Comparative study of Fe 2 B 3 O 7 and Mn 4 (B 6 O 13 (OH)): a new structure linking melilite and johachidolite

: The two compounds Fe 2 B 3 O 7 ( a = 663.61(4), c = 438.39(3) pm, space group: P 42 1 m ) and Mn 4 (B 6 O 13 (OH)) ( a = 445.14(1), b = 683.56(2), c = 1330.34(3) pm, β = 90.42(1) ° , space group: P 2 1 / c ) were synthesized under high-pressure/ high-temperature conditions of 9 GPa and 800 ° C. Their crystal structures have been determined via single-crystal X-ray di ﬀ raction and the occurring side-phases have been identi ﬁ ed. Fe 2 B 3 O 7 crystallizes in the melilite structure type, whereas the novel structure of Mn 4 (B 6 O 13 (OH)) can be derived from the structures of the minerals melilite or johachidolite.


Introduction
The substance class of melilites contains minerals and members of synthetic origin with the general formula X 2 Z[T 2 A 7 ], where X is a large cation (often alkali, alkaline earth, rare earth cation), Z and T stand for tetrahedrally coordinated smaller cations, and A refers to the charge compensating anion. The two most prominent representatives are the minerals gehlenite (Ca 2 Al[AlSiO 7 ]) and åkermanite (Ca 2 Mg[Si 2 O 7 ]) and the series of solid solutions with the general formula "(Ca,Na) 2 (Al,Mg,Fe 2+ )[(Al,Si)SiO 7 ]". Due to their intrinsic photophysical properties, melilites containing boron are promising candidates as non-linear optical (NLO) materials [1]. Just recently CdTb 1-x Sm x GaB 2 O 7 (0 ≤ x ≤ 0.2) was also proposed as a tunable phosphor for n-UV LEDs [2]. In recent years, our group has published two borates that are homeotypic with the melilite structure type, namely Sc 1.67 B 3 O 7 [3] and In 1.2 B 3 O 5.6 (OH) 1.4 [4], which both were synthesized under high-pressure/high-temperature (HP/HT) conditions. In both cases, the Z and T sites are fully occupied by boron and the X site is occupied by a cation with a rather small ionic radius, resulting in a cell volume of about two thirds of that of a regular member of the substance class.
The compound Fe 2 B 3 O 7 is shown in Figure 1 as a new representative of the melilite structure type. The general formula of melilite is X 2 Z[T 2 A 7 ], where Z forms (ZA 4 ) tetrahedra which are connected to four (T 2 A 7 ) units. Together, these two units form a layer in the crystallographic ab plane with five-membered rings of tetrahedra. The terminal vertices of the (T 2 A 7 ) units face the neighboring layers, with alternating orientation. The X atoms are coordinated as (XA 8 ) square antiprisms located between two five-membered rings of the adjacent anionic layers. Figure 1 shows this polyhedron in its lower part and Figure 7(a) (vide infra) shows this polyhedron from another perspective and in comparison to a similar coordination polyhedron in Mn 4 (B 6 O 13 (OH)).
The structure type CaAlB 3 O 7 appears in two cases: the mineral johachidolite with the sum formula CaAlB 3 O 7 [5,6], and LaNiB 3 O 7 [7]. This structure type consists of a phyllosilicatelike layer of corner-sharing (BO 4 ) tetrahedra that are connected and charge-compensated by layers of cations. The coordination of the cations is octahedral and pentagonal antiprismatic for the smaller and larger cations, respectively. Figure 2 shows the johachidolite structure type. The structural motifs of the tetrahedra within the layer are comparable to that of the melilite structure: (BO 4 ) tetrahedra connect (B 2 O 7 ) units, whose terminal oxygen atoms point towards the next layer, with the orientation of the (B 2 O 7 ) diborate units alternating. These two motifs together form the layer, generating four-and six-membered rings that share two tetrahedra with the neighboring rings. The layers are exactly staggered above each other, with layers of alternating, charge-compensating cations in between. They form distorted (AlO 6 ) octahedra and (CaO 10 ) pentagonal antiprisms, with the former residing between two four-membered rings and the latter lying between two six-membered rings. Figure 2 shows both of these two polyhedra in the lower part and Figure 7(b) and (c) (vide infra) depicts the polyhedra from another perspective and in comparison to similar coordination polyhedra in Mn 4 (B 6 O 13 (OH)).
Herein, we report the synthesis of a third compound with a melilite-type structure containing only boron in the Z and T positions which is the first example with a fully occupied site X, namely Fe 2 B 3 O 7 . Accordingly, the charge compensating cation must have a mixed valence. Furthermore, a novel borate with the formula Mn 4 (B 6 O 13 (OH)) is presented, which shows the similar Fundamental Building Block (FBB) as either melilite or johachidolite. However, the resulting borate layers show an arrangement which is different from that of either of these two minerals. This extraordinary property and the structural relationship of this compound to the melilite and johachidolite structure types are discussed. (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.

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-L 3 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 Mn 4 (B 6 O 13 (OH)) were refined freely and the occupation eventually set to 0.5, as the free refinement resulted in values close to 0.5.
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 Fe 2 B 3 O 7 and CSD-2282605 for Mn 4 (B 6 O 13 (OH)).

Single-crystal IR spectroscopy
To further characterize the hydroxyl groups in the compound Mn 4 (B 6 O 13 (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 BaF 2 window, and 90 scans were recorded. Atmospheric effects were corrected using the OPUS 7.2 software [17].

Description of the products and phase identification
The mixed valence compound Fe 2 B 3 O 7 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 Fe 5 B 12 O 25 (OH) is another unknown borate, which will be presented elsewhere. Mn 4 (B 6 O 13 (OH)) forms orange-red crystals. The two other products, Mn 5 B 12 O 25 (OH) and Mn 3 B 7 O 13 (BO 4 ), are hitherto also unknown and will also be published elsewhere. Figures 3 and 4 show the Rietveld refinement of Fe 2 B 3 O 7 and Mn 4 (B 6 O 13 (OH)), respectively. The asterisk in Figure 4 marks a signal at 2θ = 8.2°, due to the grease used to prepare a flat sample.

Crystal structures
The structure of Fe 2 B 3 O 7 belongs to the melilite type crystallizing in the tetragonal space group P42 1 m (no. 113) with two formula units (Z = 2) in the unit cell. In terms of the general formula of melilite, which is  (1) [3,4]. A list of structure refinement details based on the singlecrystal data is given in Table 1. Lists of atomic coordinates,   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 Fe 2+ and Fe 3+ cations, which would exactly balance the negative charge of the anionic borate layer (B 3 O 7 ) 5-. When the structure of Fe 2 B 3 O 7 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.  Figure 5 shows the crystal structure of Mn 4 (B 6 O 13 (OH)). It consists of layers of cornersharing (BO 4 ) tetrahedra and charge compensating cations between the layers.
A list containing details of the structure refinement based on the single-crystal data of Mn 4 (B 6 O 13 (OH)) is given in Table 1.
Lists containing the atomic coordinates, displacement factors, interatomic distances, angles, and charge calculations of Mn 4 (B 6 O 13 (OH)) are given in Tables 7-11.
The same two structural motifs as in melilite and johachidolite are distinguishable: (B 2 O 7 ) diborate units, which are connected to four (BO 4 ) tetrahedra, which in turn are connected to four of these diborate units. Every second (B 2 O 7 ) 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-, fiveand six-membered rings of corner-sharing (BO 4 ) tetrahedra. This results in different coordination spheres of the Table : Atomic coordinates, equivalent isotropic displacement parameters U eq (Å  ), and occupancy factors of Fe  B  O  . U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses).

Atom
Wyckoff position  charge compensating cations between the borate layers: Mn1 is surrounded by two four-membered rings, resulting in a distorted (Mn1O 6 ) 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 (Mn2O 8 ) 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 sixmembered rings. In johachidolite, this position is occupied by a cation with a larger ionic radius, forming a (CaO 10 ) pentagonal antiprism, as shown in Figure 7(c). In Mn 4 (B 6 O 13 (OH)), this distorted pentagonal antiprism also exists, but the Mn3 position inside is split and coordinated as a monocapped (Mn3O 7 ) 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.

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 Mn 4 (B 6 O 13 (OH)). For all three structures, the anionic borate layers are stacked exactly congruently above each other. For melilite The bold values represent the averages. and johachidolite this is along the c axis, and for Mn 4 (B 6 O 13 (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 (BO 4 ) tetrahedron and one (B 2 O 7 ) 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 fivemembered 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 Mn 4 (B 6 O 13 (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 Table : Atomic coordinates, equivalent isotropic displacement parameters U eq (Å  ), and occupancy factors of Mn  (B  O  (OH)). U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses).

Atom
Wyckoff position Atom 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 7 compares the coordination polyhedra of the charge compensating cations of Mn 4 (B 6 O 13 (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 Mn 4 (B 6 O 13 (OH)), as its layers consist of four-, fiveand six-membered borate rings. According to charge distribution calculations (Table 11)   as well as the H1 position, which is not within the Mn3O 7 polyhedron.

Infrared spectroscopy
To experimentally confirm the partial protonation of the anionic borate layer of Mn 4 (B 6 O 13 (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.

Conclusions
This publication extends the field of melilite-like transition metal borates. It presents the first transition metal melilitelike borate with a fully occupied cation position, Fe 2 B 3 O 7 , consisting of anionic borate layers, charge compensated by equal amounts of Fe 2+ and Fe 3+ cations between the layers. The now reported novel borate Mn 4 (B 6 O 13 (OH)) also features anionic borate layers. The structural arrangement of the borate layers can be described as a regularly disordered  variant of either the melilite structure type or the johachidolite structure type. This is demonstrated by the possibility of describing the anionic borate layers with the Fundamental Building Blocks of either the melilite or the johachidolite structure type. The structure of Mn 4 (B 6 O 13 (OH)) therefore can be considered as a pseudo-melilite or pseudojohachidolite structure, indicating a structural link between these two otherwise unrelated minerals.