Flux growth in a horizontal configuration: an analogue to vapor transport growth

J.-Q. Yan,1, ∗ B. C. Sales,1 M. A. Susner,1, 2, 3 and M. A. McGuire1

1Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA2Aerospace Systems Directorate, Air Force Research Laboratory 1950 Fifth St.,

Building 18, Wright-Patterson Air Force Base, OH 45433 USA3UES, Inc. 4401 Dayton Xenia Rd. Beavercreek, OH 45432 USA

(Dated: June 9, 2017)

Flux growth of single crystals is normally performed in a vertical configuration with an uprightrefractory container holding the flux melt. At high temperatures, flux dissolves the charge forminga homogeneous solution before nucleation and growth of crystals take place under proper super-saturation generated by cooling or evaporating the flux. In this work, we report flux growth in ahorizontal configuration with a temperature gradient along the horizontal axis: a liquid transportgrowth analogous to the vapor transport technique. In a typical liquid transport growth, the chargeis kept at the hot end of the refractory container and the flux melt dissolves the charge and transfersit to the cold end. Once the concentration of charge is above the solubility limit at the cold end,the thermodynamically stable phase nucleates and grows. Compared to the vertical flux growth,the liquid transport growth can provide a large quantity of crystals in a single growth since thecharge/flux ratio is not limited by the solubility limit at the growth temperature. This technique iscomplementary to the vertical flux growth and can be considered when a large amount of crystalsare needed but the yield from the conventional vertical flux growth is limited. We applied thistechnique to the growth of IrSb3, Mo3Sb7, MnBi from self flux, and the growth of FeSe, CrTe3,NiPSe3, FePSe3, and InCuP2S6 from a halide flux.

I. Introduction

Flux growth is a compelling method for the discoveryof new materials and for the growth of crystals for sci-entific studies as well as technological applications.1–6 Ina typical flux growth, the mixture of target compoundand a flux with a low melting point is heated to hightemperatures where the flux completely dissolves otherstarting materials, thereby forming a homogeneous solu-tion. Then, the temperature is slowly lowered to gen-erate supersaturation necessary for the nucleation andgrowth of crystals. Crystal growth can also take placeunder a supersaturation generated by evaporating theflux, which takes advantage of the large vapor pressuresof some fluxes. A refractory container, such as the Al2O3-based Canfield crucible set,7 is needed to hold the hightemperature melt. The refractory crucible is normallyplaced in an upright position to keep the melt inside andno temperature gradient, either vertical or radial, is pur-posely generated. In the following text, we refer to thistype of flux growth in a vertical configuration as ”con-ventional vertical flux growth”.

In this paper, we report on flux growth in a horizon-tal configuration with a temperature gradient purposelyapplied along the horizontal axis. This technique is sim-ilar to vapor transport growth but here the flux meltbehaving as the transport agent. We thus name it liquidtransport growth. Quite different from that in the con-ventional vertical flux growth, the charge/flux ratio is notlimited by the solubility of charge in the flux at given tem-peratures. Thus the liquid transport growth can producea large quantity of single crystals in a single growth whichis otherwise impossible for the conventional vertical fluxgrowth. This technique is complementary to the conven-

tional vertical flux growth and can be employed whena large amount of crystals are needed for some specialmeasurements but the yield from the conventional verti-cal flux growth is limited. We show the growth detailsusing this technique of IrSb3, Mo3Sb7, and MnBi out ofself flux, FeSe, CrTe3, NiPSe3, FePSe3, and InCuP2S6

out of an AlCl3-KCl eutectic flux. In addition, the reac-tion between transition metal oxides and the AlCl3-KCleutectic flux can provide clean small RuCl3 crystals andOsCl4 starting materials for further crystal growth of tri-halides. The successful growth of these proof of princi-ple compounds suggests that the liquid transport growthworks well for a large variety of compounds. Tempera-ture and the temperature gradient along the horizontalaxis are critical growth parameters which determine thecomposition of the crystallizing phase which is thermo-dynamically stable at given temperatures and also affectthe growth kinetics.

II. Liquid transport vs vapor transport

Figure 1 illustrates the principles of vapor transportgrowth and liquid transport growth. A horizontal tem-perature gradient is applied for both techniques. In atypical vapor transport growth, a volatile substance (theso-called transport agent) is sealed in, for example, aquartz tube together with the raw materials. The sealedampoule is then placed inside of a tube furnace with awell-defined temperature profile (See Fig. 1(a)). At hightemperatures, the volatile transport agent reacts withthe nonvolatile starting materials into some intermedi-ate molecular species that can diffuse to the cold end.Once the partial pressure of the intermediate molecular

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FIG. 1. (color online) (a) Temperature profile along the hor-izontal axis. (b)Schematic picture of vapor transport growth.(c) Schematic picture of liquid transport growth.

species is high enough, crystallization takes place at thecold end and the transport agent is released. As thetransport agent is not consumed in the growth, only asmall amount of transport agent is needed. For example,an empirical amount of 3-5mg per cm3 is normally used.For detailed description of this technique, see Reference[8,9].

Liquid transport growth is similar to the vapor trans-port technique as illustrated in Fig. 1(c). The charge iskept at the hot end of the growth ampoule. In additionto the charge, a large amount of flux is added to par-tially fill the growth ampoule. At high temperatures, theflux dissolves the charge at the hot end and transfers thedissolved charge to the cold end driven by the composi-tion gradient. At the cold end, once the concentrationof the dissolved charge goes beyond the solubility limit,the thermodynamically stable phase starts to precipitate.As in the conventional vertical flux growth, the flux canbe the constituent of the desired compound, i.e., the so-called self flux. In this case, the flux will be consumedduring the crystal growth. The flux can also have com-positions quite different from the starting materials andthe desired crystal. In this case, the flux is not consumedduring crystal growth but works as a liquid transportagent.

We first present the growth details of some proof ofprinciple compounds in Section III and then discuss thegrowth mechanism of the liquid transport growth in Sec-tion IV.

III. Liquid transport growth examples

A. liquid transport growth from self flux

1. IrSb3

IrSb3 crystallizes in the Skutterudite structure (spacegroup Im3), which can host excellent thermoelectricproperties upon filling the voids in the structure.10 Re-

FIG. 2. (color online) Phase diagram of Ir-Sb from Reference[12]. Other versions of phase diagram in the same databasesuggest the peritectic plateau extends to ∼85% Sb. Our fluxgrowths using the vertical configuration support the versionshown here.

cently, isostructural RhSb3 was proposed to be a newzero-gap three-dimensional Dirac semimetal.11 This sug-gests that the unfilled skutterudite compounds providea new materials playground for the study of topologicalDirac semimetals. Figure 2 shows the Ir-Sb binary phasediagram.12 The peritectic plateau extends to a composi-tion of ∼97% Sb, leaving a narrow composition range forthe growth of IrSb3 out of Sb flux and the liquidus linehas a large composition dependence in this range. Nev-ertheless, we have managed to grow IrSb3 single crystalsout of Sb flux using the vertical configuration. However,less than 3 % Ir is allowed in the melt according to thephase diagram, which limits the amount of crystals onemay obtain from a single growth. We thus tried fluxgrowth in a horizontal configuration.

The starting materials are Ir powder (Alfa Aesar, -22 mesh, 99.99%) and Sb shot (Alfa Aesar, 1-3 mm,99.9999%). In a typical growth, about 1.5 g Ir pow-der and 50 g Sb shot were loaded into a quartz tube of16 mm inner diameter and 1.5 mm wall thickness. Thequartz tube was sealed under vacuum (see Fig. 3(b)).The growth ampoule was then put in a single zone tubefurnace with the Ir powder located at the hot end. Itis worth mentioning that mixing of Ir and Sb startingmaterials should be minimized. Figure 3(a) shows thetemperature profile along the tube. After keeping thehot end (x=0) at 860◦C for two weeks, the furnace wasturned off. Figure 3(c) shows the ampoule after growth.IrSb3 crystals were found within about ∼3 cm of the coldend of the tube. To isolate the crystals from the ex-tra Sb flux, we sealed the part cut from the cold end ina Canfield crucible set under vacuum and heated it to650◦C. After dwelling at this temperature for two hours,the extra Sb flux was removed by centrifuging. Fig-ure 3(d) shows some millimeter-sized IrSb3 crystals left in

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FIG. 3. (color online) Liquid transport growth of IrSb3. (a)The temperature profile along the horizontal axis where thethermocouple is located at 0 cm. (b) The growth ampouleinside of the single zone tube furnace before heating up; theIr powder is located at the hot end (x=0). (c) The ampouleafter growth on a mm grid; single crystals were found in the2-3 cm range from the cold end. (d) IrSb3 single crystals ona mm grid after removing the extra Sb flux as described intext.

the growth crucible after decanting. The crystals are ofsimilar size as those obtained from the conventional ver-tical flux technique. In such a growth, we obtain about2.8 g of IrSb3 crystals. This is much more than what onecan grow using the conventional vertical flux growth. In atypical vertical flux growth using a 2 ml Canfield crucibleset, about 0.4 g of IrSb3 crystals can be obtained startingwith a 4 g batch containing 0.18 g of Ir as the charge. Theliquid transport grown IrSb3 crystals exhibit the sametransport properties as those grown in the conventionalvertical configuration. The detailed physical propertiesof IrSb3 crystals will be reported elsewhere as it is not

FIG. 4. (color online) Phase diagram of Mo-Sb from Refer-ence[ 12]. The dashed curve and line are added following ourflux growths and Differential Scanning Calorimetry measure-ments.

the focus of the present work.

2. Mo3Sb7

Despite a low superconducting transition temperatureof 2.30 K, the Zintl compound Mo3Sb7 attracted muchattention due to possible interplay between superconduc-tivity, structural instability, and magnetism. With ap-propriate doping, Mo3Sb7 also shows good thermoelec-tric performance. The first Mo3Sb7 single crystal largeenough for physical property measurements was grownby melting Sb in a thick Mo tube by Bulowski et al.13

This work motivated us to grow millimeter sized Mo3Sb7

single crystals out of Sb flux.14 In a typical flux growthusing the conventional vertical configuration, Mo slug orreduced powder and Sb shot were mixed in the molar ra-tio of Mo:Sb = 1:49 and placed in a 2 ml Al2O3 crucible.A catch crucible of the same size containing quartz woolwas mounted on top of the growth crucible and both weresealed in a silica ampoule under approximately 1/3 at-mosphere of argon gas. The sealed ampoule was heatedto 1000◦C in 5 hours and homogenized for 12 hours be-fore cooling to 700◦C over 100 hours in a programmablebox furnace. At 700◦C, the Sb flux was decanted fromthe Mo3Sb7 crystals. A similar procedure has been suc-cessfully employed to grow doped single crystals.15

The above self flux technique can produce Mo3Sb7

single crystals with the largest dimension around 3 mm.However, the small fraction of Mo in the starting com-position and the low yield of sizeable pieces make it verytime consuming and expensive to grow enough pieces forinelastic neutron scattering measurements for the studyof lattice dynamics and possible magnetic resonance.

Figure 4 shows the Mo-Sb binary phase diagram.12 The

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FIG. 5. (color online) Liquid transport growth of Mo3Sb7.(a)The temperature gradient along the horizontal axis. (b) Theampoule before growth with Mo and premelted Sb sealed in aquartz tube. (c) Single crystals buried in Sb flux. (d) Mo3Sb7

crystals on a millimeter grid after mechanically removing theSb flux using a surgical blade.

successful growth of Mo3Sb7 crystals out of Sb flux sug-gests the reported phase diagram misses the liquidusline. Our Differential Scanning Calorimetry measure-ments suggest the peritectic temperature is above 950◦C.We necessarily modified the phase diagram by adding adashed line denoting the peritectic reaction and a dashedcurve as the liquidus line. Mo3Sb7 is the only compoundin the Mo-Sb binary system. Following the idea of liquidtransport technique described in Section II, we would ex-pect Sb flux reacts with Mo at the hot end and depositesMo3Sb7 at the cold end as long as the cold end is kept

below the peritectic temperature.

Figure 5(a) shows the temperature profile used in theliquid transport growth of Mo3Sb7. The temperatureprofile along the horizontal axis was measured after thefurnace dwells at the setup temperature for at leasthalf an hour. First, about 6.5 g of Mo slugs (Alfa Ae-sar, 99.95%) and 50 g of Sb shot (Alfa Aesar, 1-3 mm,99.9999%) were loaded into a quartz tube of 12 cm length,14 mm inner diameter and 1.5 mm wall thickness. Simi-lar to the aforementioned growth of IrSb3, the mixing ofMo and Sb starting materials should be avoided. UsingSb ingots premelted in tubes of same inner diameter canhelp control the tube filling and avoid mixing of Mo slugand Sb shot. The quartz tube was then sealed under vac-uum and the sealed ampoule was put in a single zone tubefurnace with Mo located at the hot end (Fig. 5(b)). Thefurnace was set up at 815◦C and kept at this temperaturefor a week before furnace cooling to room temperature.

Mo3Sb7 crystals were found in the whole tube andburied in Sb flux. Figure 5(c) shows the picture of onesmall portion of ingot with Mo3Sb7 crystals buried in-side of Sb flux. Sb flux around crystals can be mechani-cally removed using a surgical blade. Over 15 g Mo3Sb7

crystals with the largest dimension up to 5 mm can beisolated from the Sb flux as shown in Figure 5(d). Thelargest crystals are normally found at the cold end andthe average size of the crystals decreases from the coldend to the hot end. Crystals at the cold end are of com-parable size to those grown in the vertical configuration.Crystals from different positions were purposely pickedup and characterized by measuring x-ray powder diffrac-tion, magnetic susceptibility, and electrical resistivity.No sample variation was observed. In addition, singlecrystals grown in this technique have the same physicalproperties as those grown out of Sb flux using traditionalvertical configuration.14,15

Compared to the conventional vertical flux growth, theliquid transport growth produces a much larger quantityof Mo3Sb7 crystals of comparable dimension in a singlegrowth. In a typical vertical flux growth using a 2 mlCanfield crucible set, about 0.2 g of Mo3Sb7 crystals canbe obtained starting with a 4 g batch with 0.06 g of Moas the charge.

3. MnBi

MnBi shows unusual magnetic and magneto-opticalproperties that has attracted interests for decades. Thereis recent revived interest in this compound because ofits potential as a rare-earth-free permanent magnet athigh temperatures and as a hard phase in nanocompos-ite magnets.16 Figure 6 shows the Mn-Bi binary phasediagram. MnBi melts incongruently at 355◦C and canbe precipitated out of Bi1−xMnx (3≤x≤10) melt inthe temperature range 262∼355◦C. Compared to IrSb3

and Mo3Sb7, the composition range appropriate for fluxgrowth is wider but with a narrower temperature range.

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FIG. 6. (color online) Phase diagram of Mn-Bi from Refer-ence[ 12].

FIG. 7. (color online) Liquid transport growth of Mn-Bi.(a) ampoule after growth. The ampoule was tilted at about300oC when Bi is still in liquid state. MnBi was found inabout 2-3 cm range from the cold end. (b) The SEM pictureof the cross-section of the precipitation at the cold end.

One more important feature that distinguishes MnBifrom IrSb3 and Mo3Sb7 is that the composition of MnBi(50% Mn) is quite different from that of the coexistingliquid (3-10% Mn). This limits the yield of conventionalvertical flux growth following the lever rule. We thustried the flux growth using the horizontal configuration.

The starting materials are Bi shot (Alfa Aesar, spher-ical, 1-5 mm, 99.999%) and Mn pieces (Alfa Aesar,99.95%). Mn was arc melted before using to remove thesurface oxide layer. In a typical growth, about 1g of Mnwas first loaded into a quartz tube of 9 mm inner diam-eter and 1.5 mm wall thickness. Then about 10 cm ofthe tube was filled with Bi shot. The ampoule was thensealed under vacuum and put into a single zone tubefurnace with the hot end at 350◦C and the cold end at300◦C. After one week, the ampoule was taken out of

the furnace and cooled to room temperature. Figure 7(a)shows the ampoule after the growth. MnBi precipitationwas found within about 3 cm of the cold end. X-ray pow-der diffraction found both MnBi and Bi at the cold end.MnBi crystals are much smaller than those grown usingthe conventional vertical configuration. As the crystalsare small, we thus looked at the fracture surface of theas-grown ingot at the cold end using scanning electronmicroscope (SEM). Figure 7(b) shows the SEM picturein which submillimeter-sized MnBi crystals are separatedby Bi flux. As discussed Section IV, the growth of MnBicrystal is limited by the viscosity of Bi melt at such lowgrowth temperatures.

MnBi is not the only compound in the Mn-Bi bi-nary system and MnBi is stable only in the temperaturerange 262∼355◦C. Thus the growth temperature shouldbe carefully controlled to avoid the precipitation of otherphases. For example, above 355◦C, Mn1.06Bi will formaccording to the phase diagram shown in Fig. 6. We willfurther discuss the importance of temperature in sectionIV.

B. Liquid transport growth from a halide flux

1. FeSe superconductor

Among all Fe-based superconductors, FeSe has thesimplest chemical formula and structure. However, crys-tal growth has been challenging because the supercon-ducting tetragonal phase is stable only below ∼450◦Cin a narrow compositional range and the superconduc-tivity is extremely sensitive to the nonstoichiometry.17

A KCl-AlCl3 eutectic flux, which melts around 120◦C,was found to be ideal for the growth of high qualityFeSe single crystals of millimeter size.18 The growth wasperformed in a tube furnace with the sealed quartz am-poule sitting horizontally in a temperature gradient. Ourgrowth of FeSe crystals follows the procedures reportedby Chareev et al.18 but starting with pre-reacted FeSe.The starting materials are iron powder (Alfa Aesar, -22mesh, 99.998%) and selenium shot (Alfa Aesar, amor-phous, 2-6 mm, 99.999%). The starting materials in themolar ratio of Fe:Se=1.063:1 were sealed in a quartz tubeunder vacuum and heated in a box furnace to 250◦C in 10hours, kept at this temperature for 12 hours, then heatedto 750◦C in 24 hours and dwelled for 16hours, and finallyheated to 1100◦C in 20 hours and homogenized at thistemperature for 20 hours before furnace cooling to roomtemperature. The premelted FeSe was ground into coarsepowder and loaded into a quartz tube with an inner di-ameter of 19 mm and thickness of 1.5 mm. Then AlCl3and KCl in a ratio of AlCl3:KCl=2:1 were introducedinto the ampoule. The step is done inside of a dry glovebox filled with He as AlCl3 is rather hygroscopic. Thepremelted FeSe is kept at one end of the tube and notmixed with the halide flux. The ampoule was then sealedunder vacuum and Fig. 8(a) shows a picture of such an

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FIG. 8. (color online) Crystal growth of FeSe out of a KCl-AlCl3 eutectic flux.(a) the ampoule before growth. (b) theampoule during growth with some crystals at the cold endinside of the clear melt. (c) the ampoule after growth. (d)crystals at the cold end. A cluster of plate-like crystals werenormally found at the cold end. The left panel shows a singlepiece of FeSe crystal on a mm grid.

ampoule before growth. The ampoule was then put in-side of a single zone horizontal tube furnace with theFeSe powder staying at the center of the furnace. Thefurnace was then slowly heated up to 400◦C and kept atthis temperature for two weeks. The temperature at thecold end is determined to be 375◦C. Figure 8(b) showsa picture of the ampoule during growth. The residualstarting materials at the hot end and a cluster of grownFeSe crystals at the cold end can be observed through theclear flux melt. After two weeks, the furnace was shutoff and cooled to room temperature. Figure 8(c) showsthe picture of the ampoule after cooling to room tem-perature. We extended the growth to two months andfound there is always some residual starting material atthe hot end. The crystals were then washed out of fluxand cleaned as described by Chareev et al.18 Figure 8(d)shows the cluster of crystals at the cold end and the pic-ture of one piece of plate-like crystals on a mm grid. Ourtransport measurements under hydrostatic pressure con-firmed that FeSe crystals grown in this way are of goodquality.19,20

2. van der Waals magnets and ferroelectrics

The low melting temperature of KCl-AlCl3 eutecticmixture and the successful growth of FeSe crystals sug-gest that KCl-AlCl3 flux can be employed to grow otherchalcogenides with low melting or decomposition tem-peratures. We thus tried the growth of CrTe3 out ofthe KCl-AlCl3 eutectic flux using the horizontal config-uration. CrTe3 is an antiferromagnetic semiconductorwith magnetic layers made up of lozenge shaped Cr4tetramers. According to the Cr-Te phase diagram, CrTe3melts incongruently at 480◦C. The peritectic plateau ex-tends to 97% Te. The liquid coexisting with CrTe3 hasa very narrow the composition range (97%-98% Te) ina very narrow temperature range (445-480◦C). This nar-row composition and temperature ranges allow conven-tional vertical flux growth but with a rather low yield.21

Liquid transport growth using self flux is expected towork well for this compound but with the concerns oflow growth temperature and the difficulty of extractingthe crystals from the residual Te flux. We thus tried theliquid transport growth out of the halide flux since CrTe3is not hygroscopic and crystals can be easily washed outof the halide flux. In our growth, about 100 mg of pre-fired CrTe3 powder , KCl, and AlCl3 were loaded into asilica tube (9 mm I.D., 1.5 mm wall thickness) that wasthen sealed under vacuum. As for the growth of FeSe,the chemical loading was performed inside of a dry glovebox filled with He. The ampoule was heated in a hori-zontal furnace with the hot end held at 450◦C and thecold end at 425◦C. Millimeter sized platelike crystals isobtained after several days to one week(see Fig. 9 (a)).These CrTe3 crystals have comparable in-plane dimen-sion but are much thinner than those grown of the Teflux in the vertical configuration. The details of crys-tal growth and physical properties have been reportedelsewhere.21

The same flux technique has been employed togrow other van der Waals functional compounds wheretwo dimensional layers are held together by van derWaals bonding, such as transition metal selenophos-phates (MPSe3, M=transition metal) and thiophos-phates (MPS3). In these compounds the functionalityis determined by the type of metal present on the cationsublattice, yielding materials with functionalities rangingfrom magnetic to ferroelectric. While the vapor trans-port method using I2 as the transport agent can growsizable single crystals of MPS3, it does not work well forsome MPSe3 members as P-Se mixture melts at ratherlow temperatures. By using a similar procedure and thesame temperature profile used for the growth of CrTe3,we have been growing out of the KCl-AlCl3 eutectic fluxmillimeter sized single crystals of NiPSe3 and FePSe3,and ferroelectric CuInP2S6. Figure 9 (b) and (c) show thecrystal pictures of FePSe3 and CuInP2S6, respectively.

In the above growths of FeSe, CrTe3, MPSe3, andInCuP2S6, the KCl-AlCl3 mixture does not change thecomposition of the starting materials and is expected to

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FIG. 9. (color online) Pictures of compounds grown out of aKCl-AlCl3 eutectic flux: (a) CrTe3 in a mm grid, (b) FePSe3,(c) CuInP2S6, and (d) SEM picture of a cluster of OsCl4 cubesdeposited at the cold end.

work as transport media only. In some special cases, thehalide salt mixture can chemically react with the start-ing materials and deposit at the cold end small halidecrystals. A good example of this is RuCl3. α-RuCl3 hasattracted intense attention recently as a candidate withproximate Kitaev quantum spin liquid behavior.22 Con-sidering possible reaction between oxides and the halideflux, we tried the growth of RuCl3 starting with RuO2

powder in the KCl-AlCl3 mixture. About 1 gram ofRuO2 was first dried at 900◦C for overnight and thensealed in a quartz tube together with the KCl-AlCl3 eu-tectic mixture. The ampoule was heated in a horizontalfurnace with the hot end at 420◦C and the cold end at380◦C. After 8 days, plate-like crystals of submillimetersize form with the majority at the cold end and somein the middle of the ampoule. X-ray powder diffractionconfirms that both α- and β-RuCl3 of comparable frac-tion are obtained. We did not try to optimize the fluxgrowth because large α-RuCl3 crystals up to 0.5 gram perpiece can be obtained using the sublimation behavior ofRuCl3 at high temperatures.23 However, considering thecommercial RuCl3 powder is always contaminated withRuOCl2, Ru, or RuO2, reacting RuO2 with the KCl-AlCl3 eutectic flux can be employed to provide phasepure RuCl3 starting materials for further crystal growth.We also tried to react OsO2 with the KCl-AlCl3 eutecticflux with the OsO2 powder at the hot end of 400◦C. Thetemperature at the cold end is kept at 370◦C. After 6days, the flux turns to be brown with some black depo-sition at the cold end. After washing in water, only theblack deposition is left. Figure 9(d) shows the SEM pic-ture of a cluster of small cubes deposited at the cold end.

X-ray powder diffraction at room temperature shows theblack deposition is OsCl4 (space group Cmmm).24 AsOsCl3 is reported to be dark brown and hygroscopic,25

the color change of the flux suggests that OsCl3 mightform but is washed away. The liquid transport growthtechnique can be employed to grow halide crystals or toprovide necessary starting materials for crystal growth.This approach can be considered when growing variouslayered transition metal halides.26

In all the above growths using the KCl-AlCl3 eutec-tic flux, the temperature of the hot end is kept around400◦C. Higher growth temperatures above 500◦C mightlead to tube failure possibly due to the reaction betweenquartz tube and halide flux and the vapor pressure of thehalide flux.

IV. Discussion

Growth mechanism Figure 1 illustrates the principlesof vapor transport and liquid transport growth. Thedifference between these two techniques is the state oftransport medium. For the vapor transport growth, asmall amount of transport agent in a vapor state worksas the transport agent at high temperatures. While forthe liquid transport growth, the growth ampoule is par-tially filled with a large amount of flux and the flux meltis expected to transport the charge from the hot end tothe cold end. For both techniques, crystals are expectedto first appear at the cold end. This has been well es-tablished for vapor transport growth. Two observationsin our proof of principle growths are important in under-standing the growth mechanism of the liquid transportgrowth: (1) IrSb3 (see Fig. 3(c)), MnBi (Fig. 7(a)), FeSe(Fig. 8(c)), CuInP2S6, FePSe3, and OsCl4 single crystalsare found at the cold end of the growth ampoule. This ob-servation confirms that crystal growth first occurs at thecold end and suggests the flux melt behaves as the trans-port agent; (2) Mo3Sb7 crystals were found in the wholeampoule for the growth starting with a large amount ofstarting Mo. This indicates that the growth front gradu-ally moves from the cold end to the hot end and crystal-lization can take place in the whole ampoule with enoughgrowth time and starting materials.

With the above observation and analysis, the detailedgrowth process can be summarized as the schematic inFig. 10: (1) Fig. 10(a) illustrates the state when the fluxjust melts with very little charge dissolved into the fluxmelt. (2) As time proceeds, the flux melt starts to dis-solve the charge. The dissolved charge diffuses from thehot end to the cold end due to the compositional gradi-ent. The dashed curve in Fig. 10(b) shows the concen-tration of dissolved charge after some time of dwelling athigh temperatures. (3) With more extended stay at hightemperatures, the concentration of the dissolved chargeat the cold end goes beyond the solubility limit. Un-der enough supersaturation, nucleation occurs and thenuclei grow with the charge transported from the hot

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FIG. 10. (color online) Possible mechanism of liquid trans-port growth. (a) The growth ampoule when flux first meltswith little charge dissolved. (b)The ampoule in early stage ofthe growth highlighting the diffusion of dissolved charge fromthe hot end to the cold end. The dashed curve shows possiblecomposition gradient of charge in flux at a certain time. (c)Crystals start to precipitate at the cold end once the concen-tration of charge is above the solubility limit. Panel (d) and(e) illustrate that different crystals can be obtained by con-trolling the temperature profile. (d) Different crystals (crystalA and B) can precipitate in one single growth depending onthe temperature profile along the horizontal axis. (e) one hy-pothetical phase diagram that determines the stable phasesat different positions in (d).

end by the flux melt as illustrated in Fig. 10(c). Withthe grown crystals taking the space at the cold end, thegrowth fronts gradually moves to the hot end.

Temperature is critical Temperature is one critical pa-rameter in liquid transport growth as in other high tem-perature growths. Thermodynamically, it determines thephase that will precipitate out of the melt. In the growthof Mo3Sb7 which is the only compound in the Mo-Sbbinary system, Mo3Sb7 is expected to nucleate as longas the temperature at the cold end is above the meltingtemperature of Sb. However, for systems with more than

one compounds, such as Ir-Sb (Fig. 3) or Mn-Bi (Fig. 7),the growth temperature should be carefully selected tocrystallize the desired phase. By controlling the temper-ature gradient along the horizontal axis, it is possible tocrystallize different phases at different positions of thegrowth ampoule. This is well illustrated with Fig. 10(d)and (e). Fig. 10(e) shows a hypothetical phase diagramwith two phases A and B which would grow out of fluxmelt in different temperature intervals. Fig. 10(d) illus-trates that crystals A and B can grow above and belowthe peritectic temperature, respectively, in one single liq-uid transport growth.

Kinetically, the temperature determines the solubilityand diffusivity of solute inside of the melt and the super-saturation, thus the morphology and dimension of thecrystals. The growth process illustrated in Fig. 10 sug-gest that the diffusion transport of solute in the flux meltis prerequisite for the crystallization. We can thus expectthat the growth rate is proportional to the diffusion coef-ficient of solute in the flux melt, and the solubility differ-ence between the hot end and the cold end, and inverselyproportional to the diffuse length between the charge andthe growth front. The values of solubility for given tem-peratures can be taken from the liquidus curve. From thephase diagrams shown in Fig. 2, 4, and 6, the compo-sitional gradient for the growth of MnBi is much largerthan that for Mo3Sb7 and IrSb3. The fact that MnBicrystals are small suggests that the diffusion coefficient,which is a function of both temperature and position inliquid transport growth, is the major factor affecting thisgrowth. The small crystals and quite some Bi inclusionsmight both result from the low growth temperature be-low 300◦C. At such a low temperature, the viscosity of Biis large and the diffusivity of solute is small. This is sup-ported by the observation in a conventional flux growththat stirring the melt by shaking the crucible whelps toproduce larger crystals.16

Temperature gradient is necessary In all our liquidtransport growths, a temperature gradient along the hor-izontal axis is always applied and this temperature gra-dient is believed to be necessary for the completion ofthe growth. Without the temperature gradient, a ho-mogeneous melt is expected at high temperatures andrandom nucleation would occur once the charge concen-tration is beyond the solubility limit. The temperaturegradient along the horizontal axis makes it possible forthe nucleation first occurs at the cold end and the growthfront gradually moves from the cold end to the hot endof the growth ampoule. The observation of IrSb3 (Fig. 3)and MnBi (Fig. 7) crystals only in the 2-3 cm range fromthe cold end supports that the crystallization first occursat the cold end and then gradually moves to the hot end.

However, the temperature gradient might affect thestoichiometry of the as-grown crystals for those com-pounds with a temperature dependent composition. Forexample, the fraction of Te in 1T’-MoTe2 changes from66%at to 68%at when cooling from 1180◦C to 998◦C ac-cording to the available phase diagram. Liquid transport

9

growth with horizontal temperature gradient is not idealfor 1T’-MoTe2 as the growth will result in crystals withdifferent stoichiometry and thus different physical prop-erties. This is supported by the fact that MoTe2 crys-tals grown at a fixed temperature show a larger resid-ual resistivity ratio and magnetoresistance than thosegrown by cooling the self flux in the temperature range998◦C∼1180◦C.27 The temperature gradient may also af-fect the composition of dopants if the distribution coeffi-cient of the dopant has a large temperature and compo-sition dependence. This effect should be taken into con-sideration when liquid transport technique is employedto grow some doped compositions. On the other hand,this effect of the temperature gradient can be employedto fine tune the stoichiometry of crystals.

For all the growths reported in this work, the chargealways stays at the hot end. However, if the solubilityincreases with decreasing temperature, the charge shouldbe kept at the cold end. This is analogous to what hap-pens in vapor transport growth: the starting materialsare kept at the cold end and crystals form at the hotend of the growth ampoule when the reaction betweenthe charge and the transport agent is exothermic; thetemperature gradient should be reversed if the reactionis endothermic.

Comparison with the vertical flux growth Compared tothe conventional vertical flux growth, the following fea-tures of the liquid transport growth are noteworthy:

(1) The charge/flux ratio is not limited by the solu-bility limit at given temperatures. In the vertical fluxgrowth, the charge/flux ratio is determined by the liq-uidus line at given temperatures. In contrast, a muchlarger quantity of charge can be used in the liquid trans-port growth. This feature allows the liquid transportgrowth to produce a large quantity of crystals in a singlegrowth. For all three incongruent melting compoundsgrown out of a self flux reported in Section III A, thecomposition of the peritectic liquid coexisting the desiredphase gives the maximum fraction of charge allowed with-out precipitating other phases in a vertical flux growth.This can limit the yield of crystals in a single growth asdescribed below.

(2) Crystal growth occurs before fully dissolving thecharge. For the vertical flux growth, it is important tostay at temperatures above the liquidus line at a givencomposition to form a homogeneous melt before cooling.For the liquid transport growth, the flux dissolves thecharge and the compositional gradient along the hori-zontal axis drives the diffusion transport.

(3) A temperature gradient is applied along the hor-izontal axis and mixing of charge and flux should beavoided. This temperature gradient is important in driv-ing the diffusion transport of charge from the hot endto the cold end of growth ampoule and driving the crys-tal growth in opposite direction. Random or uniformdistribution of charge in flux could impede the diffusiontransport of charge and might induce random nucleationinside of the growth ampoule.

It should be noted that most compounds mentionedin this work can be grown out of flux using the conven-tional vertical configuration. The conventional verticalflux growth is still the more widely used technique whichshould be considered first. The liquid transport growthis a complementary technique and is recommended whena large amount of crystals are needed but the yield fromthe conventional flux growth is limited. Both IrSb3 andMo3Sb7 melt incongruently and the peritectic plateauextends to very high Sb content according to the avail-able phase diagrams. This limits the fraction of chargein the melt thus the yield of crystals when the conven-tional vertical flux is performed. For example, a Mo:Sbratio of 1:49 is used for the growth of Mo3Sb7 and a Ir:Sbratio of 3:97 for IrSb3. MnBi is a good example that thelow yield from the vertical flux growth is caused by thelarge compositional difference between the solid phaseand the coexisting liquid phase. According to the Mn-Bi phase digram, the composition of MnBi (50% Mn) isquite different from that of the coexisting liquid (3-10%Mn). Following the lever rule, this limits the yield of aconventional vertical flux growth. There is no phase dia-gram or other thermodynamic data about the solubilityof chalcogenides or halides in the KCl-AlCl3 eutectic flux.However, the observation that there is always some left-over starting materials in the hot end suggests that thesolubility is limited. Liquid transport growth of chalco-genides and halides in the KCl-AlCl3 eutectic flux canproduce a large amount of crystals in a single growth asit is not limited by the solubility at given temperatures.

Extracting crystals from the flux For liquid transportgrowth from a halide flux, the flux can be washed awayin water or other solvents in case the crystals are mois-ture sensitive. For liquid transport growth from a metalflux, we tried two different ways to separate the crystalsfrom the flux. For IrSb3, we loaded the part from thecold end of the liquid transport growth ampoule into aCanfield crucible set, and centrifuged it after melting theflux at high temperatures. This approach does not workif more than one phase is obtained (Fig. 10(d)) in theampoule or the stoichiometry of crystals is sensitive tothe temperature gradient. For Mo3Sb7, the flux wrap-ping the crystals can be removed mechanically using asurgical blade since the crystals are large and mechani-cally strong. This mechanical extraction does not workfor soft crystals such as MoTe2, which is soft and exfo-liable. Other methods of extracting crystals from solidflux might be needed.28

This is not horizontal Bridgman This growth tech-nique described in this work is different from the horizon-tal Bridgman method, or modified Bridgman techniqueperformed in a horizontal temperature gradient, or thehorizontal gradient-freeze technique29. The fundamentaldifference lies in whether the charge and flux are wellmixed and a homogeneous melt is obtained before gen-erating supersaturation. As in the vertical flux growth,the Bridgman technique involves melting all starting ma-terials (both charge and flux for the modified Bridgman

10

technique) at high temperatures to form a homogeneousmelt before applying a temperature gradient to generatesupersaturation. For the liquid transport growth, theamount of charge is always more than the flux can dis-solve at the growth temperature and a homogeneous meltis never obtained before precipitation occurs at the coldend.

The other prominent difference is the presence of acompositional gradient along the horizontal axis. Forthe modified Bridgman technique performed in a hori-zontal configuration, the melt far away from the growthfront is homogeneous in composition and the composi-tional gradient exists only near the growth front. Thenucleation and growth takes place once the temperatureof the cold end is below the liquidus line. In contrast, thecompositional gradient in the horizontal transport tech-nique is inherent and the crystallization takes place oncethe concentration of solute is above the solubility limit.The magnitude of the composition difference along thehorizontal axis depends on the slope of the liquidus linein phase diagram.

Possible role of vapor phase It is worth mentioningthat the sealed ampoules are only partially filled with theflux. It is likely the vapor phase might also play a rolein species transport. In the growth of IrSb3, Mo3Sb7,and MnBi, the vapor pressure of starting materials orother intermediate binary phases are small at the growthtemperatures employed in the growth. Single crystalsare found inside of the flux. For the growth out of halideflux, according to the pseudo-binary AlCl3-KCl phase di-agram, there is a certain vapor pressure around 400◦C.This vapor pressure might be involved in the mass trans-port in the growth of FeSe as FeSe crystals are foundboth below and above the melt at the cold end. Becauseof this, Bohmer et al. called growth using this techniqueas vapor transport growth and they purposely tilted thegrowth ampoule.30 However, in all other growths we per-formed, single crystals are observed only inside of themelt which does not suggest a significant role of vaporphases in the growth. The possible effect or involvementof the vapor phases deserves further investigation in theliquid transport growth.

V. Summary

In summary, we report the flux growth in a horizon-tal configuration with a temperature gradient along thehorizontal axis. At high temperatures, the flux melt dis-solves the charge at the hot end and transports to the coldend where crystallization occurs after the concentrationof solute is above the solubility limit. The growth princi-ples involved are similar to those in the vapor transportgrowth. We thus name it liquid transport growth. Thistechnique can provide a large amount of crystals in a sin-gle growth as the charge/flux ratio is not limited by thesolubility limit at given temperatures. This is supportedby the growths of proof-of-principle compounds reported

in this work. This technique can grow crystals of compa-rable dimension as the conventional flux growth. Tem-perature is a key growth parameter which determines thephases that would precipitate and the crystal size. Thetemperature gradient along the horizontal axis is neededto drive the diffusion transport from the hot end to thecold end of the growth ampoule and to drive the crystalgrowth in an opposite direction.

Our proof-of-principle compounds reported in thiswork either melt incongruently or have a large vapor pres-sure at high temperatures. We would expect the liquidtransport growth can also be employed to grow congru-ent melting compositions at easily accessible tempera-tures much lower than the melting point. We also expectthat this technique works well for the flux growth of ox-ides. This liquid transport technique is complementaryto the conventional vertical flux growth and can be em-ployed when a large amount of crystals are needed butthe yield from the conventional vertical flux growth islimited. When the temperature has to be low in orderto precipitate the right phase, the large viscosity of themelt and the low diffusivity of solute in the melt mightlimit the size of the final crystals. This concern should betaken into consideration when selecting the appropriateconfiguration, horizontal vs vertical, for the flux growthof desired crystals.

The present work is more focused on understandingthe principles of liquid transport growth. We did nottake efforts to optimize the growth conditions. More fu-ture studies are needed to understand the effects of someimportant growth parameters, such as temperature andtemperature gradient along the horizontal axis, the fluxfilling of the growth ampoule, the dimension of the am-poule, the charge/flux ratio, and the tilting angle of thegrowth ampoule. Our present understanding of liquidtransport growth suggests that the diffusion of the solutein the flux melt might control the growth rate. How-ever, dissipation of the latent heat away from the crystalsurface or reactions at the crystal-melt interface couldsignificantly affect the growth rate in some cases. Thekinetics of liquid transport growth also deserves furtherstudy.

VI. Acknowledgment

This research was supported by the U.S. Departmentof Energy, Office of Science, Basic Energy Sciences, Ma-terials Sciences and Engineering Division. Manuscriptpreparation was partially funded by the Air Force Re-search Laboratory under an Air Force Office of ScientificResearch grant (LRIR No. 14RQ08COR) and a grantfrom the National Research Council.

This manuscript has been authored by UT-Battelle,LLC, under Contract No. DE-AC0500OR22725 with theU.S. Department of Energy. The United States Govern-ment retains and the publisher, by accepting the arti-cle for publication, acknowledges that the United States

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Government retains a non-exclusive, paid-up, irrevoca-ble, world-wide license to publish or reproduce the pub-lished form of this manuscript, or allow others to do so,for the United States Government purposes. The De-

partment of Energy will provide public access to theseresults of federally sponsored research in accordancewith the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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