New Fluoride Borate Solid–Solution Series Ba4–xSr3+x(BO3)4–yF2+3y

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New Fluoride Borate Solid−Solution Series Ba4−xSr3+x(BO3)4−yF2+3y S. V. Rashchenko,†,‡ T. B. Bekker,*,† V. V. Bakakin,§ Yu. V. Seryotkin,†,‡ V. S. Shevchenko,† A. E. Kokh,† and S. Yu. Stonoga† †

Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, 3 Academician Koptyug Avenue, 630090 Novosibirsk, Russia ‡ Novosibirsk State University, 2 Pirogov Street, 630090 Novosibirsk, Russia § Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3 Academician Lavrentiev Avenue, 630090 Novosibirsk, Russia S Supporting Information *

ABSTRACT: A lot of attention has been paid recently to the non-centrosymmetric fluoride borates with chemical formula BaMBO3F (M = Ca, Mg, Zn). A new BaSrBO3F phase has been synthesized by the solid-state reaction, and single crystals have been grown in the BaSrBO3F−NaF system. According to the X-ray single-crystal structure analysis, the compound crystallizes in the space group P63mc. The distinguished feature of the determined structure is the simultaneous cationic (Ba2+ ↔ Sr2+) and anionic (3F)3− ↔ (BO3)3− isomorphisms, implying the existence of the solid-solution series Ba 4 − x Sr 3 + x (BO 3 ) 4 − y F 2 + 3 y . The Ba 4 Sr 3 (BO 3 ) 4 F 2 and Ba3Sr4(BO3)4F2 end-members have been synthesized by solid-state reaction. The existence of Ba4Sr3(BO3)3F5 and Ba3Sr4(BO3)3F5 end-members has not been confirmed. d22 = 0.74 × d36KDP. The potential of Nd3+:BaCaBO3F as a new self-frequency doubling laser crystal was evaluated.9 Structures of the new fluoride borates BaZnBO3F and BaMgBO3F were described by Li and Chen.10 The single BaMgBO3F crystal with dimensions of 26 × 19 × 4 mm3 was grown from the LiF flux by the top-seeded solution growth technique.11 We have tried to obtain a new fluoride borate compound BaSrBO3F and investigate its crystal structure.

1. INTRODUCTION In the last few decades, a number of fluoride borates crystals possessing nonlinear optical properties have been discovered. They reveal a wide optical transparency window, high polarizability, and laser damage threshold. The KBe2BO3F2 (KBBF) crystal has a large birefringence (Δn = 0.077), effective second harmonic generation coefficient deff = 2.5 × d36KDP, transparency range from 155 to 3660 nm, and gives direct access to deep ultraviolet.1,2 However, the development of KBBF crystal growth for nonlinear optics (NLO) is limited because of the strong layering habit and the toxic element in its composition. Also, it decomposes at a relatively low temperature (approximately 825 °C). Another fluoride borate crystal is BaAlBO3F2 (BABF).3 Its structure has some similarities with that of KBBF with a smaller distance between the layers and filled interlayer space. The ultraviolet absorption edge of BABF is 165 nm, and it is a promising NLO crystal for practical application in high-power UV-light generation. High-quality BABF crystals sized up to 50 × 40 × 25 mm3 were successfully grown using LiF−B2O3−NaF flux by the middle-seeded solution growth technique.4,5 A lot of attention has been paid to the non-centrosymmetric fluoride borates with chemical formula BaMBO3F, where M is a divalent cation. In 1994 Keszler et al.6 reported on the structural study of the BaCaBO3F compound. Optical and NLO properties of BaCaBO3 F crystals grown by the Kyropoulos method were investigated:7,8 they are transparent in the 220−2500 nm range with nonlinear optical coefficient © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Solid-State Synthesis of BaSrBO3F. The solid-state synthesis of BaSrBO3F compound was performed in air in a platinum crucible with periodic sample grinding. Commercially available BaCO3, H3BO3, SrF2, and SrCO3 of high purity grade were used as starting reagents. At first, HBO2 was obtained from H3BO3 by annealing at 150 °C for 12 h. Next, all the reagents were mixed thoroughly in stoichiometric amounts, calculated for the final product weight of 5 g, and successively annealed at 840 and 900 °C for 24 h at each temperature. The powder patterns of the samples annealed at 840 and 900 °C were identical, which confirmes the completeness of the reaction (Figure 1). The profile fitting and indexing, performed with Stoe WinXPOW software, have shown the hexagonal symmetry of the obtained phase with cell parameters a = b = 10.885(3) Å and c = 6.9921(13) Å. Received: February 6, 2012 Published: April 25, 2012 2955

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WinGX suite13 was used for structure solution and refinement. The details of data collection and structure refinement are summarized in Table 1. All structural data including atomic coordinates, isotropic or equivalent displacement parameters, and refined site occupancies are listed in Table 2.

Table 1. Parameters of Single-Crystal Data Collection and Structure Refinement refined stoichiometry formula weight space group a = b (Å) c (Å) calculated density (g/cm3) absorption coefficient μ (mm−1) F(000) θ range (°) hkl limits measured reflections unique reflections reflections with I > 2σ(I) Rint refined parameters Flack parameter R factors (I > 2σ(I)) R factors (all data) residual electron density (e/ Å3)

Figure 1. Powder patterns of BaSrBO3F compound obtained by the solid-state synthesis (a) and crystals grown in the BaSrBO3F−NaF system (b). 2.2. Crystal Growth. The experiment on the crystal growth was carried out in a furnace with precise (±0.1 °C) temperature control (Eurotherm 2604). The single crystals were grown on a platinum loop in the BaSrBO3F−NaF system in air. The initial composition of the high temperature solution was 50 mol % BaSrBO3F and 50 mol % NaF. The solution (40 g) was melted in a platinum crucible (40 mm in diameter) through the solid-stage synthesis. Heating rate was 25 °C/h; the maximum heating temperature was 990 °C. At that temperature the melt was kept for 24 h for homogenization with periodic mechanical stirring. The liquidus temperature of about 950 °C was determined by visual polythermal analysis. At that temperature a platinum loop was placed into the central part of the melt surface to induce spontaneous crystallization. From the moment of detection of spontaneous microcrystals, the solution was cooled at a rate of 2 °C/ day for 5 days in order to increase crystal size. A grown polycrystalline aggregate of long prismatic crystals is shown in Figure 2. The strong similarity was detected between powder patterns of grown crystals and solid-phase synthesis product (Figure 1).

Ba3.12Sr3.88(BO3)3.65F3.05 (Z = 2) 1041.07 P63mc (No. 186) 10.87255(18) 6.94718(11) 4.861 23.020 911 2.16−37.07 −18 ≤ h ≤ 18; −18 ≤ k ≤ 18; −11 ≤ l ≤ 11 24018 1353 1287 0.0600 54 0.024(12) R1 = 0.0346; wR2 = 0.0828 R1 = 0.0376; wR2 = 0.0844 max 2.071; min −3.301; average 0.409

Three cation positions (Ba, Sr, and M) were localized. The first two ones are occupied by Ba2+ and Sr2+, respectively, whereas isomorphic M position is statistically populated with both cations. The refined stoichiometry of M position is Ba0.12Sr0.88. Besides cationic isomorphism, the anionic one was revealed as well. It consists in 3F− ↔ (BO3)3− substitution, similar to that in Eu3(BO3)2+xF3−3x14 and αMg2(BO3)1+xF1−3x15 compounds. This substitution occurs in the anion group “4X” with geometry of trigonal pyramid, whose vertices can be occupied either by fluorine, or a (BO3)3− group can appear instead of each fluorine triangle, except a basal one (Figure 3). As a result, two types of 4X groups are statistically distributed over the structure: (4F)4− and (BO3F)4−. The corresponding boron atom is situated in the B2 site, disordered around the 3-fold axis (Figure 3). The refinement, performed with charge balance constraints, gave the stoichiometry of 4X group as (4F)0.35(BO3F)0.65. It is necessary to note that fluorine and oxygen atoms occupy different sites in the base of the 4X pyramid. The X2O oxygen site is closer to the B2 boron site, giving a B−O distance of 1.38(2) Å, typical for (BO3)3− anion. The X2F fluorine site, split by mirror plane into two statistically occupied symmetry equivalents, is more distant from B2 (1.59(3) Å). Herewith, three competing local sites (X2O and two symmetry equivalents of X2F) are populated statistically in each basal vertex of the 4X pyramid, causing strong delocalization of electron density, apparent from the Fourier difference map (Figure 3). The described disordering in the 4X anion group raises the reasonable question: is the space group actually P63mc or a lower symmetry is more appropriate? In order to clarify this point, we performed the trial structure solution and refinement in the P1 space group. All atomic positions appearing in the P63mc space group (including symmetry equivalents) were localized as well in the P1 one as independent positions. It clearly demonstrates the correctness of the P63mc space group for the investigated structure. Because of the presence of large heavy cations (Ba2+ and Sr2+), the studied crystal structure may be easily described on the basis of cation sublattice with anion-filled cavities.16 The sublattice consists of folded nets perpendicular to the c-axis described in Well’s classification among semiregular ones.17 Each net contains Ba3 and Sr3 triangles

Figure 2. Polycrystalline aggregate grown in the BaSrBO3F−NaF system. 2.3. Single-Crystal Structure Analysis. A quality single crystal with dimensions of 0.13 × 0.09 × 0.04 mm3 was selected using a polarizing microscope from the aggregate grown on Pt-loop. X-ray diffraction data were collected on an Oxford Diffraction Gemini R Ultra single-crystal diffractometer (CCD-detector, graphite-monochromatized MoKα radiation) using ω-scan technique with scan width of 1° per frame. Data reduction was performed with Oxford Diffraction CrysAlisPro software, and the P63mc space group was selected as the most appropriate one on the basis of systematic absences and intensities of reflections. The SHELX-97 software12 in 2956

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Table 2. Positional Parameters of Ba3.12Sr3.88(BO3)3.65F3.05 Structure Ba Sr M O1 O2 B1 F1 X1 X2O X2F B2 Ba Sr M O1 O2 B1 F1 X1 a

occ

x

y

z

Ueq

1 1 Ba: 0.118(13) Sr: 0.882(13) 1 1 1 1 O: 0.65(6) F: 0.35(6) 0.43(4) 0.284(19) 0.216(19)

0.72943(4) 0.47203(4) 2/3

0.864715(19) 0.52797(4) 1/3

0.46423(4) 0.11800(11) 0.22322(16)

0.01436(10) 0.01894(14) 0.0116(3)

0.6929(8) 0.9050(5) 0.6279(7) 0 1/3

0.8465(4) 0.5933(4) 0.8140(3) 0 2/3

0.0821(9) 0.3159(6) 0.9054(10) 0.3084(17) 0.1186(15)

0.0244(11) 0.0219(7) 0.0101(9) 0.0302(18) 0.0231(18)

0.4061(10) 0.461(2) 0.391(3)

0.5939(10) 0.6259(18) 0.6956(17) U23

U11

U22

U33

0.01985(16) 0.0180(2) 0.0110(3) 0.029(3) 0.0266(17) 0.008(2) 0.028(2) 0.026(3)

0.01372(12) 0.0180(2) 0.0110(3) 0.029(2) 0.0205(16) 0.0111(18) 0.028(2) 0.026(3)

0.01156(15) 0.0183(3) 0.0129(4) 0.015(2) 0.0254(18) 0.010(2) 0.035(5) 0.017(4)

0.410(3) 0.440(3) 0.310(4) U13

0.00133(7) 0.00651(11) 0 −0.0026(11) 0.0020(13) 0.0008(9) 0 0

0.00266(15) −0.00651(11) 0 −0.005(2) 0.0111(15) 0.0015(18) 0 0

0.031(4)a 0.035(4)a 0.010b U12 0.00993(8) 0.0070(2) 0.00548(14) 0.0147(14) 0.0170(14) 0.0038(10) 0.0138(12) 0.0131(14)

Uiso are given for X2O and X2F sites, refined isotropically. bUiso of B2 site was fixed equal to the Ueq of B1 site.

Figure 3. The 4X anion group with all possible sites (a), and with (4F)4− (b) and (BO3F)4− (c) stoichiometry realized. The electron density maps (d and e) were plotted by the difference Fourier synthesis with zero occupancies of both X2O and X2F sites (d) and B2 site (e). The maxima in (d), corresponding to X2O and X2F sites, are labeled as X2.

• Ba6 octahedron (Figure 6a) is formed by Ba3 triangles from

lying at slightly different c-levels, and M positions centering hexagonal loops (Figure 4). Any net may be obtained from the neighbor one with turning by 180° around the cell origin and following translation by half of the c-parameter. Herewith, each Ba3 triangle appears above the same one, whereas Sr3 triangles alternate along the c-axis with the hexagonal Ba3Sr3M loops. Neighbor nets thus form layers with anion-filled cavities (Figure 5). These cavities are of three following types:

adjacent nets. This octahedron is occupied by the F1 fluorine site, shifted along the c-axis toward three basal Ba atoms. • Ba4Sr4M three-capped trigonal prism (Figure 6b) is occupied by the (BO3)3− group (B1, O1, and O2 sites). Such configuration provides a favorable octahedral coordination for each O2− anion. 2957

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Figure 4. Cation sublattice of Ba3.12Sr3.88(BO3)3.55F3.05 structure: semiregular net (a) and nets sequence (b). The Ba, Sr, and M positions are given as dark, gray, and light spheres, respectively; the unit cell is shown with dashed line.

Figure 5. Ba3.12Sr3.88(BO3)3.65F3.05 structure in the aspect of cation sublattice. Sr2+ and anionic (4F)4− ↔ (BO3F)4− isomorphisms. The formulas of assumed end-members in such solid solution series may be written as follows:

• Sr3+3 truncated trigonal pyramid is formed by a Sr3 triangle situated above another three Sr atoms from the hexagonal loop. This cavity is filled by the 4X anion group (Figure 6c). Two adjacent cations in M position complement coordination of the latter. Of particular interest, therefore, is the case in which the 4X group consists of four fluorine anions. Each of them has unusual one-sided coordination and is situated near three neighbors of the same negative charge. The similar phenomenon appears in other compounds with (BO3)3− ↔ 3F− substitution.14,15 We suppose that such configurations may be stabilized with F−−F− attractive interaction, which is well studied for organic compounds only.18 The refined stoichiometry of studied sample, Ba3.12Sr 3.88(BO3)3.65F3.05, differs from the intended BaSrBO 3F composition. In the light of details given above, the structural formula of Ba3.12Sr3.88(BO3)3.65F3.05 may be written as Ba3Sr3(BO3)3F × [Ba0.12Sr0.88][(BO3F)0.65(4F)0.35], where “Ba3Sr3(BO3)3F” describes the non-isomorphic part of the structure (i.e., atomic sites Ba, Sr, B1, O1, O2, and F1). The square brackets represent cationic Ba2+ ↔

• • • •

Ba3Sr3(BO3)3F Ba3Sr3(BO3)3F Ba3Sr3(BO3)3F Ba3Sr3(BO3)3F

× × × ×

[Ba][4F] → Ba4Sr3(BO3)3F5 [Sr][4F] → Ba3Sr4(BO3)3F5 [Ba][BO3F] → Ba4Sr3(BO3)4F2 [Sr][BO3F] → Ba3Sr4(BO3)4F2

All intermediate members have the formula Ba4−xSr3+x(BO3)4−yF2+3y (0 ≤ x ≤ 1; 0 ≤ y ≤ 1) and may be visualized in the isomorphism square (Figure 7), where x and y correspond to the horizontal and vertical axes, respectively. Notably, the center of this square, Ba3.5Sr3.5(BO3)3.5F3.5 stoichiometry, actually corresponds to the “BaSrBO3F” phase, obtained with the solid-state synthesis. 2.4. Syntheses of End-Members of Ba4−xSr3+x(BO3)4−yF2+3y Solid-Solution Series. In order to determine the existence of assumed end-members in the discovered solid-solution series, the solid-state syntheses of “Ba 4 Sr 3 (BO 3 ) 3 F 5 ”, “Ba 3 Sr 4 (BO 3 ) 3 F 5 ”, “Ba4Sr3(BO3)4F2”, and “Ba3Sr4(BO3)4F2” stoichiometries have been 2958

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• Ba4Sr3(BO3)4F2: a = b = 10.965(5) Å; c = 7.0088(21) Å • Ba3Sr4(BO3)4F2: a = b = 10.863(4) Å; c = 6.9170(18) Å

Figure 8. Powder patterns of Ba4Sr3(BO3)4F2 (a) and Ba3Sr4(BO3)4F2 (b) compounds, and products of solid state synthesis of Ba4Sr3(BO3)3F5 (c) and Ba3Sr4(BO3)3F5 (d) stoichiometries. The peaks of (Ba,Sr)F2 compound are marked with triangles. The difference in cell parameters is caused by cationic isomorphism. As the Ba2+ cation is larger than Sr2+, the Ba4Sr3(BO3)4F2 end-member has greater cell parameters than Ba3Sr4(BO3)4F2. The syntheses of “Ba4Sr3(BO3)3F5” and “Ba3Sr4(BO3)3F5” stoichiometries, in contrast, have not confirmed the existence of corresponding end-members. Along with reflections of Ba4−xSr3+x(BO3)4−yF2+3y structure, the strong peaks of (Ba,Sr)F2 phase were present in both powder patterns (Figure 8c,d). The second annealing step at 900 °C has not led to any changes in the diffraction patterns, confirming the absence of “Ba4Sr3(BO3)3F5” and “Ba3Sr4(BO3)3F5” fluorine-rich end-members in the Ba4−xSr3+x(BO3)4−yF2+3y solid-solution series.

Figure 6. The coordination of anions in Ba3.12Sr3.88(BO3)3.65F3.05 structure: (a) F1 site, (b) (BO3)3− group, and (c) (4X)4− group. The X2O and X2F sites are generalized in (c) as X2 spheres.

3. DISCUSSION Though the model of Ba4−xSr3+x(BO3)4−yF2+3y solid-solution series has been suggested for the first time in the present work, the crystal growth, structure, and properties of Ba3Sr4(BO3)3F5 compound were recently reported by Zhang et al.19 Since our experiments clearly demonstrate that phase with such stoichiometry does not exist, special efforts have been taken to clarify this inconsistency. As reported Zhang et al.,19 the solution for “Ba3Sr4(BO3)3F5” crystal growth was prepared from NaF flux and the product of solid-state synthesis of “Ba3Sr4(BO3)3F5” stoichiometry. The latter, as we showed above, consists of the Ba4−xSr3+x(BO3)4−yF2+3y phase mixed with a significant amount of (Ba,Sr)F2 fluoride. So, it seems more reliably that Ba4−xSr3+x(BO3)4−yF2+3y crystals have been grown from such solution but not “Ba3Sr4(BO3)3F5” ones. The assumption of incorrect stoichiometry has caused some problems in the structure refinement, reported in details by Zhang et al.19 For example, a fluorine position with inappropriate Ueq of 0.126 Å2 was refined at the place where three statistically populated sites (X2O and two symmetry equivalents of X2F) are actually present (Figure 3). However, the most inconsistent point in the paper of Zhang et al.19 is the powder pattern of “Ba3Sr4(BO3)3F5” phase, obtained by solid-state synthesis. As we stated above, such a phase does not exist, and Ba4−xSr3+x(BO3)4−yF2+3y mixed with a significant amount of (Ba,Sr)F2 appear as products of

Figure 7. The isomorphism square of Ba4−xSr3+x(BO3)4−yF2+3y solidsolution series. The Ba2+ ↔ Sr2+ substitution in M position is shown along the horizontal axis; the (4F)4− ↔ (BO3F)4− anion isomorphism in (4X)4− group is shown along the vertical axis. 1 − “BaSrBO3F” (Ba3.5Sr3.5(BO3)3.5F3.5); 2 − Ba3.12Sr3.88(BO3)3.65F3.05. Filled circles represent the experimentally obtained compounds; open circles represent phases whose existence has not been confirmed. performed in the same way as the “BaSrBO3F” compound described above. The powder patterns of “Ba4Sr3(BO3)4F2” and “Ba3Sr4(BO3)4F2” stoichiometries correspond to the solved crystal structure of Ba4−xSr3+x(BO3)4−yF2+3y solid-solution series (Figure 8a,b). The following cell parameters were obtained after profile fitting and indexing: 2959

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(5) Hu, Z.; Yue, Y.; Chen, X.; Yao, J.; Wang, J.; Lin, Z. Solid State Sci. 2011, 13, 875−878. (6) Keszler, D. A.; Akella, A.; Schaffers, K. I.; Alekel, T. A. Mater. Res. Soc. Symp. Proc. 1994, 15−22. (7) Zhang, G.; Liu, H.; Wang, X.; Fan, F.; Fu, P. J. Cryst. Growth 2006, 289, 188−191. (8) Wang, X.; Zhang, G.; Zhao, Y.; Fan, F.; Liu, H.; Fu, P. Opt. Mater. 2007, 29, 1658−1661. (9) Zhao, W.; Zhou, W.; Song, M.; Wang, G.; Du, J.; Yu, H.; Chen, J. Opt. Mater. 2011, 33, 647−654. (10) Li, R. K.; Chen, P. Inorg. Chem. 2010, 49, 1561−1565. (11) Zhao, J.; Xia, M.; Li, R. K. J. Cryst. Growth 2011, 318, 971−973. (12) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122. (13) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838. (14) Antic-Fidancev, E.; Corbel, G.; Mercier, N.; Leblane, M. J. Sold State Chem. 2000, 153, 270−274. (15) Brovkin, A. A.; Rozdestvenskaya, I. V.; Rykova, E. A. In Second Russian National Crystal Chemical Conference Proceedings; Chernogolovka, May 22−26, 2000; Section of Crystal Chemistry of the Scientific Council on Structure and Reactivity and the Institute of Problems of Chemical Physics of Russian Academy of Sciences, 2000; http:// margot.icp.ac.ru/conferences/old/NCCC/abstracts/Brovkin.html. (16) Borisov, S. V. J. Struct. Chem. 1996, 37, 773−779. (17) Wells, A. F. Structural Inorganic Chemistry; Oxford University Press: New York,1975, Fig. 3.9. (18) Alkorta, I.; Elguero, J. Struct. Chem. 2004, 15, 117−120. (19) Zhang, G.; Liu, Z.; Zhang, J.; Fan, F.; Liu, Y.; Fu, P. Cryst. Growth Des. 2009, 9, 3137−3141.

corresponding solid-state synthesis. The powder pattern, reported by Zhang et al.,19 however, does not demonstrate any (Ba,Sr)F2 peaks. In order to examine such inconsistency, we have performed a series of repeated syntheses, reproducing thoroughly the scheme described by Zhang et al.19 All of them coincide absolutely with our previous data, demonstrating the strong peaks of (Ba,Sr)F2 compound (Figure 9), which casts serious doubts on the results reported by Zhang et al.19

Figure 9. Powder patterns of “Ba3Sr4(BO3)3F5” stoichiometry, obtained according to the solid-state synthesis scheme, described by Zhang et al.19 The strong peaks of (Ba,Sr)F2 compound are marked with triangles.

4. CONCLUSIONS New solid-solution series Ba4−xSr3+x(BO3)4−yF2+3y (space group P63mc) has been discovered. It reveals both cationic (Ba2+ ↔ Sr2+) and anionic (3F)3− ↔ (BO3)3− isomorphism. The Ba4Sr3(BO3)4F2 and Ba3Sr4(BO3)4F2 end-members have been synthesized by solid-state reaction. The existence of Ba4Sr3(BO3)3F5 and Ba3Sr4(BO3)3F5 end-members has not been confirmed.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Crystallographic information file. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to Elena N. Nigmatulina, who kindly performed electron microprobe analysis. REFERENCES

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