Expanding transition metal borate chemistry to include main group elements: high-pressure synthesis and structural relation of β -MgB 4 O 7

: The high-pressure/high-temperature synthesis of β -MgB 4 O 7 at 9 GPa and 1200 ° C is presented. The determination of its crystal structure by single-crystal X-ray di ﬀ raction has shown that the compound is isotypic to the transition metal borates with the structure type of β - M B 4 O 7 ( M = Mn – Zn). β -MgB 4 O 7 crystallizes in the orthorhombic space group Cmcm with the lattice parameters a = 10.848(2), b =6.513(2), and c =5.144(1) Å.The structural relation between the normal-pressure phase α -MgB 4 O 7 to the here presented high-pressure phase β -MgB 4 O 7 and to other reference compounds is discussed.

The series of transition metal borates with the sum formula MB 4 O 7 (M = Mn-Zn) in the β-ZnB 4 O 7 structure type were synthesized at pressures between 7.5 and 10.5 GPa and temperatures between 550 and 1250 °C with B 2 O 3 and the respective metal oxides as starting materials.As it is known from some other borates such as boracite, Mg 2+ and divalent, late 3d metal cations can be exchanged to form isotypic compounds.Furthermore, a theoretical study calculated the pressure needed for the phase transition of α-ZnB 4 O 7 to β-ZnB 4 O 7 to be 4.8 GPa and thus showed that the two phases are related [21].
Up to date, only four different anhydrous magnesium borates are known: α-MgB 4 O 7 [2], Mg 3 (BO 3 ) 2 [22], monoclinic Mg 2 B 2 O 5 [23], and triclinic Mg 2 B 2 O 5 [24].The crystal structure of the normal-pressure phase α-MgB 4 O 7 consists of two interpenetrating, three-dimensional networks.These networks are composed of an equal amount of planar (BO 3 ) triangles and (BO 4 ) tetrahedra. Figure 1 depicts this crystal structure and highlights the two interpenetrating networks in different colors.The Fundamental Building Block (FBB), which resembles the entire anionic network, is depicted in Figure 2 (left).It consists of a central (B 2 O 7 ) unit, and two interconnecting (BO 3 ) triangles.Each FBB is connected to four further FBBs.
Here, we report the successful synthesis of β-MgB 4 O 7 under high-pressure/high-temperature (HP/HT) conditions, its crystal structure determination and an attempt to describe the structural relation between the normal-and the high-pressure phase.

Experimental section 2.1 Synthesis
For the synthesis of β-MgB 4 O 7 , H 3 BO 3 (Roth, 99.8%), MgCO 3 • 5 H 2 O (Strem Chemicals, 99%) and Ga(NO 3 ) 3 • 9 H 2 O (Strem Chemicals, 99.99%) were ground and homogenized in a stoichiometric ratio of 6:3:1 in an agate mortar.The starting material was put into a Pt capsule and placed in an α-BN (Henze Boron Nitride Products AG) crucible with a fitting lid of the same material.The synthesis was performed via HP/ HT methods using a Walker-type multianvil module within a 1000 t hydraulic press (Max Voggenreiter GmbH) in a 14/8 assembly.The compression was carried out in a two-step process consisting of six steel wedges as outer and eight tungsten carbide cubes (Hawedia) as inner anvils.Further details concerning this kind of setup are described in the literature [25][26][27].
The synthesis conditions were 9 GPa and 1200 °C.Pressure was built up in 200 min, followed by heating to the desired temperature within 10 min.The temperature of the reaction mixture  was held for 30 min, cooled to 800 °C within 200 min, and then quenched to room temperature.Decompression to ambient conditions was performed within 600 min.The crystalline products were isolated from the beforehand cleaned Pt capsule for further characterization.

Structure identification and determination by X-ray diffraction
The product of the synthesis was a colorless, hygroscopic paste, which contained big, colorless crystals.Without the addition of Ga(NO 3 ) 3 • 9 H 2 O as a third starting material, the crystallinity of the product was drastically reduced.It is assumed, that the hygroscopic paste basically contains Ga(NO 3 ) 3 • 9 H 2 O.The crystalline product was ground thoroughly and a flat powder sample was analyzed by X-ray diffraction on a STOE Stadi P powder diffractometer.The measurement was performed in transmission geometry with Ge(111)-monochromatized MoKα 1 radiation (λ = 0.7093 Å) within a range of 2θ = 2-70°, a step size of 0.015°and a Mythen 1 K detector.The TOPAS 4.2 software [28] was used for the Rietveld refinement as is shown in Figure 3.One side product is H 3 BO 3 , which accounts for 12.3% of the sample.The reflection at 2θ = 8.2°marked with an asterisk in Figure 3 stems from grease, which was used for preparing the flat sample.Two reflections from an unidentified side product are visible at 2θ angles of 11.5 and 12.0°, marked with two dots in Figure 3.
Further details of the crystal structure investigation may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service: https://www.ccdc.cam.ac.uk/structures/?by quoting the deposition number CSD-2243588.

Results and discussion
3.1 Crystal structure β-MgB 4 O 7 is isotypic to the high-pressure borates with 3d transition metals β-MB 4 O 7 (M = Mn-Zn) and crystallizes in the orthorhombic space group Cmcm (no.63).The unit cell contains four formula units (Z = 4) and has a volume of 363.5(2)Å 3 , with lattice parameters of a = 10.848( 2), b = 6.513 (2), and c = 5.144(1) Å, which fall within the range of the isotypic compounds.Details of the single-crystal structure refinement are listed in Table 1.Atomic coordinates, displacement parameters, interatomic distances and angles are given in Tables 2-5.Bond valence sums (BVS) were calculated with the bond-length/bond-strength concept [34,35], and also the charge distribution characteristics obtained using the charge distribution in solids (CHARDI) concept [36,37] are given in Table 6.The nomenclature of structural features in the following section is adapted to borates in reference to the nomenclature of silicates [38], i.e. the connection of whole (BO 4 ) tetrahedra is accounted for instead of single atoms.
β-MgB 4 O 7 is solely built from (BO 4 ) tetrahedra, which are connected via common vertices forming a threedimensional network with two different channels along Multi-scan Final R/wR (I >  σ(I)) ./.Final R/wR (all data) ./.Goodness-of-fit on F  . Largest diff.peak/hole, e Å − ./−. the crystallographic c axis; six-membered rings form the cation containing channels, while the smaller channels, which are built up from four-membered rings, remain empty.A picture of the crystal structure is shown in Figure 4.The structure contains two crystallographically different boron and three oxygen atoms.All magnesium atoms reside on the same position.The Fundamental Building Block (FBB) is depicted in Figure 2 (right); it represents the whole anionic network and contains a central (B1 2 O 7 ) unit and two bridging (B2O 4 ) tetrahedra, which each connect four of the aforementioned (B1 2 O 7 ) units.An interesting structural motif of this structure is the oxygen atom O2, which is threefold coordinated (O [3] ) by boron atoms.This does not occur in the corresponding normal-pressure phase α-MgB 4 O 7 .
As the normal-pressure phase shows a lower symmetry than the high-pressure phase, and the two phases are structurally related, some atoms on different crystallographic positions in α-MgB 4 O 7 share the same position in β-MgB 4 O 7 , as listed in Table 7.To compare the multiplicity of the Wyckoff sites it must be noted that the number of formula units is Z = 8 for the normal-pressure phase and Z = 4 for the high-pressure phase.

Structural relation
Based on the structure derived by single-crystal X-ray diffraction, we hereby propose a possible topological relation between the normal-and high-pressure phase.
As shown by temperature-depended PXRD for β-ZnB 4 O 7 , a phase transformation between these structure types exists [19].The phase transition between the normal and the high-pressure phase is of a reconstructive naturebonds are broken and formedso a description on the basis of a group-subgroup relation is not possible, even though Pbca is a subgroup of Cmcm (via Pbcm as an intermediate group).However, the comparison of the two crystal Table : Atomic coordinates, equivalent isotropic displacement parameters U eq (Å  ), and Wyckoff positions of β-MgB  O  .U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses).

Atom
x y z U eq Wyckoff position For simplicity, the number of formed and broken bonds is reduced to the minimally needed amount and the distances between atoms of newly formed bonds is kept as short as possible.α-MgB 4 O 7 is different from β-MgB 4 O 7 in four main points, which need to be addressed: (1) The occurrence of planar (BO 3 ) triangles.
To become a (BO 4 ) tetrahedron, a (BO 3 ) triangle obviously needs to become bonded to another oxygen atom.To obtain a threefold bonded oxygen atom (O [3] ), one oxygen atom of the normal-pressure phase needs to bind to another boron atom.While the FBB of α-MgB 4 O 7 consists of two three-membered rings, which share two (BO 4 ) tetrahedra with each other, the structure of β-MgB 4 O 7 does not contain this structure element.Therefore, at least one bond within a three-membered ring must be broken.To keep the structural integrity of the (BO 4 ) tetrahedra, another B-O bond must be introduced for each broken bond.Based on these necessities, which are simply based on the two different bonding situations of the anionic borate parts in the two polymorphs, the structural transformation between α-MgB 4 O 7 and β-MgB 4 O 7 could be described in the following way (atom names are written for α-MgB 4 O 7 ).The two interpenetrating networks, which are not parallel to each other in the normal-pressure phase, need to be aligned parallel along the crystallographic a axis, as depicted in Figure 5.The boron atoms B2 and B4 are the main subjects of the connectivity change.For B2, the bond B2-O4 is broken, to open the three-membered ring of the FBB, which shares two (BO 4 ) tetrahedra with another threemembered ring.Two other bonds are needed to recreate a (B2O 4 ) tetrahedron: B2-O1 and B2-O7, the former creating an O [3] atom, the latter to compensate for the bond cleavage; both of these are part of FBBs of the other interpenetrating network, thus merging them together.B4 undergoes the same procedure: cleavage of B4-O7, formation of B4-O3 (forming an O [3]  To emphasize this, the bonds not present in the corresponding structure are shown as dashed lines. The coordination of the charge compensating Mg 2+ cations is in a square pyramidal shape in both cases.However, in the normal-pressure phase, the polyhedron is strongly distorted, as it shares an edge with a (BO 4 ) tetrahedron.As the two interpenetrating networks undergo rotation during the phase transition, three of the four oxygen atoms forming the base of the square pyramid change their connectivity, while the apex remains the       This structural transformation might be a possible pathway for the transition between the normal-and the high-pressure phase, as the number of broken and formed bonds is kept as low as possible, which would be in favor of the kinetics of the reaction.However, this relation is purely descriptive and at the moment there is no experimental evidence to prove this assumption.

Conclusions
In this work, we synthesized the magnesium borate β-MgB 4 O 7 , which crystallizes in the β-ZnB 4 O 7 structure type, also found with the 3d transition metal borates with Mn-Zn [17][18][19] via a HP/HT synthesis at 9 GPa and 1200 °C.A transformation pathway on the atomic level is proposed, which formulates the rearrangement from a structure with two interpenetrating, anionic networks (a-MgB 4 O 7 ) to one with a single anionic network (β-MgB 4 O 7 ).According to the pressure-coordination rule, all threefold coordinated boron atoms become fourfold coordinated and the formation of an oxygen atom which is bonded to three boron atoms (O [3] ) is observed.

Figure 1 :
Figure 1: Crystal structure of α-MgB 4 O 7 , view along the crystallographic c axis.The two interpenetrating anionic networks are colored in blue and orange; trigonal planar (BO 3 ) triangles are colored lighter than the (BO 4 ) tetrahedra.

Figure 2 :
Figure 2: Fundamental Building Blocks of α-MgB 4 O 7 (left) and β-MgB 4 O 7 (right).One central FBB and all four adjacent, covalently connected FBBs are shown (left).Due to the higher connectivity in β-MgB 4 O 7 , six FBBs are omitted and only indicated by dashed bonds to the central FBB (right).The top representation depicts all atoms, while the bottom one only shows the central boron atoms.

Figure 3 :
Figure 3: Rietveld refinement of the powder X-ray diffraction pattern.The signal at 2θ = 8.2°(asterisk) stems from the grease used for preparing the flat sample, the two at 2θ = 10.5 and 11.0°from an unidentified side product (dots).
atom) and B4-O4 (compensation for bond cleavage).A graphical representation of the structural relation of α-MgB 4 O 7 and β-MgB 4 O 7 is shown in Figure 6.Bonds which are subject of change are colored in rose and green, while rose bonds are present in α-MgB 4 O 7 and green bonds in β-MgB 4 O 7 .

Figure 4 :
Figure 4: Crystal structure of β-MgB 4 O 7 , view along the crystallographic c axis.The two crystallographically different (BO 4 ) tetrahedra are colored in dark and light blue.
bridging oxygen atom of the (B 2 O 7 ) unit.In β-MgB 4 O 7 , the square pyramid shares its four vertices of the base with two (B 2 O 7 ) units of the same former network, which are closer and parallel to each other in the high-pressure phase.

Figure 7
depicts the coordination situation of the Mg 2+ cations in both modifications.

Figure 5 :
Figure 5: Slice of the structure within the crystallographic ac plane.The corresponding eight FBBs and Mg 2+ cations are depicted.While all FBBs are parallel in β-MgB 4 O 7 (right), the normal-pressure phase α-MgB 4 O 7 (left) shows a zig-zag orientation with an opening angle of 62°.

Figure 6 :
Figure 6: Visualization of the relation between the normal-pressure modification α-MgB 4 O 7 and the high-pressure modification β-MgB 4 O 7 .The FBBs of the two interpenetrating networks are colored in blue and orange.For clarity some of the bonds, which are only present in one of the modifications, are omitted.

Figure 7 :
Figure 7: Coordination of the Mg 2+ cations with oxygen atoms and the relevant surrounding borate network.Left: α-MgB 4 O 7 , where the B-O bonds that are to be formed during the phase transition are also indicated.Right: β-MgB 4 O 7 .The colors are the same as in Figure6, the circles help to guide the eye.

Table  :
Single-crystal data and structure refinement of β-MgB  O  .

Table  :
Interatomic distances and mean values (Å) in β-MgB  O  (standard deviations in parentheses).The bold values represent the average bond length of the repsective polyhedron.

Table  :
Interatomic angles and mean values (deg) in β-MgB  O  (standard deviations in parentheses).The bold values represent the average angle within the respective polyhedron.
structures allows the description of a possible structural transformation process of α-MgB 4 O 7 into β-MgB 4 O 7 under high-pressure conditions.

Table  :
Relations of the crystallographically different atoms in α