Comparative study of Fe₂B₃O₇ and Mn₄(B₆O₁₃(OH)): a new structure linking melilite and johachidolite

2.1 Synthesis

Fe₂B₃O₇ was synthesized by thoroughly grinding and homogenizing H₃BO₃ (≥99.8 %, Carl Roth), B₂O₃ (99.9 %, Strem Chemicals), Fe₂O₃ (≥99 %, Sigma-Aldrich) and SrCO₃ (99 %, Alfa Aesar) in a stoichiometric ratio of 8:8:5:2 in an agate mortar. The starting materials for Mn₄(B₆O₁₃(OH)) were H₃BO₃, B₂O₃, and Mn₂O₃ (99 %, Sigma Aldrich) in a stoichiometric ratio of 4:10:5. In both cases, the mixture of the starting materials was encapsulated in a Pt container, which was placed in an α-BN (Henze Boron Nitride Products AG) crucible with a fitting lid of the same material. For the experiments, 18/11 assemblies were used. Synthesis was performed via HP/HT methods using a Walker-type multianvil module in a 1000 t hydraulic press (Max Voggenreiter GmbH). The compression was carried out in a two-step process using six steel wedges and eight tungsten carbide cubes (Hawedia) as outer and inner anvils, respectively. Further details concerning this kind of setup are described in the literature [8], [9], [10].

The synthesis conditions for both substances were 9 GPa and 800 °C. Pressure was built up during 240 min, followed by heating to the desired temperature within 10 min. The reaction temperature was held for 30 min, gradually lowered to 400 °C within 200 min and afterwards to room temperature by quenching for a few minutes. Decompression to ambient conditions was performed within 720 min. The crystalline products were isolated from the beforehand cleaned Pt capsules for further characterization.

2.2 Phase identification and structure determination by X-ray diffraction

Both crystalline products were ground thoroughly and flat powder samples were analyzed by X-ray diffraction on a STOE Stadi P powder diffractometer. The measurements were performed in transmission geometry with Ge(111)-monochromatized MoK-L₃ radiation (λ = 70.93 pm) within a range of 2θ = 2–70°, a step size of 0.015° and a Mythen 1 K detector. The Topas 4.2 software [11] was used for the Rietveld refinement.

Suitable single crystals were isolated under a polarization microscope (Leica 125M) and measured with a Bruker D8 Quest diffractometer equipped with a Photon 300 CMOS detector. Multiscan absorption corrections were performed with Sadabs-2014/5 [12]. The position and occupation of Mn3 and H1 in Mn₄(B₆O₁₃(OH)) were refined freely and the occupation eventually set to 0.5, as the free refinement resulted in values close to 0.5.

The program WinGX-2018.1 [13] was used for refinement of the structures with the ShelXT-2014/4 [14] and ShelXL-2018/3 [15, 16] routines, respectively. Table 1 summarizes the crystal data and numbers pertinent to data collection and structure refinement.

Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service: https://www.ccdc.cam.ac.uk/structures/? by quoting the deposition numbers CSD-2282606 for Fe₂B₃O₇ and CSD-2282605 for Mn₄(B₆O₁₃(OH)).

2.3 Single-crystal IR spectroscopy

To further characterize the hydroxyl groups in the compound Mn₄(B₆O₁₃(OH)), we performed single-crystal IR spectroscopy. The spectrum was collected using a Bruker Vertex 70 FT-IR spectrometer equipped with a KBr beam splitter, a liquid nitrogen-cooled Mercury Cadmium Telluride (MCT) detector, and a Bruker Hyperion 3000 microscope. Measurements were performed in absorbance mode, covering the spectral range of 600–4000 cm⁻¹ with a resolution of 4 cm⁻¹. A Globar (silicon carbide rod) was used as the mid-infrared source, and the radiation was focused on the sample using a 15× IR objective. The single crystal was placed on a BaF₂ window, and 90 scans were recorded. Atmospheric effects were corrected using the Opus 7.2 software [17].

3.1 Description of the products and phase identification

The mixed valence compound Fe₂B₃O₇ forms black crystal blocks. Four other side phases occur during the synthesis, which could be identified by assigning all reflections of the powder X-ray diffraction pattern. The by-product Fe₅B₁₂O₂₅(OH) is another unknown borate, which will be presented elsewhere.

Mn₄(B₆O₁₃(OH)) forms orange-red crystals. The two other products, Mn₅B₁₂O₂₅(OH) and Mn₃B₇O₁₃(BO₄), are hitherto also unknown and will also be published elsewhere. Figures 3 and 4 show the Rietveld refinement of Fe₂B₃O₇ and Mn₄(B₆O₁₃(OH)), respectively. The asterisk in Figure 4 marks a signal at 2θ = 8.2°, due to the grease used to prepare a flat sample.

Figure 3:

Rietveld refinement of Fe₂B₃O₇. The side phases present in the sample could be identified as Fe₈B₁₅O₂₈(OH)₈, FeCO₃, SrB₄O₇, and an unpublished compound with the formula Fe₅B₁₂O₂₅(OH).

Figure 4:

Rietveld refinement of Mn₄(B₆O₁₃(OH)). The side phases labeled with Mn₅B₁₂O₂₅(OH) and Mn₃B₇O₁₃(BO₄) could be identified and will be published elsewhere. The signal at 2θ = 8.2°, marked with an asterisk, stems from grease used to prepare a flat sample.

3.2 Crystal structures

The structure of Fe₂B₃O₇ belongs to the melilite type crystallizing in the tetragonal space group P 4 ‾ 2₁ m (no. 113) with two formula units (Z = 2) in the unit cell. In terms of the general formula of melilite, which is X ₂ Z[T ₂ A ₇], the occupation of the positions is as follows: X = Fe^2+/3+, Z = T = B³⁺, A = O²⁻. The crystal structure of Fe₂B₃O₇ is shown in Figure 1. The lattice parameters are a = 663.61(4) and c = 438.39(3) pm, resulting in a cell volume of V = 0.1930(1) nm³. This volume is comparable to that of Sc_1.67B₃O₇ and In_1.2B₃O_5.6(OH)_1.4 (0.1968 and 0.2012 nm³) [3, 4], but is significantly smaller than that of åkermanite or gehlinite (0.3011 and 0.3002 nm³). This is due to the fact, that the X, Z, and T positions are all occupied by elements with rather small ionic radii compared to other cations in melilite-type structured compounds. In the case of Fe₂B₃O₇, this results in a split position of the O3 atom, which is the central oxygen atom of the diborate unit. The corresponding atoms in Sc_1.67B₃O₇ and In_1.2B₃O_5.6(OH)_1.4 show very large anisotropic displacement parameters, suggesting that this is a general trend in borates with the melilite structure type [3, 4].

A list of structure refinement details based on the single-crystal data is given in Table 1. Lists of atomic coordinates, displacement parameters, interatomic distances, angles, and charge calculations for Fe₂B₃O₇ are given in Tables 2 –6. Bond valence sums (BVS) were calculated for both substances using the bond-length/bond-strength concept [18, 19], with the charge of iron set to either 2+ or 3+, and a charge distribution using the charge distribution in solids (CHARDI) concept [20, 21]. Since there is only one crystallographic position for the Fe1 cation, and the bond valence sums and charge distribution calculations suggest a charge close to 2.5 (Table 6), it is assumed, that the X position is occupied by Fe²⁺ and Fe³⁺ cations, which would exactly balance the negative charge of the anionic borate layer (B₃O₇)^5–. When the structure of Fe₂B₃O₇ is refined with the occupation of Fe1 set to free refinement, the result is very close to one, which is consistent with the mixed valence of the cation.

Mn₄(B₆O₁₃(OH)) crystallizes in the monoclinic space group P2₁/c (no. 14) with two formula units (Z = 2) in the unit cell. The lattice parameters are a = 454.14(1), b = 683.56(2), c = 1330.34(3) pm and β = 90.418(1)°, which results in a cell volume of V = 0.4130(1) nm³. Figure 5 shows the crystal structure of Mn₄(B₆O₁₃(OH)). It consists of layers of corner-sharing (BO₄) tetrahedra and charge compensating cations between the layers.

Figure 5:

Crystal structure of Mn₄(B₆O₁₃(OH)); view along [ 1 ‾ 00] (top) and [0 1 ‾ 0] (bottom). The interconnecting (BO₄) tetrahedra are colored turquoise, the (B₂O₇) units are colored blue.

A list containing details of the structure refinement based on the single-crystal data of Mn₄(B₆O₁₃(OH)) is given in Table 1. Lists containing the atomic coordinates, displacement factors, interatomic distances, angles, and charge calculations of Mn₄(B₆O₁₃(OH)) are given in Tables 7 –11.

The same two structural motifs as in melilite and johachidolite are distinguishable: (B₂O₇) diborate units, which are connected to four (BO₄) tetrahedra, which in turn are connected to four of these diborate units. Every second (B₂O₇) unit is protonated at one terminal oxygen atom. This is consistent with the disordered Mn3 position, as two diborate units surround this position with their terminal oxygen atoms. As one of the Mn3 positions is too close to one hydrogen position, the other one is occupied. Compared to the two structure types above, the layers consist of four-, five- and six-membered rings of corner-sharing (BO₄) tetrahedra. This results in different coordination spheres of the charge compensating cations between the borate layers: Mn1 is surrounded by two four-membered rings, resulting in a distorted (Mn1O₆) octahedron, just like for Al in johachidolite, shown at the top of the lower part of Figure 5 and also in Figure 7(e). Mn2 is surrounded by two five-membered rings and forms a (Mn2O₈) square antiprism, like the X site in melilite, seen in the middle of the lower part of Figure 5 and also in Figure 7(d). Mn3 is sandwiched between two six-membered rings. In johachidolite, this position is occupied by a cation with a larger ionic radius, forming a (CaO₁₀) pentagonal antiprism, as shown in Figure 7(c). In Mn₄(B₆O₁₃(OH)), this distorted pentagonal antiprism also exists, but the Mn3 position inside is split and coordinated as a monocapped (Mn3O₇) trigonal prism, which is shown at the very bottom of Figures 5 and 7(f). A hydrogen atom fills the other part of the distorted pentagonal antiprism.

3.3 Comparison of the crystal structures

We compared the anionic borate layers and the coordination of the charge compensating cations to understand the differences and similarities between the three structures of melilite, johachidolite and Mn₄(B₆O₁₃(OH)).

For all three structures, the anionic borate layers are stacked exactly congruently above each other. For melilite and johachidolite this is along the c axis, and for Mn₄(B₆O₁₃(OH)) along the a axis. The structural features of these layers are compared using the Fundamental Building Blocks (FBBs) concept, as described for borates by Burns, Grice and Hawthorne [22]. While all three borate layers could be completely tiled by the combination of one (BO₄) tetrahedron and one (B₂O₇) diborate unit, this simplistic unit does not represent the structural features of the layers. By doubling the number of tetrahedra used, the resulting FBB already contains some information about the rings present. Thus, the FBB of melilite can be described as 6□:<5□>□, a five-membered ring with one extra tetrahedron attached, as shown in Figure 6(b). The FBB of johachidolite is 6□:□<4□>□, a four-membered ring with two tetrahedra attached to different members of the ring, as depicted in Figure 6(e). The anionic layers can be constructed by arranging FBBs with common corners, as shown in Figure 6(a) and (d). Since the FBBs contain a ring of a certain size, the FBB is not interchangeable for melilite and johachidolite, as the former exhibits only five-membered rings, while the latter exhibits only four- and six-membered rings. In both melilite and johachidolite, all the FBBs have the same orientation, resulting in a uniform pattern. The anionic borate layer in Mn₄(B₆O₁₃(OH)) can be described by either of the two FBBs, but not by a combination of the two. However, unlike melilite and johachidolite, in both cases the orientation of the FBBs within the layer alternates; the differently oriented FBBs are outlined in black in Figure 6(c) and (f).

Figure 6:

Anionic borate layer of melilite and it’s FBB (a, b), as well as of johachidolite (d, e). The anionic borate layer of Mn₄(B₆O₁₃(OH)) can be built with either FBBs (c, f). All FBBs are chemically the same and the coloring just helps as a guide to the eye.

Figure 7:

Comparison of the cationic polyhedra in Fe₂B₃O₇ (a) and johachidolite (b, c) with Mn₄(B₆O₁₃(OH)) (d–f). The blue edges are shared with a (BO₄) tetrahedron. For Mn3 and H1 (f), both positions and their total environment are shown with occupancies of 50 % each.

Figure 7 compares the coordination polyhedra of the charge compensating cations of Mn₄(B₆O₁₃(OH)) with those of melilite and johachidolite. As the anionic borate layers are congruent, the ring sizes of the cation coordinating layers are the same, resulting in the same number of oxygen atoms contributing to the coordination polyhedra from both layers. All polyhedra are distorted; the coordination polyhedra occurring in melilite are square antiprisms (a). In johachidolite, octahedra (b) and pentagonal antiprisms (c) occur. All of these coordination polyhedra are also found in Mn₄(B₆O₁₃(OH)), as its layers consist of four-, five- and six-membered borate rings. According to charge distribution calculations (Table 11), Mn1 (Figure 7, e; octahedron) is trivalent, Mn2 (Figure 7, d; square antiprism) and Mn3 (Figure 7, f; monocapped trigonal prism, filling a part of the pentagonal antiprismatic cavity) are divalent. Due to the split position of Mn3 and the 50 % occupancy of H1, only one of the two Mn3 positions is occupied in each pentagonal antiprism, as well as the H1 position, which is not within the Mn3O₇ polyhedron.

3.4 Infrared spectroscopy

To experimentally confirm the partial protonation of the anionic borate layer of Mn₄(B₆O₁₃(OH)), a single-crystal IR spectrum was measured. The spectrum is depicted in Figure 8. The stretching mode of the hydroxyl group results in the strong band between 3420 and 3300 cm⁻¹. Furthermore, the absorbance between 2550 and 1350 cm⁻¹ is probably a polarization effect of the measurement due to measuring a single crystal, which broadens the O–H stretching mode. This phenomenon is reported in literature [23, 24]. The bands lower than 1400 cm⁻¹, the so-called fingerprint region, are hard to assign, as here the Mn–O and B–O vibrations and other modes overlap.

Figure 8:

Single-crystal IR spectrum of Mn₄(B₆O₁₃(OH)) from 600 to 4000 cm⁻¹.