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Phase equilibria and b-BaB2O4 crystal growth in the BaB2O4–BaF2 system

Tatyana Bekker,*a Alexander Kokha and Pavel Fedorovb

Received 16th January 2011, Accepted 11th March 2011

DOI: 10.1039/c1ce05071k

The phase formation in the BaB2O4–BaF2 system has been studied. According to the results obtained,

the BaB2O4–BaF2 system is quasi binary with the eutectic at 760 �C, 41 mol% of BaB2O4, and 59 mol%

of BaF2. The fast pyrohydrolysis of BaF2 in air causes the gradual transformation of the BaB2O4–BaF2

system into the BaB2O4–BaO system and results in the co-crystallization of BaB2O4 and Ba5B4O11

phases, making the growth of large high-quality b-BaB2O4 crystals in air impossible.

Introduction

BBO (b-BaB2O4) crystal is a nonlinear optical material widely

used in the commercial laser systems. It combines a number of

unique properties.1–4 For instance, BBO is the only NLOmaterial

that can be used for the fifth harmonic generation (5HG) of Nd:

YAG lasers at 213 nm. Frequency-doubling and -tripling of

ultrashort pulse lasers (Ti:sapphire) are the applications in which

BBO shows more preferable characteristics than other nonlinear

optical crystals. However, some applications such as large

aperture laser systems have remained limited.

Due to the phase transition at 925 �C the main method for

growing BBO crystals is crystallization from a high temperature

solution by the top seeded solution growth technique. The most

promising results in terms of crystal quality have been obtained

using Na2O as the solvent and some compositions in the BaO–

B2O3–Na2O ternary system.5–10 However, the growth process in

such boron-enriched systems is hindered by the high viscosity of

the melt. This results in the constitutional undercooling and

related cellular growth, preventing the formation of large high-

quality crystals.

Fluoride solvents that allow a reduction in the viscosity of the

melt are of great interest for growing BBO crystals. Thus, the

viscosity of the BaB2O4–NaF system is 15% below the viscosity

of the BaB2O4–Na2O system in the corresponding growth

temperature range.11 W. Chen et al. reported the growth of a b-

BaB2O4 crystal with a diameter of 100 mm, height of 40 mm and

mass of 800 g in the BaB2O4–NaF system; it was used to produce

an optical element 21 � 14 � 8 mm3 in size.12 The reproducibility

of this result was not discussed. Our investigations of the

BaB2O4–NaF system showed that it could not be considered as

quasi binary. It includes a primary crystallization area of the new

compound Ba2Na3[B3O6]2F (hexagonal system, P63/m, a¼ 7,346

aInstitute of Geology andMineralogy Siberian Branch of Russian Academyof Science, Novosibirsk, Russia. E-mail: tatyana_bekker@mail.ru; Fax:+7 383 333394; Tel: +7 383 333394bGeneral Physics Institute Russian Academy of Science, Moscow, Russia.E-mail: ppfedorov@yandex.ru

3822 | CrystEngComm, 2011, 13, 3822–3826

(1) �A, c ¼ 12,637(2) �A) and belongs to the BaB2O4–(NaBO2)2–

(NaF)2–BaF2 ternary mutual system. Chemical processes in the

melt in the BaB2O4–NaF system are responsible for the essential

decrease of the yield coefficient during the growth of b-BaB2O4

crystals.13,14

Another fluoride compound BaF2 was also considered as

a candidate for the solvent. For b-BaB2O4 crystal growth the

BaF2 solvent was first used by C. Chen et al.15 Jiang et al. grew

large transparent b-BaB2O4 crystals from the composition

55.6 mol% BaB2O4 and 44.4 mol% BaF2.16 The eutectic

temperature determined by differential-thermal analysis was

752 �C. The liquidus temperature for several compositions was

determined by means of visual polythermal analysis (see Fig. 1).

Feigelson et al.5 carried out limited experiments with BaF2

additions to the BaB2O4–Na2O system. Modest viscosity reduc-

tions were achieved with additions of up to 10 wt% BaF2.

Kaplun et al. investigated the phase equilibria in the BaB2O4–

BaF2 system by the vibration method of phase analysis and

thermal analysis17 and the results reported were quite different

from those by Jiang et al.16 According to the Kaplun et al. data,

the composition used for b-BaB2O4 crystal growth by Jiang et al.

belongs to the BaF2 primary crystallization area. The determined

eutectic coordinates were 57 mol% BaB2O4, 43 mol% BaF2 and

a temperature of 890 �C. The temperature range of b-BaB2O4

crystallization was very small - from 925 to 890 �C and the

corresponding concentration range was from 61 to 57 mol%

BaB2O4. Several growth experiments were carried out by spon-

taneous crystallization and TSSG technique.

Disagreements in the interpretation of the phase equilibria in

the BaB2O4–BaF2 system impelled us to carry out further

investigations.

Experimental

The phase equilibria in the BaB2O4–BaF2 system were studied

using the modified method of visual polythermal analysis (VPA),

solid-phase synthesis, X-ray powder diffraction analysis (XRD),

and differential thermal analysis (DTA). We used commercially

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 The phase equilibria in the BaB2O4–BaF2 system. 1, the data

obtained by VPA; 2, the data obtained by DTA; 3, the data obtained by

Jiang et al.16 by VPA; 4, the data obtained by Jiang et al.10 by DTA.

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available BaCO3, H3BO3 and BaF2 of high-purity grade as

starting materials.

The modified VPA method allows the determination of the

liquidus temperature in a high-temperature solution of specified

composition.18 The experiments were performed in a precise

furnace with a thermal field of high symmetry and stability. The

solution (40 g) was prepared in a platinum crucible (40 mm

diameter) through the intermediate stage of solid-phase

synthesis. The solution was then overheated and kept for several

hours for homogenization. The furnace temperature was

decreased stepwise by 5 �C increments; at each temperature stage

the solution was kept for 1–2 h while the seed material was

introduced. Chips of spontaneously grown crystals that were

0.5–1 mm in transverse size were used, compositionally corre-

sponding to the crystallization field of the given phase, as the seed

material. We estimate the accuracy of the modified VPA at�5 �Cand believe that this method mostly excludes the errors stemming

from the melt undercooling.

The solid-phase synthesis was performed in platinum crucibles

with periodic sample grinding. The ratio and amount of the

components were determined on the basis of the calculated

composition to reach 5 g total weight of the final product. Each

sample obtained was characterized with its XRD pattern.

The XRD patterns were recorded with the use of a DRON-3

diffractometer (CuKa irradiation). The DTA experiments were

carried out in air in a platinum crucible. Thermocouples were

calibrated against the melting points of ground single crystals of

This journal is ª The Royal Society of Chemistry 2011

NaCl (800 �C) and LiF (845 �C). The accuracy of determination

of the thermal effects was �5 �C.Chemical analysis was performed by Energy Dispersion

Spectroscopy technique with the use of a JSM-6480LV Electron

Microscope (JEOL) with INCA Oxford Instruments device.

Results and discussion

Phase equilibria in the BaB2O4–BaF2 system

Modified VPA was applied to study the concentration range

from 30 to 65 mol% of BaF2. The obtained liquidus temperatures

are presented in Table 1 and Fig. 1. Two compositions were

investigated by DTA (see Fig. 1). The results obtained are in

a good agreement with those by Jiang et al.16 The annealing

temperature and time for the solid-state synthesis were 720 �Cand 2 d. The criterion of reaction completion was the constant

ratio of peak intensities. All the XRD patterns of the samples

contained reflections of b-BaB2O4 and BaF2. Thus, the BaB2O4–

BaF2 system could be considered as quasi binary with the eutectic

at 59 mol% BaF2 and a temperature of 760 �C.

Growth of b-BaB2O4 crystals in the BaB2O4–BaF2 system

We performed three successive growth cycles on b-BaB2O4

crystal growth in the BaB2O4–BaF2 system. The crystals were

grown by the top-seeded solution growth technique in a precise

furnace with a heat field of 3-fold axis symmetry L3.19 A high-

temperature solution of 0.7 kg 54.5 mol% BaB2O4–45.5 mol%

BaF2 composition was melted in a platinum crucible (80 mm in

diameter and 80 mm in height). A crystal was grown on a 5 �5 mm2 seed oriented along the optical axis. After determining the

equilibrium temperature, the seed was allowed to grow using

constant one-sided rotation (1 rpm). The cooling and pulling

rates varied from 1 to 2 K d�1 and 0.43 to 0.17 mm d�1, respec-

tively, during the crystal growth.

To repeatedly use the prepared solution, we added BaB2O4

synthesized from metaboric acid H3BO3 and BaCO3 after each

growth cycle. The weight of the added material corresponded to

the grown crystal weight.

Table 2 contains the experimental data on b-BaB2O4 crystal

growth in the BaB2O4–BaF2 system. Due to the sharp incline of

the liquidus line, the theoretical value of the yield coefficient in

the BaB2O4–BaF2 system is relatively small 1.58 g/(kg �C). Theexperimental value of the yield coefficient turned out to be much

less than the theoretical one. It was 1.05 g/(kg �C) for the first

growth cycle and decreased to 0.72 g/(kg �C) for the two

following ones.

The crystal grown in the first growth cycle with a weight of

62.4 g is shown in Fig. 2. It doesn’t have light scattering centres in

most of the volume. We would like to pay attention to the

morphology of the crystal. Normally, the faces appearing on the

BBO crystals are rhombohedra R{102} and r{014}, trigonal

prisms m1{100} or m2{�100}, hexagonal prism a{2�10} and

monohedron c(001) or c(00�1).20 The crystal grown in the

BaB2O4–BaF2 system exhibits three rhombohedral and six

ditrigonal pyramidal faces (Fig. 2). The measured Bragg angle q

(DRON-3 diffractometer, CuKa irradiation) was 8.8� for the

rhombohedral faces and 20.9� for the ditrigonal pyramidal faces.

According to the structural data, the observed faces belong to

CrystEngComm, 2011, 13, 3822–3826 | 3823

Table 1 Study of the BaB2O4- BaF2 System by VPA

meltcomposition (mol%)

Crystallization onset temperaturedetermined by VPA, �C

meltcomposition (mol%)

Crystallization onset temperaturedetermined by VPA, �C

70 BaB2O4–30 BaF2 >970 50 BaB2O4–50 BaF2 86565 BaB2O4–35 BaF2 960 45 BaB2O4–55 BaF2 81560 BaB2O4–40 BaF2 945 40 BaB2O4–60 BaF2 80555 BaB2O4–45 BaF2 897 35 BaB2O4–65 BaF2 845

Fig. 2 Photographs of the b-BaB2O4 crystal grown in the BaB2O4–BaF2

system (side and upper view) and schematic drawing of the crystal

morphology.

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rhombohedra R{102} and ditrigonal pyramid {215} simple

forms, correspondingly. In the second cycle, a cracked unfaceted

crystal was grown. The XRD analysis of the crystal exhibited

a set of peaks corresponding to b-BaB2O4. A hard polycrystalline

aggregate of 32 mm in diameter and 5 mm in height was grown in

the third growth cycle. Three parallel thin plates were cut from

the central part of the aggregate as shown in Fig. 3 and investi-

gated by XRD analysis. For the identification of the compounds

we used the existing data bank (International center for

diffraction data). The XRD pattern of sample (a) corresponds to

nearly pure b-BaB2O4 phase. Several additional peaks of very

low intensity can be seen. The XRD patterns of samples (b) and

(c) exhibit clearly revealed sets of peaks corresponding to the b-

BaB2O4 and Ba5(BO3)2(B2O5) phases. The compound

Ba5(BO3)2(B2O5) (Ba5B4O11, BaO : B2O3 ¼ 2.5 : 1) with space

group P212121, a ¼ 9.590, b ¼ 16.66, c ¼ 22.92 �A (PDF 00-058-

0115) melts incongruously at 1170 �C (Fig. 4).21 The crystal

structure of Ba5B4O11 is described in detail in ref. 22. Thus, the

aggregate is composed of co-crystallized b-BaB2O4 and

Ba5B4O11 phases and the relative content of the Ba5B4O11 phase

increases during the growth process.

Based on the weight loss of the initial high temperature solu-

tion we estimated the volatility of the BaB2O4–BaF2 system at

temperatures from 940 to 840 �C as 25 mg/h.

We believe that the results obtained are connected with the

process of pyrohydrolysis.23,24 This process occurs at high

temperature in air and consists of the fluorides reaction with

water, with the elimination of HF. In our case the fluoride ion in

BaF2 is replaced by an O2� anion according to the reaction

shown in eqn 1:

BaF2 + H2O / BaO + 2HF[ (1)

The pyrohydrolysis of BaF2 during crystallization causes the

gradual transformation of the BaB2O4–BaF2 system into the

BaB2O4–BaO system. The excess of barium oxide forming

accumulates in the high temperature solution over the course of

time and reacts with BaB2O4 according to the reaction shown in

Table 2 Experimental data on b-BaB2O4 crystal growth in the BaB2O4–BaF

Growthcycle

cooling range/�C

pullingrange/mm

Crystalweight/g

Crystaand h

1 85 15.13 62.4 53�20

2 85 15.71 42.5 47�153 35 17.6 17.5 32�5

3824 | CrystEngComm, 2011, 13, 3822–3826

eqn 2. As a result, we observe the co-crystallization of b-BaB2O4

and Ba5B4O11 phases.

2BaB2O4 + BaO / Ba5B4O11 (2)

Thus, the initial growth composition of 54.5 mol% BaB2O4–

45.5 mol% BaF2 could finally transform into 54.5 mol% BaB2O4–

45.5 mol% BaO or 35.3 mol% B2O3–64.7 mol% BaO composi-

tion. According to the phase equilibria along the B3B2O6–

BaB2O4 section (Fig. 521), the composition containing 64.7 mol%

BaO corresponds to the Ba5B4O11 primary crystallization area.

The eutectic temperature of the B3B2O6–BaB2O4 section is 905�7 �C.We could suggest that the much higher eutectic temperature

of 890 �C reported in ref. 17 for the BaB2O4–BaF2 system is due

to the long-term exposure of the high temperature solution to air

and, correspondingly, the accumulation of the high-temperature

Ba5B4O11 compound in the solution.

On the surface of the grown aggregate we observed a lot of

cracks and small spherical particles (see Fig. 5). Table 3 shows

the results of the energy-dispersive X-ray fluorescence analysis.

According to the results of EDXRF analysis, the aggregate

surface lacked boron. The X-ray powder diffraction pattern of

the thin sample cut from the surface, contains b-BaB2O4 and

Ba5B4O11 phases, and also a set of peaks of low temperature a-

modification of BaCO3, which is stable up to 810 �C (Fig. 6). The

a-BaCO3 peaks are rather broad, which could be connected with

2 system

l diametereight/mm

yield coefficient/g/(kg �C) Crystal characteristic

1.05 faceted crystal, poor quality (seeFig. 2)

0.72 unfaceted block crystal0.72 white polycrystalline aggregate

This journal is ª The Royal Society of Chemistry 2011

Fig. 3 X-ray diffraction patterns of the aggregates grown in the

BaB2O4–BaF2 system. 1, b-BaB2O4; 2, Ba5B4O11.

Fig. 4 Phase equilibria along the Ba3B2O6–BaB2O4 section.21 1, the data

obtained by DTA; 2, the data obtained by VPA; 3, the data taken from

Ref. 25.

Fig. 5 Photographs of the surface of the polycrystalline aggregate grown

from the high-temperature solution in the BaB2O4–BaF2 system.

Table 3 The results of the energy-dispersive X-ray fluorescence analysis

Element

Spectrum 1 Spectrum 2

wt% at% wt% at%

C Ka 6.94 19.27 6.08 17.05O Ka 31.60 65.82 32.22 67.82Ba La 61.45 14.91 61.69 15.13

Fig. 6 The X-ray diffraction patterns of the surface layer of the aggre-

gate grown in the BaB2O4–BaF2 system. 1, b-BaB2O4; 2, Ba5B4O11; 3,

a-BaCO3.

This journal is ª The Royal Society of Chemistry 2011

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the small size of the blocks of coherent scattering. We believe that

the barium oxide forming in the high-temperature solution due

to the process of pyrohydrolysis could react with carbon dioxide

contained in air according to the reaction shown in eqn 3:

BaO + CO2 / BaCO3 (3)

This results in b-BaCO3 crystallization on the aggregate

surface. While cooling of the grown aggregate phase transition

into low–temperature a-BaCO3 modification occurs.

Conclusions

According to the data obtained, the BaB2O4–BaF2 system is

quasi binary with the eutectic coordinates 750 �C, 41 mol%

BaB2O4, and 59 mol% BaF2. The fast pyrohydrolysis of BaF2 in

air causes a gradual transformation of the BaB2O4–BaF2 system

into the BaB2O4–BaO system and results in the co-crystallization

of BaB2O4 and Ba5B4O11 phases. It seems that b-BaB2O4 crystals

of good quality can be grown in the BaB2O4–BaF2 system only in

an inert gas atmosphere.

CrystEngComm, 2011, 13, 3822–3826 | 3825

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Acknowledgements

The authors wish to thank Mr Gontar’ I.V. for carrying out

chemical analysis. The investigations are partly supported by

RFBR grant # 09-02-12261-ofi_m.

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