BO2, F - Crystal Growth

Article pubs.acs.org/crystal

The Ternary Reciprocal System Na, Ba // BO2, F T. B. Bekker,*,† P. P. Fedorov,‡ and A. E. Kokh† †

Institute of Geology and Mineralogy Siberian Branch of Russian Academy of Science, 630090 Novosibirsk, Russia General Physics Institute Russian Academy of Science, 119991, Moscow, Russia

‡

ABSTRACT: Differential-thermal analysis, solid-state synthesis, X-ray diffraction, modified visual polythermal analysis, and spontaneous crystallization were used to study the interaction between the components of the Na, Ba // BO2, F ternary reciprocal system. It has been found that the projection of the liquidus surface consists of five fields of primary crystallization of the NaF, NaBO2, BaB2O4, BaF2, and Ba2Na3[B3O6]2F phases. The types of univariant and invariant processes have been studied. The characteristics of β-BaB2O4 (BBO) crystal growth in the BaB2O4− (NaBO2)2, BaB2O4−BaF2, BaB2O4−(NaF)2, and BaB2O4−Ba2Na3[B3O6]2F systems have been determined.

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INTRODUCTION Barium borate β-BaB2O4 (BBO) single crystals are widely used for laser frequency conversion in the UV/visible range. Though nonlinear optical properties of BBO have been known for a long time, and some industrial applications such as large aperture laser systems have remained limited due to the difficulties in growing large-size bulk crystals. Because of the phase transition at 925 °C, BBO crystals are commonly grown from high-temperature solutions by the topseeded solution growth technique. Main solvents used are compositions in the BaO−B2O3−Na2O ternary system,1−5 the BaB2O4−NaF system,6−8 the BaB2O4−BaF2 system,9−11 and mixed sodium oxide−fluorine compositions.12 The information on solvents used has been often contradictory and segmental. We suggest that the mentioned solvents should be considered as one quaternary reciprocal system Na, Ba, B // O, F. It should be represented as a trigonal prism with corresponding oxide (Na2O−BaO−B2O3) and fluoride ((NaF)2−BaF2−BF3) concentration triangles at the bottom and the top, respectively (Figure 1a). On the BaB2O4−NaF section, which had been considered as quasi-binary,8 a primary crystallization area of the new compound Ba2Na3[B3O6]2F (hexagonal system, P63/m, a = 7.346(1) Å, c = 12.637(2) Å) was found.13,14 The BaB2O4− NaF section and the new compound belong to the Na, Ba // BO2, F ternary reciprocal system which is one of the prism sections (Figure 1b). The system is formed by two salts of metaboric (HBO2) and fluoric (HF) acids, respectively. This paper has presented the results on the phase equilibria and BBO crystal growth in the Na, Ba // BO2, F system, which has not been investigated earlier.

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Figure 1. Quaternary reciprocal system Na, Ba, B // O, F and the position of the ternary reciprocal system Ba, Na // BO2, F (highlighted section) (a); the ternary reciprocal system Ba, Na // BO2, F (b). polythermal analysis (VPA), spontaneous crystallization on the platinum loop, and X-ray diffraction (XRD). We used commercially available BaCO3, H3BO3, BaF2, and NaF of high purity grade as starting materials. Solid-phase synthesis was performed in a platinum crucible with periodic sample grinding. The ratio and amount of the components corresponded to the calculated composition and weight (5 g) of the final product. The annealing temperature and time were chosen individually for each sample. Maximum annealing temperature was 750 °C. Each sample obtained by solid-phase synthesis was characterized with its XRD pattern. The criterion of reaction completion was the constant ratio of peak intensities. All samples investigated by thermal analysis (DTA) were prepared through the solid-phase synthesis. The DTA experiments were both carried out in the dry atmosphere (Ar passed through the zeolite) and in air (at ambient humidity) in platinum crucibles (Q-1500

EXPERIMENTAL PROCEDURES

Received: July 11, 2011 Revised: November 29, 2011 Published: November 29, 2011

The phase equilibria were studied using solid-phase synthesis, differential-thermal analysis (DTA), the modified method of visual © 2011 American Chemical Society

129

dx.doi.org/10.1021/cg2008705 | Cryst. Growth Des. 2012, 12, 129−134

Crystal Growth & Design

Article

Figure 2. Phase equilibria along the BaB2O4−(NaF)2 (a) and BaF2−(NaBO2)2 (b) sections. 1, the data obtained by VPA; 2, the data obtained by DTA, BNBF−Ba2Na3[B3O6]2F.

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derivatograph thermoanalyzer; samples about 300−700 mg). The ambient relative humidity was between 50 and 60%. The heating rate was 10 °C/min; the maximum heating temperature depends on the composition under investigation and was not higher than 950 °C. Thermocouples were calibrated against the melting points of ground single crystals of NaCl (800 °C) and LiF (845 °C).The results of the investigations of the same samples in the dry atmosphere and in air have been identical. Experiments by the modified VPA and spontaneous crystallization on the platinum loop were conducted in air. The modified VPA method allows the determination of the liquidus temperature in a hightemperature solution of specified composition.15 The experiments were carried out in a precise furnace with a thermal field of high symmetry and stability (±0.1 °C). The solution (40 g) was melted in platinum crucible (40 mm diameter) through the solid-phase synthesis.16 Heating rate was 50 °C/h; the maximum heating temperature depends on the composition under investigation and was not higher than 950 °C (except the composition in the BaB2O4− Ba2Na3[B3O6]2F system). The melt was kept for homogenization for several hours. Chips of spontaneous crystals, compositionally corresponding to the crystallization field of the given phase, were placed on the melt surface. Dissolution or growth of the chips was observed to decide the temperature of the liquidus. After this, the furnace temperature was correspondingly changed (decreased or increased) and the search was continued. The temperature at which chips lay on the melt surface for several hours neither dissolving nor growing was considered as the equilibrium temperature. We estimate the accuracy of VPA at ±5 °C and believe that this method excludes the errors arising from melt undercooling. The process of determination of equilibrium temperature was not longer than one day. For each composition investigated by VPA, a new hightemperature solution was prepared. X-ray diffraction patterns of the solidified melt have not revealed any additional phases related to the process of pyrohydrolysis. To induce spontaneous crystallization, a platinum rod with a loop was placed in the central part of the melt surface at liquidus temperature. From the moment of detection of spontaneous microcrystals, the solution was cooled at a rate of 2 °C/day for 5− 10 days in order to increase crystal size. The platinum loop with grown crystals was extracted from the melt and cooled. Crystals without inclusions were selected for XRD. XRD patterns were recorded with the use of a DRON-3 diffractometer (CuKα irradiation).

RESULTS AND DISCUSSION

Phase Equilibria in the Na, Ba // BO2, F System. Boundary Systems. The compounds BaF2, NaF, NaBO2, and BaB2O4 melt congruently at 1373, 997, 970, and 1100 °C, respectively. The Ba2Na3[B3O6]2F compound melts congruently at 835 °C.17 Four boundary systems are quasi-binary and do not contain any intermediate phases. The eutectic coordinates for the NaF−BaF2 system e1 are 64 mol % NaF, 36 mol % BaF2 (or 47 mol % (NaF)2, 53 mol % BaF2), 825 °C;18 for the NaBO2−NaF system e2 - 60 mol % NaBO2, 40 mol % NaF, 831 °C;19 for the BaB2O4−(NaBO2)2 system e3 - 44 mol % (NaBO2)2, 56 mol % BaB2O4, 831 °C.20 Special attention should be given to the BaB2O4−BaF2 boundary system because of the disagreements in the interpretation of the phase equilibria. According to Jiang et al.,9 the eutectic temperature determined by differential-thermal analysis was 752 °C (the composition was not indicated), while eutectic coordinates determined by the vibration method of thermal analysis by Kaplun et al.10 were 57 mol % BaB2O4, 43 mol % BaF2 and a temperature of 890 °C. According to our investigation, the eutectic coordinates e4 are 760 °C, 41 mol % BaB2O4, 59 mol % BaF2.21 Diagonal Sections. Diagonal sections in the ternary reciprocal system are the sections in the composition square through the reciprocal salt pairs (Figure 1b).22,23 The distinguished feature of the investigated system is the interaction between both reciprocal salt pairs, BaB2O4 and NaF, NaBO2 and BaF2, with the formation of the new compound Ba2Na3[B3O6]2F. Figure 2a shows phase equilibria along the BaB2O4−(NaF)2 section. It has been found that at concentrations less than 33. (3) mol % (NaF)2 the products of the solid-state reactions are Ba2Na3[B3O6]2F, BaF2, and BaB2O4; at concentrations higher than this value the phases are Ba2Na3[B3O6]2F, BaF2, and NaF. The chemical reactions in the solid state along the BaB2O4− (NaF)2 section can be represented as follows: 130



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Table 1. Coordinates of the Invariant Points in the Na, Ba // BO2, F System composition (mol %) invariant point

indication

(NaBO2)2

BaB2O4

BaF2

peritectic peritectic eutectic eutectic

P1 P2 E1 E2

26 15 39 45

18 41 5 53

56

(NaF)2 45

56 8

characteristic temperature, °C 750 760 735 780

Figure 3. Phase equilibria along the Ba2Na3[B3O6]2F−BaF2 (a) and Ba2Na3[B3O6]2F−(NaF)2 (b) sections. 1, the data obtained by VPA; 2, the data obtained by DTA; BNBF−Ba2Na3[B3O6]2F.

Ba2Na3[B3O6]2 F + BaF2 ↔ BaB2O4 + P1

at x ≤ 33.(3), (100 − x)BaB2O4 + x(NaF)2 2 2 → x Ba2Na3[B3O6]2 F + x BaF2 3 3

(1a)

while at concentrations higher than 33 mol % (NaF) 2 crystallization is completed by the eutectic reaction E1↔ Ba2Na3[B3O6]2F + NaF + BaF2 at 735 °C. Coordinates of P1 and E1 are indicated in Table 1. The results of the investigation of the phase equilibria along the BaF2−(NaBO2)2 section are presented in Figure 2b. It has been found that at concentrations less than 40 mol % (NaBO2)2 the products of the solid-state reactions are Ba2Na3[B3O6]2F, NaF, and BaF2, at concentrations higher than this value the phases are Ba2Na3[B3O6]2F, NaF, and NaBO2. The chemical reactions in the solid state along the BaF2− (NaBO2)2 section can be represented as follows:

+ (100 − 3x)BaB2O4

at x ≥ 33.(3), (100 − x)BaB2O4 + x(NaF)2 100 − x 100 − x → Ba2Na3[B3O6]2 F + BaF2 3 3 3x − 100 x(NaF)2 + 2 where x is the molar fraction in percent of (NaF)2. With the stoichiometric ratio of BaB2O4/(NaF)2 = 2:1 (x = 33.(3)), the reaction proceeds completely with the formation of Ba2Na3[B3O6]2F and BaF2. The liquidus of the BaB2O4−(NaF)2 section consists of the primary crystallization fields of the BaB2O4, Ba2Na3[B3O6]2F, and NaF phases. The concentration interval from 33 to 50 mol % (NaF)2 corresponds to the Ba2Na3[B3O6]2F primary crystallization field. There are three regions of secondary crystallization involving the liquid phase in the subliquidus area. Thermal effects at about 800 °C (at concentrations less than 33 mol % (NaF)2) and 755 °C (at concentrations higher than 50 mol % (NaF)2) correspond to univariant equilibrium of Ba2Na3[B3O6]2F melting. At concentrations less than 33 mol % (NaF)2, crystallization is completed by the peritectic reaction at 750 °C:

at x ≤ 60, (100 − x)BaF2 + x(NaBO2 )2 x x 300 − 5x → Ba2Na3[B3O6]2 F + (NaF)2 + 3 2 3 BaF2 at x ≥ 60, (100 − x)BaF2 + x(NaBO2 )2 100 − x 3 → Ba2Na3[B3O6]2 F + (100 − x) 2 4 5x − 300 (NaF)2 + (NaBO2 )2 2 where x is the molar fraction in percent of (NaBO2)2. 131



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Ba2Na3[B3O6]2F. Phase reaction 1a proceeds at a temperature of 750 °C. Phase equilibria along the Ba2Na3[B3O6]2F−(NaF)2 section are shown in Figure 4b. This section has also been found to be quasi-binary in the solid state. The liquidus includes the primary crystallization area of the NaBO2 compound, and at 760 °C peritectic reaction 2a proceeds. According to the results obtained, the Ba2Na3[B3O6]2F− BaB2O4 and Ba2Na3[B3O6]2F−(NaBO2)2 systems are quasibinary with the eutectics e 5 at 810 °C, 85 mol % Ba2Na3[B3O6]2F (Figure 5a), 15 mol % BaB2O4 and e6 at 800 °C, 62 mol % Ba2Na3[B3O6]2F, 38 mol % BaB2O4, respectively. A number of additional sections were investigated to define more precisely the coordinates of nonvariant points and the position of the isotherms. The results have been combined in Figure 5 showing the projection of the liquidus surface of the Na, Ba // BO2, F system on the concentration square. It consists of five fields of primary crystallization of the NaF, NaBO2, BaB2O4, BaF2, and Ba2Na3[B3O6]2F phases, which are separated by univariant curves and eight invariant points (four binary eutectics, two ternary peritectics, and two ternary eutectics). β-BaB2O4 Crystal Growth in the Ba, Na // BO2, F System. The β-BaB2O4 crystals were grown by the top-seeded solution growth technique in a precise furnace with a heat field of 3-fold axis symmetry L3.25 We performed several successive growth cycles on β-BaB2O4 crystal growth in the BaB2O4− NaBO 2 , BaB 2 O 4 −BaF 2 , BaB 2 O 4 −NaF, and BaB 2 O 4 − Ba2Na3[B3O6]2F systems in air. Commercially available BaCO3, H3BO3, NaF, and BaF2 of high-purity grade were used as starting materials. The solution was melted in a platinum crucible through the stages of solid-phase synthesis.16 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 with continuous rotation 1 rpm. The cooling and pulling rates varied from 0.4 to 2 °C/day and 0.5 to 0.1 mm/day, respectively. To repeatedly use the prepared solution, we added BaB2O4 synthesized from metaboric acid HBO2 and BaCO3 after each growth cycle. The mass of the added material corresponded to the grown crystal mass. Initial compositions of the high temperature solutions, the theoretical and experimental values of the yield coefficient are shown in Table 2. The yield coefficient is the weight of an overgrown crystal under 1 °C temperature reduction and 1 kg initial charge. The theoretical value was calculated under the assumption of a linear temperature dependence of the liquidus line in the interval of crystallization. The common feature of βBaB2O4 crystal growth in the investigated systems was the absence of constitutional undercooling and corresponding cellular growth that is the characteristic of the BaO−B2O3− Na2O ternary system.4 The BaB2O4−(NaBO2)2 system is of the eutectic type. Both theoretical and experimental values of the yield coefficient are relatively high. The experimental value of the yield coefficient as well as the growth onset temperature do not change significantly in the successive runs, which implies the chemical stability of the system. However, all grown crystals contained visible white inclusions up to 500 μm in size. The nature and composition of the inclusion require further investigations. The BaB2O4−BaF2 system is of the eutectic type. Because of the sharp incline of the liquidus line, the theoretical value of the

With the ratio of BaF2/(NaBO2)2 = 2:3 (x = 60), the reaction proceeds completely with the formation of Ba2Na3[B3O6]2F and BaF2. According to the results of the modified VPA followed by spontaneous crystallization on the platinum loop and XRD analysis, the liquidus of the BaB2O4−(NaF)2 section consists of the primary crystallization fields of four phases: BaB2O4, Ba2Na3[B3O6]2F, NaF, and BaF2 phases. There are three regions of secondary crystallization involving the liquid phase in the subliquidus area. Thermal effects at 760 °C at concentrations higher than 60 mol % (NaBO2)2 correspond to the second peritectic equilibrium P2 (P2 coordinates are indicated in Table 1) in the Ba, Na // BO2, F system:

Ba2Na3[B3O6]2 F + NaF ↔ NaBO2 + P2

(2a)

At 735 °C at concentrations less than 60 mol % (NaBO2)2 the ternary eutectic reaction mentioned above takes place. Solid-State Triangulation of the Na, Ba // BO2, F System. The suggested solid-state triangulation scheme of the Na, Ba // BO2, F system is based on the existence of the Ba2Na3[B3O6]2F ternary compound, which causes the division of the concentration square into four secondary phase triangles, namely, BaB2O4−BaF2−Ba2Na3[B3O6]2F, Ba2Na3[B3O6]2F− BaF 2 −(NaF)2 , Ba 2 Na 3 [B 3O 6 ]2F−(NaF)2−(NaBO 2 ) 2, and Ba2Na3[B3O6]2F−(NaBO2)2−BaB2O4. Phase equilibria along four sections Ba2 Na 3 [B3 O 6 ] 2 F−BaF 2 , Ba 2 Na 3 [B 3 O 6 ]2 F− (NaF)2, Ba2Na3[B3O6]2F−BaB2O4, and Ba2Na3[B3O6]2F− (NaBO2)2 have been investigated. Solid-phase synthesis at different molar ratios of the components of the Ba2Na3[B3O6]2F−BaF2 system showed that the reaction products are Ba2Na3[B3O6]2F and BaF2; that is, the section is quasi-binary in the solid state (Figure 4a).24

Figure 4. Phase equilibria in the Ba, Na // BO2, F ternary reciprocal system.

The liquidus of the Ba2Na3[B3O6]2F−BaF2 section consists of the primary crystallization fields of the Ba2Na3[B3O6]2F, BaB2O4, and BaF2 phases. The composition of 50 mol % Ba2Na3[B3O6]2F, 50 mol % BaF2, being the intersection point of the BaB2O4−(NaF)2 and Ba2Na3[B3O6]2F−BaF2 sections, corresponds to the primary crystallization field of 132



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Figure 5. Phase equilibria in the BaB2O4−Ba2Na3[B3O6]2F system (a). 1, the data obtained by VPA; 2, the data obtained by DTA. Side view (b) and axial section (c) of the BBO crystal (340 g) grown in the BaB2O4−Ba2Na3[B3O6]2F system.

Table 2. Compositions Used for the Growth of the β-BaB2O4 Crystal composition of the initial high temperature solution mass % mol %

BaO

B2O3

70 BaB2O4−30 (NaBO2)2 54.5 BaB2O4−45.5 BaF2 80 BaB2O4−20 (NaF)2 60 BaB2O4−40 Ba2Na3[B3O6]2F

54.9 41.5 62.9 56.3

35.6 18.8 28.5 32.8

NaF

Na2O

BaF2

theoretical value of the yield coefficient, g/(kg °C)

maximum experimental value of the yield coefficient, g/(kg °C)

3.81 1.58 3.66 3.23

3.22 1.05 2.76 2.85

9.5 39.7 8.6 6.5

4.4

yield coefficient in the BaB2O4−BaF2 system is small, 1.58 g/ (kg °C). Three successive growth runs were carried out. The experimental 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. In the third growth run polycrystalline aggregate formed by two phases β-BaB2O4 and Ba5B4O11 was obtained.21 Another specific feature of the growth in the BaB2O4−BaF2 system was the increase in the growth onset temperature, which in the third growth run was 21 °C higher than that in the first one. We believe that the results obtained are connected with the process of pyrohydrolysis.26−28 This process occurs at high temperature and consists of the fluorides reaction with the water vapor from the ambient with the formation of HF. According to refs 26 and 27, NaF and BaF2 are relatively stable to the pyrohydrolysis compared to other fluorides. We have not found any evidence of pyrohydrolysis of samples investigated by DTA and VPA at relatively short exposure time to high temperature. However, when we deal with BBO crystal growth with the average duration of growth cycle of 75 days and the initial crystallization temperature of 925 °C, the pyrohydrolysis of BaF2 becomes notable. In the BaB2O4−BaF2 system, the fluoride ion in BaF2 is replaced by an O2− anion according to the reaction 1:

BaF2 + H2O → BaO + 2HF↑

(1)

The pyrohydrolysis of BaF2 during crystallization causes the gradual transformation of the BaB2O4−BaF2 system into the BaB2O4−BaF2−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 reaction 2.

2BaB2O4 + 3BaO → Ba5B4 O11

(2)

As a result, we observe the cocrystallization of two phases. The formation of the Ba5B4O11 is probably responsible for the increase of the growth onset temperature. It seems that βBaB2O4 crystals of good quality can be grown in the BaB2O4− BaF2 system only in a dry atmosphere. While growing crystals in the BaB2O4−(NaF)2 section, we observed an essential decrease in the yield coefficient (from 2.76 to 2.02 g/(kg °C)) and an increase of the growth onset temperature in the three successive runs as well. The magnitude of the growth onset temperature change was about 13 °C. As it has been shown, in the BaB2O4−(NaF)2 system the BaB2O4, BaF2 , and Ba 2Na 3[B3O 6]2F phases are present in the concentration range of BBO primary crystallization due to the chemical reaction between BaB2O4 and NaF. Further pyrohydrolysis of BaF2 probably also results in the formation of the Ba5B4O11 compound in the system over the course of time. 133



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Most promising results in terms of β-BaB2O4 crystal growth were obtained in the BaB2O4−Ba2Na3[B3O6]2F system. Phase equilibria in the system are shown in Figure 5a. The experimental value of the yield coefficient is relatively high and remains fairly constant in the in successive growth runs. The growth onset temperature is also constant (within ±3 °C). These mean that the BaB2O4−Ba2Na3[B3O6]2F system does not undergo significantly the process of pyrohydrolysis. Figure 5b,c shows photographs of the crystal of 340 g in weight grown in a crucible of 100 mm in diameter and the crystal axial section. The weight of initial high temperature solution was 1.5 kg. The temperature interval of crystallization and the pooling ranges were 85 °C and 20.5 mm, respectively. The grown crystals do not contain the so-called middle column29 with numerous gas and solid inclusions in the central area of the crystal.

(18) Byhalova, G. A.; Berezhnaja, V. T.; Bergman, A. G. Russ. J. Inorg. Chem. 1961, 6 (10), 1196−1198. (19) Petit, M. G.; Jaeger, M. C. R. Acad. Sci. 1957, 244 (13), 1734− 1737. (20) Huang, Q.-Z.; Luang, J. K. Acta Phys. Sin. 1981, 30, 559. (21) Bekker, T. B.; Kokh, A. E.; Fedorov, P. P. CrystEngComm 2011, 13, 3822−3826. (22) Bergman, A. G.; Dombrovskaya, N. S. Zhurn. Rossiisk. fiz.-khim. ob-va. 1929, V. LXI (8), 1451−1578, (in Russian). (23) Findlay, A. The Phase Rule and Its Application; Dover Publication: Mineola, NY, 1951, 494 p. (24) Bekker, T. B.; Kononova, N. G.; Kokh, A. E.; Kuznetsov, S. V.; Fedorov, P. P. Crystallogr. Rep. 2010, 55 (5), 877−881. (25) Kokh, A. E.; Bekker, T. B.; Vlezko, V. A.; Kokh, K. A. J. Cryst. Growth 2011, 18, 602−605. (26) Domange, L. Ann. Chimie (Paris) 1937, 225−297. (27) Warf, J. C.; Cline, W. C.; Tevebaugh, R. D. Anal. Chem. 1954, 26, 342−346. (28) Kuznetsov, S. V.; Osiko, V. V.; Tkatchenko, E. A.; Fedorov, P. P. Russ. Chem. Rev. 2006, 75 (12), 1065−1082. (29) Kokh, A. E.; Popov, V. N.; Bekker, T. B.; Kononova, N. G.; Kokh, K. A.; Mokrushnikov, P. V. J. Cryst. Growth 2005, 275 (1−2), e669−e674.

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CONCLUSIONS The liquidus surface of the Na, Ba // BO2, F ternary reciprocal system consists of a primary crystallization area of the NaF, NaBO2, BaB2O4, BaF2, and Ba2Na3[B3O6]2F phases, which are separated by univariant curves and eight invariant points (four binary eutectics, two ternary peritectics, and two ternary eutectics). The characteristics of β-BaB2O4 crystal growth in the Na, Ba // BO2, F system have been determined. A new flux for growing a high quality large BBO crystal has been proposed.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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REFERENCES

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BO2, F - Crystal Growth

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