Jurnal Baso4 Anor

Journal of Crystal Growth 234 (2002) 533–538

Flux growth of baryte-type BaSO4 from chloridic alkalinemetal solvents

D. Ehrentraut*, M. Pollnau

Department of Microtechnique, Institute of Applied Optics, Swiss Federal Institute of Technology,

CH-1015 Lausanne, Switzerland

Received 14 May 2001; accepted 29 August 2001

Communicated by T. Hibiya

Abstract

The growth of BaSO4 from high-temperature fluxes of both chloridic alkaline metal and alkaline-earth metal solventshas been investigated. Two binary alkaline-metal solvent systems containing LiCl and the additive ternary system withCsCl–KCl–NaCl where optimized with respect to solute concentration and growth temperature. It is found that the

solution of BaSO4 in these solvents increases with decreasing average radius of the solvent cation. In addition, thesolubility of BaSO4 is increased in the LiCl-containing fluxes by a chemical reaction between solute and solvent. Binarysolvents of LiCl–KCl and LiCl–CsCl delivered prismatic crystals of 2 cm size, whereas the CsCl–KCl–NaCl flux prefersthe growth of short-prismatic crystals. The crystals grown under optimized conditions were crack free, colorless, and

transparent. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 61.72.Qq; 81.05.Je; 81.10.Dn

Keywords: A1. Solubility; A1. Solvents; A2. Growth from solution; B1. Barium compounds; B1. Lithium compounds

1. Introduction

Transition-metal (TM) ions are widely usedas activator ions in laser crystals. Prominentexamples are Al2O3 : Cr

3+, Ca2+co-dopedY3Al5O12 : Cr

4+, and Al2O3 : Ti3+, and the valence

states of TM ions vary significantly (for example,the valence states of Cr vary theoretically fromCr2+ to Cr6+).An interesting but so far rather poorly investi-

gated material is the baryte type of BaSO4. First,

this host offers the possibility to substitute thesulphur ion in the SO2�

4 -tetrahedron by fourfold-coordinated TM ions in their high valence states(typically TM5+ or TM6+). Second, BaSO4 formssolid solutions with SrSO4 [1] and CaSO4 [2]. Inthis series, the refractive index n decreases withdecreasing size of the cation. For example, in thesolid solution BaxSr1�xSO4 n ranges from n ¼1:636 [3] (x ¼ 1) to n ¼ 1:631 [4] (x ¼ 0:78). Thisfeature offers the possibility to create waveguidestructures by adjusting a lower n for the substratecompared to the active layer. The disadvantage ofBaSO4 is that it cannot be grown from its meltbecause of thermal decomposition [5]. In addition,

*Corresponding author. Fax: +41-21-6933701.

E-mail address: [email protected] (D. Ehrentraut).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 6 9 3 - 1

BaSO4 undergoes a phase transition [6] at 10901Cto a high-temperature a-form. Therefore, onlygrowth techniques from solutions are applicable.Group I (alkaline metal) and group II (alkaline-earth metal) chlorides are suitable solvents foralkaline tungstates, molybdates, and sulphates [7].These chlorides are non-toxic and available inlarge quantities of adequate purity.We investigated the growth of BaSO4 crystals

from different fluxes of chloridic alkaline metalsand alkaline-earth metals. Our main emphasiswhen searching for a suitable solvent was put onthe solubility of BaSO4 in the solvent. Differ-ences of the valence states between solute andsolvent [8] and a lower ionic radius of thesolvent compared to the solute are necessary toachieve a high solubility. Furthermore, the solu-bility should be sufficiently high at lower tempera-tures, because TM ions have the tendency toreduce their valence states with increasing growthtemperatures.In this study, we report on flux optimization for

the growth of large BaSO4 crystals. Mixtures ofLiCl, NaCl, KCl, RbCl, CsCl, CaCl2, and MgCl2were investigated systematically as suitable sol-vents for BaSO4 at temperatures lower than 6001C.Large BaSO4 crystals will be used as substrates in afollow-up layer-growth process by means ofliquid-phase epitaxy (LPE) to fabricate opticalthin films activated with TM ions in their highoxidation states (TM5+ or TM6+) at temperaturesbelow 6001C.

2. Experimental procedure

We used chemicals with purities from 3 to 4.5N.The chemicals were dried at 1501C for 24 h. Afterweighing and mixing, they were filled into Al2O3crucibles and immediately heated for melting. Theflux experiments were carried out under ambientatmosphere in a resistance heated furnace whichprovided a precision in temperature control of0.3K. Start temperatures Tstart between 5101C and6001C for the LiCl–KCl system, 4501C and 6001Cfor the LiCl–CsCl system, and 5701C and 6001Cfor the CsCl–KCl–NaCl system were chosen. Thecooling rate from Tstart down to the solidificationtemperature Ts was 1.0Kmin

�1 for all experi-ments. Table 1 shows the quantitative variation ofthe compositions of the solvents and the concen-trations of BaSO4.The water-insoluble BaSO4 crystals can be

extracted easily from the flux after the experimentby solving with distilled water. The crystals werechecked for phase purity using X-ray powderdiffraction (Guinier-de Wolff method) with siliconas the internal standard. Optical characterizationwas carried out by means of microscopy.

3. Results and discussion

The crystal symmetry of baryte-type BaSO4is orthorhombic (Pnma) with lattice constantsa0 ¼ 8:878 (A, b0 ¼ 5:450 (A, and c0 ¼ 7:152 (A [3].

Table 1

Solidification point (Ts), growth time (tG), cooling rate, and the concentrations of BaSO4 ðcBaSO4Þ for different compositions of solvents

for the flux growth of baryte-type BaSO4

Composition (mol%) Ts (1C) Ref. in [10] tG (h) Cooling rate (Kh�1) cBaSO4(wt%)

LiCl :KCl

65 : 35 405 Fig. 1777 165 1 15.0

60 : 40 348 Fig. 1777 112–252 1 9.0–30.0

35 : 65 590 Fig. 1777 10 1 12.0–15.0

LiCl : CsCl

57 : 43 326 Fig. 1227 105–255 1 3.0–11.0

50 : 50 333 Fig. 1227 215–245 1 4.0–9.0

CsCl :KCl:NaCl

45.2 : 24.4 : 30.4 480 Fig. 3241 120 1 2.5–6.0

D. Ehrentraut, M. Pollnau / Journal of Crystal Growth 234 (2002) 533–538534

We investigated the following flux systems inmore detail because of their high yield of grownBaSO4 crystals in our preliminary experiments: thebinary LiCl–KCl and LiCl–CsCl, and the additiveternary CsCL–KCl–NaCl solvents. The latter wasreported [9] to be successful in flux-growthexperiments of TM-ion doped BaSO4. Combina-tions of the group II chlorides SrCl2 and BaCl2with group I and/or group II chlorides are not ofinterest for our purposes due to melting pointsabove 6001C [10].The parameters in our flux experiments were: (a)

concentrations c of the constituents of the solvent,(b) concentration cBaSO4

of BaSO4 in the solvent,and (c) the start temperature Tstart of the experi-ment. The variation of parameter (a) will inevi-tably change the temperature of solidification ofthe solvent.

3.1. Solubility

The solubility of BaSO4 in chloridic solventswas estimated under growth conditions. Thesolubility sBaSO4

is the value of cBaSO4; the

concentration of BaSO4, where nucleation ofBaSO4 is initiated. In each system we addeddifferent amounts of the solute BaSO4 in order todetermine the solubility of BaSO4. High super-saturation s; s ¼ ðcBaSO4

� ceÞ=ce; with ce; theequilibrium concentration of the solute, may resultin growth of dendrites (instable growth), whereas asolute concentration cBaSO4

below the critical super-saturation will not initiate crystal growth. The re-gion between the curves of ce and sBaSO4

in a typicalconcentration-versus-temperature diagram is theOstwald-Miers-region, a metastable region of s:The results of our experiments are shown in

Figs. 1a–c. The LiCl–KCl (Fig. 1a) and LiCl–CsCl(Fig. 1b) fluxes show a different behavior in thesolution of BaSO4 compared to the CsCl–KCl–NaCl flux (Fig. 1c). Each straight line in Figs. 1a–ccorresponds to one flux experiment. The differ-ences in cBaSO4

between Tstart (open squares) and Ts(solid squares) represent the amounts of BaSO4deposit obtained in each experiment. Completedissolution of BaSO4 in the solvent gives a linewithout slope, and no deposit is obtained, see line6 in Fig. 1a and lines 4, 5, and 7 in Fig. 1b. A

similar behavior, complete dissolution of BaSO4and no deposit at the end of the experiment, wasnot found in the additive ternary system and weobtained BaSO4 deposits for all concentrations ofBaSO4 between 2.5 and 6.0wt%, Fig. 1c. The twobinary solvents are distinguished by the solubilityof BaSO4, which differs by a factor of almost 2 tothe favor of LiCl–KCl. The results of Figs. 1aand b show an increase of the solubility sBaSO4

of BaSO4 with increasing LiCl concentration.This increase can partly be explained by thedecreasing average radius of the solvent cationsrþ ¼ xAþ � rAþ þ xBþ � rBþ ; with x; the mole frac-tions of the salts A, B, and r; the radii of thecations A+, B+. The values of the ionic radii aretaken from [11]. Fig. 2 displays the dependence ofsBaSO4

on rþ: The average cation radius of thesolvent increases from 0.91 (A for the 65 : 35LiCl :KCl system to 1.55 (A for the 67 : 33CsCl :NaCl system. The increase of sBaSO4

withdecreasing average cation radius is very pro-nounced. The difference in sBaSO4

between the65 : 35 LiCl :KCl solvent (symbol 1 in Fig. 2), andthe CsCl :KCl :NaCl solvent (symbol 6 in Fig. 2),is almost one order of magnitude.In addition, the solubility depends on the

chemical nature of the solvent. In certain cases, achemical reaction between solvent and solute maytake place, which changes the solubility of thesolute. Alkaline-earth metal chlorides are comple-tely dissociated in the molten state [12], and are,therefore, comparable which each other in theirsolubility of a common solute. As we will see in thefollowing, this is not the case for all the alkaline-metal solvents.The following results confirm our assumption of

the existence of chemical reaction during the pro-cess of BaSO4 dissolution, and the formation of newphases. The minimum amounts of BaSO4 to initiatecrystal growth are: 9wt% for LiCl–KCl, 5.5wt%for LiCl–CsCl, ando2.5wt% for CsCl–KCl–NaCl.However, the highest yield of crystallized BaSO4 of83.3% was achieved with the CsCl–KCl–NaClsystem, the system with the smallest sBaSO4

: The LiCl–KCl and the LiCl–CsCl systems delivered highestyields of only 55.6% and 42.8%, respectively.By use of the values [13] for the enthalpy of

formation DHf ; we calculated the following reac-

D. Ehrentraut, M. Pollnau / Journal of Crystal Growth 234 (2002) 533–538 535

tion to be energetically preferred in the LiCl-containing systems:

2LiClþ BaSO4-TLi2SO4þBaCl2:

At temperatures of 600K and above, DHf incre-ases to the favor of the products, DHf (600K)=

�6.731 kJmol�1, DHf (800K)=–131.258 kJmol�1.In contrast, in the additive ternary system nosulfate other than BaSO4 is energetically stable,because it does not contain LiCl.The shift of the chemical equilibrium toward the

formation of Li2SO4 and BaCl2 is probablyinitiated during dissociation of LiCl and BaSO4

Fig. 1. Experimental determination of the solubility of BaSO4 in group I chlorides in dependence on temperature and concentration:

(a) LiCl–KCl solvent (values for 100 : 0 are derived from [10], value for the eutectic temperature Teutectic after [LAG], (b) LiCl–CsCl

solvent (value for Teutectic after [16], and (c) CsCl–KCl–NaCl solvent (value for Teutectic after [10]. The compositions of the solvents are

given. The differences in the solidification temperatures Ts for the LiCl-containing systems in (a) and (b) are due to the different

compositions of the solvents. The values for Tstart (&) and Ts (’) are given for each curve. The difference in cBaSO4between Tstart and

Ts indicates the mass in wt% of the crystalline BaSO4 deposit of each experiment.


while heating up the flux. The concentrations ofLi2SO4 and BaCl2 increase with increasing tem-perature due to the increasing gain in DHf : Whilecooling down, the chemical reaction is probablyonly partly reversed and both formatted saltsremain in the liquid flux. Part of the formedLi2SO4 and BaCl2 remain in the flux even after itssolidification. Due to the high solubility of Li2SO4and BaCl2 in water, 3.11 and 1.78mol l

�1 at 251C[3], respectively, these components are lost duringextraction of the insoluble BaSO4 crystals (solubi-lity 10�5mol l�1 at 251C [3]).The phase diagram of the Li2SO4–BaCl2 system

[10] describes the coexistence of solid BaSO4 and aliquid solution of Li2SO4–BaCl2 below the liquidusline at temperatures much lower than 9001C. Thisis yet another indication for the hypothesis of theformation of Li2SO4 and BaCl2 during the fluxgrowth of BaSO4 from LiCl-containing solvents.

3.2. Crystal habit, quality, and size

In general, the habit of a crystal is in closerelation to its growth mechanism and the incor-

poration of impurities and inclusions, which bothdepend on the growth conditions [14]. The abilityto control the habit offers the chance to growtailor-made crystals with respect to follow-upapplications.Figs. 3a and b show typical flux-grown BaSO4

crystals. We observed prismatic habits for allthe systems. The fastest growth direction is thea-direction, ½1 0 0�; and therefore the crystals areelongated along ½1 0 0�: The b=c-ratio is almostunity for the solvents investigated. The largesta=c-ratio of 15 was found for the LiCl–CsClsolvent whereas the additive ternary CsCl–KCl–NaCl system delivered crystals with a morecompact, short prismatic habit (Fig. 3b). Thea=c-ratio was determined to be almost 2 in thelatter case. Both binary solvents seem to favor thegrowth of large b- or c-oriented substrates.Typical growth rates of the flux-grown crystals

are 10–50 mmh�1 for ½1 0 0�; and 5–10 mmh�1 for

Fig. 3. Comparison of the habits of BaSO4 crystals grown

from: (a) the LiCl–KCl solvent and (b) the CsCl–KCl–NaCl

solvent. The aspect ratios for the specimen of (a) and (b) are 8

and 2, respectively.

Fig. 2. Solubiltity of BaSO4 (sBaSO4) as function of the average

solvent cation radius rþ: The strong increase of sBaSO4with

decreasing rþ is due to the increased LiCl concentration in the

flux, which, firstly, decreases rþ; and, secondly, introduces achemical reaction between solute and solvent.

D. Ehrentraut, M. Pollnau / Journal of Crystal Growth 234 (2002) 533–538 537

½0 1 0� and ½0 0 1�: The higher values are for thecrystals grown from LiCl-containing fluxes. Nee-dle-like crystals with high optical quality could begrown at high growth rates of 500–1000 mmh�1 for½1 0 0� by use of the 35 : 65wt% LiCl–KCl solvent.The crystal size reached up to 2 cm in ð1 0 0Þ and0.3 cm in ð0 0 1Þ and ð0 1 0Þ:The crystals grown under optimized growth

conditions are clear, transparent, and colorless.The best optical quality was obtained for thecrystals grown from the additive ternary CsCl–KCl–NaCl system. A few cracks observed arepossibly due to mechanical stresses during fluxremoval and handling. BaSO4 exhibits a cleavagefrom good for f0 1 0g to perfect for f0 0 1g [15].

4. Conclusions

The solubility of BaSO4 in pure chloridicsolvents at start temperatures between 4501C and6001C was investigated. Binary LiCl–KCl solventsare favored for the growth of large ½1 0 0�-elongated crystals with high optical quality. Weobserved a high solubility of BaSO4 in LiCl-containing solvents. This effect was attributed tothe decrease in average cation radius with increas-ing LiCl contents and the formation of Li2SO4 andBaCl2 with increasing temperature. The highestgrowth rate is along ½1 0 0� for all solvent systemsinvestigated. The use of CsCl–KCl–NaCl assolvent favors the growth of short prismaticcrystals. Crystals with the best optical qualityachieved, without cracks, and without inclusionswere grown from the CsCl–KCl–NaCl flux.Furthermore, this system enables us to work atconstant crystal-growth conditions because of itschemical stability to the solute BaSO4, a fact thatis of high importance for the following LPEgrowth to achieve uniform layers of highest opticalquality.

Acknowledgements

The authors wish to thank Benoit Deveaud-Pl!edran for the outstanding help with laboratory

equipment and Ren!e-Paul Salath!e for his support.D.E. thanks Christine Klemenz for fruitful discus-sions. We also thank Karl Kr.amer and ThomasBrunold for experimental support and discussions.This work was partially supported by the SwissNational Science Foundation.

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