electronic reprint A new mechanism of anionic

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Applied Crystallography ISSN 0021-8898

A new mechanism of anionic substitution in fluoride borates Sergey V. Rashchenko, Tatyana B. Bekker, Vladimir V. Bakakin, Yurii V. Seryotkin, Alexander E. Kokh, Peter Gille, Arthur I. Popov and Pavel P. Fedorov

J. Appl. Cryst. (2013). 46, 1081–1084

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Journal of Applied Crystallography covers a wide range of crystallographic topics from the viewpoints of both techniques and theory. The journal presents papers on the application of crystallographic techniques and on the related apparatus and computer software. For many years, the Journal of Applied Crystallography has been the main vehicle for the publication of small-angle scattering papers and powder diffraction techniques. The journal is the primary place where crystallographic computer program information is published.

Crystallography Journals Online is available from journals.iucr.org J. Appl. Cryst. (2013). 46, 1081–1084

Sergey V. Rashchenko et al. · A new mechanism of anionic substitution

research papers Journal of

Applied Crystallography

A new mechanism of anionic substitution in fluoride borates

ISSN 0021-8898

Received 17 April 2013 Accepted 5 June 2013

Sergey V. Rashchenko,a,b* Tatyana B. Bekker,a Vladimir V. Bakakin,a Yurii V. Seryotkin,a,b Alexander E. Kokh,a Peter Gille,c Arthur I. Popovd and Pavel P. Fedorovd a

Sobolev Institute of Geology and Mineralogy SB RAS, 3 Koptyug Avenue, Novosibirsk, 630090, Russian Federation, bGeology and Geophysics Department, Novosibirsk State University, 2 Pirogov Street, Novosibirsk, 630090, Russian Federation, cDepartment of Earth and Environmental Sciences, Ludwig-Maximilians University Munich, 41 Theresienstrasse, Munich, 80333, Germany, and d Prokhorov General Physics Institute RAS, 38 Vavilov Street, Moscow, 119991, Russian Federation. Correspondence e-mail: [email protected]

# 2013 International Union of Crystallography Printed in Singapore – all rights reserved

A comprehensive study of the BaF2–Ba3(BO3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba7(BO3)4yF2+3y. The latter exhibited (BO3)3 $ 3F anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4yF2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3 $ 3F isomorphism, allows for the disclosure of the mechanism of (BO3)3 $ 3F heterovalent anionic substitution in fluoride borates via [(BO3)F]4 tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3 $ 3F substitution.

1. Introduction Isomorphic substitution is a very powerful and flexible tool to control the properties of materials, stabilize their preferred crystal structure and optimize the conditions of their synthesis. This phenomenon exists in various forms, such as isovalent/ heterovalent isomorphism as well as cationic/anionic and combined cationic–anionic isomorphism, that are well represented among natural and man-made materials. Anionic, specifically heterovalent anionic isomorphism, with a change in the number of ions in the unit cell [e.g. heterovalent inclusion of oxygen in tysonite lattices in LnF3–Ln2O3 systems, where Ln is a rare earth element (Sobolev et al., 1976; Fedorov, 2000)], is a much rarer phenomenon than the other types of isomorphism. Inorganic borates are quite well known for their nonlinear optical properties and transparency in the UV range of the electromagnetic spectrum. Fluoride borates stand out amongst other borates, as they possess much broader areas of transparency with the cutoff edges shifted further in the aforementioned UV range (Wu et al., 1996). Thus the implementation of (BO3)3 $ 3F anionic substitution may be a very useful tool for the design and manufacture of optical materials. Our recent study of the BaF2–Ba3(BO3)2 system led to the discovery of two intermediate phases: Ba7(BO3)4yF2+3y and Ba5(BO3)3F (Bekker et al., 2012). The former represents a solid solution series where the area of homogeneity spans between Ba7(BO3)3.79F2.63 and Ba7(BO3)3.35F3.95 compositions (0.21 < y < 0.65) as a result of (BO3)3 $ 3F anionic substitution. In contrast, no evidence of substitution was J. Appl. Cryst. (2013). 46, 1081–1084

found in the Ba5(BO3)3F phase. Such a difference between two compounds with very close chemical composition motivated us to look into what structural feature defines the possibility of (BO3)3 $ 3F substitution, and clarification of the latter is the main goal of this paper.

2. Study of the BaF2–Ba3(BO3)2 phase diagram The sub-solidus area of the BaF2–Ba3(BO3)2 system has been studied earlier using solid-state synthesis followed by X-ray powder diffraction (Bekker et al., 2012). In order to obtain a complete phase diagram of the system we carried out differential thermal analysis (DTA) experiments. All samples for DTA investigation were prepared by a solid-phase synthesis similar to protocols described by Bekker et al. (2012). The DTA experiments were performed under an argon atmosphere in platinum crucibles (Netzsch installation, ca 200 mg specimens, 5 K min1 heating rate). Thermocouples were calibrated against the melting points of ground NaCl (1073 K) and LiF (1118 K) single crystals. The results obtained are presented in Fig. 1. The BaF2– Ba3(BO3)2 phase diagram contains primary crystallization areas of BaF2, Ba7(BO3)4yF2+3y, Ba5(BO3)3F and Ba3(BO3)2 compounds. At Ba3(BO3)2 concentrations less than 46 mol% Ba3(BO3)2 + 54 mol% BaF2, the solid-state reaction products under equilibrium conditions are BaF2 and Ba7(BO3)4yF2+3y phases. At Ba3(BO3)2 concentrations from 59 mol% Ba3(BO3)2 + 41 mol% BaF2 to 75 mol% Ba3(BO3)2 + 25 mol% BaF2 [composition corresponding to Ba5(BO3)3F] the reaction doi:10.1107/S0021889813015756

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research papers products are Ba7(BO3)4yF2+3y and Ba5(BO3)3F. Two peritectic reactions take place at 1358 and 1448 K, respectively: Ba7(BO3)4yF2+3y $ Ba5(BO3)3F + P1 and Ba5(BO3)3F $

Ba3(BO3)2 + P2, where P1 and P2 are peritectic melts. The phase diagram (Fig. 1) indicates the ways to prepare the Ba7(BO3)4yF2+3y phase by both solid-state synthesis and incongruent melt crystallization. However, the distinguishing feature of the BaF2–Ba3(BO3)2 phase diagram is the gradual increase of the solidus and liquidus temperatures from 1228 to 1358 K in the area of solid solution homogeneity. Such a significant temperature increase of about 130 K means that the observed anionic isomorphism greatly influences the physical properties of the Ba7(BO3)4yF2+3y compound formed.

3. Crystal structures of Ba5(BO3)3F and Ba7(BO3)4yF2+3y

Figure 1 Phase diagram of the BaF2–Ba3(BO3)2 system. L is the high-temperature solution. FSS is the Ba7(BO3)4yF2+3y fluoride–borate solid solution. Filled circles represent the experimental DTA data (Netzsch installation, argon atmosphere, ca 200 mg samples preliminary annealed under argon, 5 K1 heating rate).

Figure 2 Crystal structure of Ba5(BO3)3F in the representation of cationcoordinated anions. F ions are in the centers of the yellow octahedra, black triangles represent (BO3)3 anions, and Ba2+ cations are shown as dark-blue spheres.

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The crystalline aggregates of Ba5(BO3)3F for structural investigation were obtained by spontaneous crystallization on a platinum loop from a melt with BaF2:Ba3(BO3)2 = 54: 46 mol% composition. In order to induce spontaneous crystallization, a platinum rod with a loop was placed in the central part of the melt surface at the liquidus temperature. From the moment of detection of spontaneous microcrystals, the solution was cooled at a rate of 2 K d1 for 5–10 d to increase the size of the crystals. Then the platinum loop with grown crystals was extracted from the melt and cooled. A high-quality single-crystal sample with dimensions of 0.26 0.13 0.05 mm was selected using a light polarizing microscope. X-ray diffraction data were collected with an Oxford Diffraction Gemini R Ultra single-crystal diffractometer (CCD detector, graphite-monochromated Mo K radiation) using the !-scan technique with a scan width of 1 per frame. Data reduction was performed with CrysAlisPro (Oxford Diffraction, Abingdon, UK) software, and space group ˚] Pnma [a = 7.60788 (12), b = 14.8299 (2), c = 10.28650 (16) A was selected as the most appropriate one by analyzing the systematic absences and intensities of the reflections. SHELX97 (Sheldrick, 2008) and WinGX (Farrugia, 1999) software were used for structure solution and refinement (see CIF1 for additional details). The structural study reveals that Ba5(BO3)3F is isostructural with the Sr5(BO3)3F phase described by Alekel & Keszler (1993). We believe that the most convenient and best fitting depiction of such structures is by the model made up of a cation sublattice with anion-filled cavities [similar to the model suggested for alkali-earth borates by Vegas (1985)]. The Ba5(BO3)3F lattice has the following three types of cationcoordinated anions: (1) F situated in octahedra face linked into columns (Fig. 2) and (BO3)3 triangles occupying either (2) three-capped trigonal prisms or (3) distorted tetragonal antiprisms. As no evidence of anionic substitution in Ba5(BO3)3F has been obtained, it is most likely that the (BO3)3 anions with such coordination are incapable of producing (BO3)3 $ 3F isomorphism. 1

Supplementary data for this paper are available from the IUCr electronic archives (Reference: NB5067). Services for accessing these data are described at the back of the journal.

A new mechanism of anionic substitution

electronic reprint

J. Appl. Cryst. (2013). 46, 1081–1084

research papers The structure of the second phase in the BaF2–Ba3(BO3)2 system, Ba7(BO3)4yF2+3y, was determined and reported by Bekker et al. (2012). It also has a known strontium-substituted analog, Ba4xSr3+x(BO3)4yF2+3y (Rashchenko et al., 2012). The following three types of cation-coordinated anions are present in the Ba7(BO3)4yF2+3y structure: (1) octahedrally coordinated F anions, (2) (BO3)3 triangles in three-capped trigonal prisms and (3) tetrahedral [X4]4 anionic groups. Anions of the first two types are similar to the anions in the Ba5(BO3)3F structure and demonstrate no anomalous features. However, the tetrahedral [X4]4 fragments, located in the large 11 vertex cavities of the cationic sublattice (Fig. 3a), represent the distinguishing feature of the Ba7(BO3)4yF2+3y structure. These [X4]4 groups include either a combination of a (BO3)3 triangle and a fluoride anion or four fluoride anions. Partially substituted Ba7(BO3)4yF2+3y solid solutions with 0.21 < y < 0.65 contain both [(BO3)F]4 and [F4]4 groups responsible for (BO3)3 $ 3F isomorphic substitution. The formation of such hollow-centered [F4]4 tetrahedra, containing only negatively charged ions, in the large cationic cavity is quite unusual, requiring some kind of F F interionic interaction for its stabilization. Actually, weak F F interactions are well documented for molecular compounds (Alkorta & Elguero, 2004; Drews et al., 2006), but they are an exotic phenomenon for ionic crystals. The aforementioned F F interaction makes [F4]4 groups functionally similar to the other four-charged tetrahedral anions, e.g. [SiO4]4. Crystallization of Ba7(BO3)3[SiO4](CN) and Sr7(BO3)3[SiO4](CN) (Schmid et al., 2003) in the same structural type with [SiO4]4 tetrahedra replacing [F4]4 fragments supports this assumption. In other words, our study of Ba7(BO3)4yF2+3y suggests that it does not demonstrate direct (BO3)3 $ 3F anionic isomorphism; rather, there is a different substitution of the type [(BO3)F]4 $ [F4]4 taking place, and it is a fourth fluoride anion in these fragments that makes the [(BO3)F]4 $ [F4]4 isomorphism possible.

the Ba7(BO3)4yF2+3y crystal structure discussed above, as the sites of anionic isomorphic substitution (Fig. 3b). It is worth noting that -Mg2(BO3)F, in contrast to its modification [-Mg2(BO3)1+yF13y, y 0.14], lacks [X4]4 groups in its crystalline structure and, as a result, does not exhibit any sign of anionic isomorphism (Nikishova et al., 1971). At the same time, natural samples of the pertsevite mineral can exhibit an even more complex anionic isomorphism, [(BO3)F]4 $ [F4]4 $ [SiO4]4, where the mineral crystalline lattice also accommodates statistically distributed oxygen-silica tetrahedra at the [X4]4 sites (Schreyer et al., 2003; Galuskina et al., 2008). Antic-Fidancev et al. (2000) suggested that the (BO3)3 $ 3F type of anionic isomorphism occurs for Eu3(BO3)2yF3+3y upon analysis of its emission spectra. Eu3(BO3)2yF3+3y has the same crystal structure as Ln3(BO3)2F3 (Ln = Sm3+, Eu3+, Gd3+; space group C2/c; Corbel et al., 1998) and apparently belongs to a broader structural class of Ln3[X4]2Z compounds along with similar halogenosilicates {[X4]4 = [F4]4, [(BO3)F]4, [SiO4]4 and Z = F, Cl, Br}, such as La, Ce, Pr and Nd chlorosilicates and La and Ce bromosilicates (Gravereau et al., 1988). The structure of Eu3(BO3)2yF3+3y fluoride borate contains both ‘isolated’ (F in the Z site) and ‘grouped’ [X4]4 anions (Fig. 3c). Therefore, the limited (BO3)3 $ 3F isomorphism observed by Antic-Fidancev et al. (2000) and their conclusions should be attributed to the

4. Discussion The discovery of such an unusual mechanism of (BO3)3 $ 3F isomorphism in tetrahedral [X4]4 anionic groups prompted us to look at the other phases with formally similar (BO3)3 $ 3F substitutions in order to elucidate structural mechanisms of anionic heterovalent isomorphism. There are only two other previously documented examples of (BO3)3 $ 3F isomorphism, namely, -Mg2(BO3)1+yF13y (Brovkin & Nikishova, 1975) and Eu3(BO3)2yF3+3y (AnticFidancev et al., 2000). The first hint of the aforementioned (BO3)3 $ 3F substitution was made by Brovkin & Nikishova (1975), who described the crystalline structure of an -Mg2(BO3)1+yF13y (y 0.14) phase, a man-made analog of the later discovered natural mineral pertsevite (Schreyer et al., 2003). A detailed study of the -Mg2(BO3)1+yF13y structure allows designation of the [X4]4 tetrahedral groups, similar to the fragments in J. Appl. Cryst. (2013). 46, 1081–1084

Figure 3 Anionic substitution in the [X4]4 tetrahedral group in fluoride borates. The top row shows [(BO3)F]4 tetrahedral groups which are replaced by [X4]4 ones in the bottom row. The fragments correspond to Ba7(BO3)4yF2+3y (a), -Mg2(BO3)1+yF13y (b) and Eu3(BO3)2yF3+3y (c) solid solutions. Yellow spheres represent F anions; (BO3)3 anions are given as a combination of red oxygen and black boron atoms; Ba2+, Mg2+ and Eu3+ cations are shown in dark blue, blue and purple, respectively. Please note that silicate analogs of all of the above compounds also contain [SiO4]4 tetrahedra in place of [X4]4 anionic groups.


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research papers presence of tetrahedral [(BO3)F]4 and [F4]4 anionic groups in the Eu3(BO3)2yF3+3y lattice. Thus, all presently known fluoride borates that exhibit isomorphism with formal (BO3)3 $ 3F stoichiometry have one similar feature of their crystalline structures, namely, the presence of tetrahedral [(BO3)F]4 anionic groups, which can be replaced by [F4]4 tetrahedra. It would seem that [(BO3)F]4 $ [F4]4 isomorphism is the only plausible mechanism of (BO3)3 $ 3F replacement of (BO3)3 anions by fluoride anions. This explains why (BO3)3 $ 3F substitution is unknown for pure orthoborates and can be observed only in fluoride borates. Our suggestion leads to the forecast of possible [(BO3)F]4 $ [F4]4 isomorphism in phases containing [(BO3)F]4 groups in their crystalline lattices, such as Gd2(BO3)F3 (Mu¨ller-Bunz & Schleid, 2002) and Yb5(BO3)2F9 (Haberer & Huppertz, 2009). Another important conclusion from the above observations includes the crystallographic similarities of the [(BO3)F]4, [F4]4 and [SiO4]4 four-charged tetrahedral anions that determine the existence of silicate analogs of fluoride borates as well as [(BO3)F]4 $ [F4]4 $ [SiO4]4 isomorphism (e.g. in pertsevite mineral). As a result, the opportunity to vary the anionic sublattice composition within the same crystal structure becomes a prospective tool to vary physical and chemical properties of fluoride borates and design novel fluoride borates and halogeno–orthoborato–orthosilicate composite materials on the basis of various orthosilicates.

The authors are very grateful to R. Simoneaux for his help in the preparation of this manuscript. This work was supported by the Russian Foundation for Basic Research (grant No. 1303-12158 to TB).

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J. Appl. Cryst. (2013). 46, 1081–1084