y.; Tamura, M.; Kinoshita, M.; Takeya, H. Growth of Large Isometric YBa2Cu3Ox Single Crystals from Coexisting Region of Solid with Melt in Y2O3 Crucibles. Jpn. J. Appl. Phys. 1993, 32, Single Crystals from Coexisting Region of Solid with Melt in Y2O3 Crucibles. Jpn. J. Appl. Phys. 1993, 32, 1091. [CrossRef] 1091. 6. Funabiki, H.; Okanoue, K.; Takano, C.; Hamasaki, K. Growth conditions and characterization of 6. Funabiki, H.; Okanoue, K.; Takano, C.; Hamasaki, K. Growth conditions and characterization of Bi2Sr2CaCu2O8+δ single crystals. J. Cryst. Growth 2003, 259, 85–89. [CrossRef] Bi2Sr2CaCu2O8+δ single crystals. J. Cryst. Growth 2003, 259, 85–89. 7. Gencer, F.; Abell, J.S. The growth of YBa2Cu3O7−δ single crystals with the aid of NaCl-KCl flux. 7. Gencer, F.; Abell, J.S. The growth of YBa2Cu3O7−δ single crystals with the aid of NaCl-KCl flux. J. Cryst. J. Cryst. Growth 1991, 112, 337–342. [CrossRef] Growth 1991, 112, 337–342. 8. Katsui, A. Crystal Growth of Superconducting Bi-Sr-Ca-Cu-O Compounds from KCl Solution. Jpn. J. 8. Katsui, A. Crystal Growth of Superconducting Bi-Sr-Ca-Cu-O Compounds from KCl Solution. Jpn. J. Appl. Appl. Phys. 1988, 27, L844. [CrossRef] Phys. 1988, 27, L844. 9. Wang, X.L.; Horvat, J.; Liu, H.K.; Dou, S.X. Spiral growth of Bi2Sr2CaCu2Oy single crystals using KCl flux 9. Wang, X.L.; Horvat, J.; Liu, H.K.; Dou, S.X. Spiral growth of Bi2Sr2CaCu2Oy single crystals using KCl flux technique. J. Cryst. Growth 1997, 173, 380–385. [CrossRef] technique. J. Cryst. Growth 1997, 173, 380–385. 10. Lee, S.; Yamamoto, A.; Tajima, S. Crystal growth of Bi2Sr2Ca2Cu3O10+x and (Bi,Pb)2Sr2Ca2Cu3O10+x by the 10. Lee, S.; Yamamoto, A.; Tajima, S. Crystal growth of Bi2Sr2Ca2Cu3O10+x and (Bi,Pb)2Sr2Ca2Cu3O10+x by the KCl flux method. J. Mater. Res. 2002, 17, 2286–2293. [CrossRef] KCl flux method. J. Mater. Res. 2002, 17, 2286–2293.

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