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Expanding transition metal borate chemistry to include main group elements: high-pressure synthesis and structural relation of β-MgB4O7

  • Leonard C. Pasqualini ORCID logo and Hubert Huppertz ORCID logo EMAIL logo
Published/Copyright: March 28, 2023
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Zeitschrift für Naturforschung B
From the journal Zeitschrift für Naturforschung B Volume 78 Issue 5

Abstract

The high-pressure/high-temperature synthesis of β-MgB4O7 at 9 GPa and 1200 °C is presented. The determination of its crystal structure by single-crystal X-ray diffraction has shown that the compound is isotypic to the transition metal borates with the structure type of β-MB4O7 (M = Mn–Zn). β-MgB4O7 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-MgB4O7 to the here presented high-pressure phase β-MgB4O7 and to other reference compounds is discussed.

Keywords: alkaline earth; borate; crystal structure; high-pressure synthesis; phase transition

1 Introduction

The sum formula MB4O7, with M standing for a divalent metal cation, comprises several different structure types, depending on the incorporated cations and the synthetic conditions. This variety leads to different normal-pressure phases, namely α-BaB4O7 [1], the α-MB4O7 structure type (M = Mg, Mn, Co, Zn, Cd, Hg) [2], [3], [4], [5], [6], [7], α-CaB4O7 [8], and trigonal β-SrB4O7 [9]. The representatives of the SrB4O7 structure type are split into normal-pressure phases with large cations (M = Sr, Pb, Eu) [10], [11], [12] and high-pressure phases (M = Ca, Sn, Hg) [13], [14], [15] for slightly smaller cations. Other high-pressure phases are β-BaB4O7 [16], the β-ZnB4O7 structure type (M = Mn–Zn) [17], [18], [19], and γ-NiB4O7 [20], which shows similarities to β-NiB4O7 in the occurrence of threefold bonded oxygen atoms (descriptive notation: O[3]) and is composed entirely of (BO4) tetrahedra, but forms a different structure.

The series of transition metal borates with the sum formula MB4O7 (M = Mn–Zn) in the β-ZnB4O7 structure type were synthesized at pressures between 7.5 and 10.5 GPa and temperatures between 550 and 1250 °C with B2O3 and the respective metal oxides as starting materials. As it is known from some other borates such as boracite, Mg2+ 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-ZnB4O7 to β-ZnB4O7 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-MgB4O7 [2], Mg3(BO3)2 [22], monoclinic Mg2B2O5 [23], and triclinic Mg2B2O5 [24]. The crystal structure of the normal-pressure phase α-MgB4O7 consists of two interpenetrating, three-dimensional networks. These networks are composed of an equal amount of planar (BO3) triangles and (BO4) 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 (B2O7) unit, and two interconnecting (BO3) triangles. Each FBB is connected to four further FBBs.

Figure 1: Crystal structure of α-MgB4O7, view along the crystallographic c axis. The two interpenetrating anionic networks are colored in blue and orange; trigonal planar (BO3) triangles are colored lighter than the (BO4) tetrahedra.
Figure 1:

Crystal structure of α-MgB4O7, view along the crystallographic c axis. The two interpenetrating anionic networks are colored in blue and orange; trigonal planar (BO3) triangles are colored lighter than the (BO4) tetrahedra.

Figure 2: Fundamental Building Blocks of α-MgB4O7 (left) and β-MgB4O7 (right). One central FBB and all four adjacent, covalently connected FBBs are shown (left). Due to the higher connectivity in β-MgB4O7, 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 2:

Fundamental Building Blocks of α-MgB4O7 (left) and β-MgB4O7 (right). One central FBB and all four adjacent, covalently connected FBBs are shown (left). Due to the higher connectivity in β-MgB4O7, 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.

Here, we report the successful synthesis of β-MgB4O7 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.

2 Experimental section

2.1 Synthesis

For the synthesis of β-MgB4O7, H3BO3 (Roth, 99.8%), MgCO3 · 5 H2O (Strem Chemicals, 99%) and Ga(NO3)3 · 9 H2O (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.

2.2 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(NO3)3 · 9 H2O as a third starting material, the crystallinity of the product was drastically reduced. It is assumed, that the hygroscopic paste basically contains Ga(NO3)3 · 9 H2O. 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 MoKα1 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 H3BO3, 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.

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).
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).

A suitable crystal was isolated under a polarization Leica 125M microscope and measured with a Bruker D8 Quest diffractometer equipped with a Photon 300 CMOS detector. Multiscan absorption correction was performed with Sadabs-2016/2 [29]. The program WinGX-2018.1 [30] was used for the calculation of the structure, which was solved and refined with the implemented Shelxt-2018/2 [31] and Shelxl-2018/3 [32, 33] routines, respectively.

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.

3 Results and discussion

3.1 Crystal structure

β-MgB4O7 is isotypic to the high-pressure borates with 3d transition metals β-MB4O7 (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) Å3, 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 (BO4) tetrahedra is accounted for instead of single atoms.

Table 1:

Single-crystal data and structure refinement of β-MgB4O7.

Empirical formula MgB4O7
Molar mass, g mol−1 179.55
Crystal system Orthorhombic
Space group Cmcm (no. 63)
Single-crystal diffractometer Bruker D8 Quest Kappa
Radiation/wavelength λ, Å MoKα/0.71073
a, Å 10.848(2)
b, Å 6.513(2)
c, Å 5.144(1)
V, Å3 363.5(2)
Formula units per cell Z 4
Calculated density, g cm−3 3.28
Crystal size, mm3 0.04 × 0.12 × 0.26
Temperature, K 293(2)
Absorption coefficient, mm−1 0.5
F(000), e 352
θ range, deg 3.65–40.25
Range in hkl −19 ≤ h ≤ +19; −11 ≤ k ≤ +11; −9 ≤ l ≤ +9
Refl. total/independent 9054/614
Rint/R σ 0.0329/0.0151
Refl. with I > 2 σ(I) 596
Data/ref. parameters 614/36
Absorption correction Multi-scan
Final R1/wR2 (I > 2 σ(I)) 0.0178/0.0516
Final R1/wR2 (all data) 0.0182/0.0520
Goodness-of-fit on F2 1.125
Largest diff. peak/hole, e Å−3 0.30/−0.30
Table 2:

Atomic coordinates, equivalent isotropic displacement parameters Ueq (Å2), and Wyckoff positions of β-MgB4O7. Ueq 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
Mg1 0 0.23130(4) 1/4 0.00537(8) 4c
B1 0.20309(5) 1/2 1 0.00411(9) 8e
B2 0.11737(5) 0.19495(7) 3/4 0.00397(9) 8g
O1 0 0.08116(7) 3/4 0.00421(9) 4c
O2 0.28097(3) 0.52341(5) 3/4 0.00386(8) 8g
O3 0.12783(2) 0.31785(4) 0.98015(5) 0.00434(7) 16h
Table 3:

Anisotropic displacement parameters Uij (Å2) of β-MgB4O7 (standard deviations in parentheses).

Atom U 11 U 22 U 33 U 12 U 13 U 23
Mg1 0.0062(2) 0.0054(2) 0.0046(2) 0 0 0
B1 0.0042(2) 0.0042(2) 0.0040(2) 0 0 −0.0003(2)
B2 0.0041(2) 0.0041(2) 0.0038(2) −0.0003(2) 0 0
O1 0.0028(2) 0.0036(2) 0.0062(2) 0 0 0
O2 0.0037(2) 0.0048(2) 0.0031(2) 0.00085(8) 0 0
O3 0.0050(2) 0.0043(2) 0.0037(2) −0.00140(6) 0.00048(6) −0.00107(6)
Table 4:

Interatomic distances and mean values (Å) in β-MgB4O7 (standard deviations in parentheses). The bold values represent the average bond length of the repsective polyhedron.

Mg1– O1 2.0352(7) B2– O3 1.4337(4) 2×
O3 2.0415(4) 4× O1 1.4733(6)
av. Mg1–O 2.04 O2 1.5698(6)
B1– O3 1.4438(4) 2× av. B2–O 1.48
O2 1.5463(4) 2×
av. B1–O 1.50
Table 5:

Interatomic angles and mean values (deg) in β-MgB4O7 (standard deviations in parentheses). The bold values represent the average angle within the respective polyhedron.

O3–Mg1–O3 85.57(2) 2× O3–B1–O2 106.66(2) 2×
O3–Mg1–O3 85.69(2) 2× O3–B1–O2 109.34(2) 2×
O1–Mg1–O3 106.03(2) 4× O3–B1–O3 111.13(4)
O3–Mg1–O3 147.94(2) 2× O2–B1–O2 113.77(4)
av. O–Mg1–O 106.3 av. O–B1–O 109.5
O1–B2–O2 104.42(4)
O3–B2–O2 109.99(3) 2×
O3–B2–O1 110.44(3) 2×
O3–B2–O3 111.35(4)
av. O–B2–O 109.4
Table 6:

Calculated charge distribution in β-MgB4O7, with the bond-length/bond-strength (ΣV) and the Chardi (ΣQ) concept.

Mg1 B1 B2 O1 O2 O3
ΣV 1.96 2.89 3.03 −1.91 −1.83 −2.06
ΣQ 1.88 2.97 3.09 −2.19 −1.65 −1.92

β-MgB4O7 is solely built from (BO4) tetrahedra, which are connected via common vertices forming a three-dimensional network with two different channels along 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 (B12O7) unit and two bridging (B2O4) tetrahedra, which each connect four of the aforementioned (B12O7) 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-MgB4O7.

Figure 4: Crystal structure of β-MgB4O7, view along the crystallographic c axis. The two crystallographically different (BO4) tetrahedra are colored in dark and light blue.
Figure 4:

Crystal structure of β-MgB4O7, view along the crystallographic c axis. The two crystallographically different (BO4) tetrahedra are colored in dark and light blue.

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-MgB4O7 share the same position in β-MgB4O7, 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.

Table 7:

Relations of the crystallographically different atoms in α-MgB4O7 and β-MgB4O7.

α-MgB4O7 (Pbca) β-MgB4O7 (Cmcm)
Atoms Wyckoff position (Z = 8) Atoms Wyckoff position (Z = 4)
Mg1 8c Mg1 4c
B2, B4 8c B1 8e
B1, B3 8c B2 8g
O2 8c O1 4c
O1, O3 8c O2 8g
O4, O5, O6, O7 8c O3 16h

3.2 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-ZnB4O7, a phase transformation between these structure types exists [19]. The phase transition between the normal and the high-pressure phase is of a reconstructive nature – bonds are broken and formed – so 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 structures allows the description of a possible structural transformation process of α-MgB4O7 into β-MgB4O7 under high-pressure conditions.

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-MgB4O7 is different from β-MgB4O7 in four main points, which need to be addressed:

  1. The occurrence of planar (BO3) triangles.

  2. The lack of oxygen atoms, which are bound to three boron atoms (O[3]).

  3. The occurrence of two interpenetrating, three-dimensional anionic borate networks.

  4. The zig–zag arrangement of the FBBs within the crystallographic ac plane (Figure 5).

Figure 5: Slice of the structure within the crystallographic ac plane. The corresponding eight FBBs and Mg2+ cations are depicted. While all FBBs are parallel in β-MgB4O7 (right), the normal-pressure phase α-MgB4O7 (left) shows a zig–zag orientation with an opening angle of 62°.
Figure 5:

Slice of the structure within the crystallographic ac plane. The corresponding eight FBBs and Mg2+ cations are depicted. While all FBBs are parallel in β-MgB4O7 (right), the normal-pressure phase α-MgB4O7 (left) shows a zig–zag orientation with an opening angle of 62°.

To become a (BO4) tetrahedron, a (BO3) 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-MgB4O7 consists of two three-membered rings, which share two (BO4) tetrahedra with each other, the structure of β-MgB4O7 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 (BO4) 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-MgB4O7 and β-MgB4O7 could be described in the following way (atom names are written for α-MgB4O7).

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 (BO4) tetrahedra with another three-membered ring. Two other bonds are needed to recreate a (B2O4) 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] atom) and B4–O4 (compensation for bond cleavage). A graphical representation of the structural relation of α-MgB4O7 and β-MgB4O7 is shown in Figure 6. Bonds which are subject of change are colored in rose and green, while rose bonds are present in α-MgB4O7 and green bonds in β-MgB4O7. To emphasize this, the bonds not present in the corresponding structure are shown as dashed lines.

Figure 6: Visualization of the relation between the normal-pressure modification α-MgB4O7 and the high-pressure modification β-MgB4O7. 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 6:

Visualization of the relation between the normal-pressure modification α-MgB4O7 and the high-pressure modification β-MgB4O7. 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.

The coordination of the charge compensating Mg2+ 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 (BO4) 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 bridging oxygen atom of the (B2O7) unit. In β-MgB4O7, the square pyramid shares its four vertices of the base with two (B2O7) 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 Mg2+ cations in both modifications.

Figure 7: Coordination of the Mg2+ cations with oxygen atoms and the relevant surrounding borate network. Left: α-MgB4O7, where the B–O bonds that are to be formed during the phase transition are also indicated. Right: β-MgB4O7. The colors are the same as in Figure 6, the circles help to guide the eye.
Figure 7:

Coordination of the Mg2+ cations with oxygen atoms and the relevant surrounding borate network. Left: α-MgB4O7, where the B–O bonds that are to be formed during the phase transition are also indicated. Right: β-MgB4O7. The colors are the same as in Figure 6, the circles help to guide the eye.

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.

4 Conclusions

In this work, we synthesized the magnesium borate β-MgB4O7, which crystallizes in the β-ZnB4O7 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-MgB4O7) to one with a single anionic network (β-MgB4O7). 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.

Corresponding author: Hubert Huppertz, Universität Innsbruck, Institut für Allgemeine, Anorganische und Theoretische Chemie, Innrain 80–82, 6020 Innsbruck, Austria, E-mail:

Acknowledgments

We thank Assoz.-Prof. Dr. Gunter Heymann for the recording the single-crystal data. LCP is grateful for the PhD scholarship of the Universität Innsbruck.

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: None declared.

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2023-02-23
Accepted: 2023-03-12
Published Online: 2023-03-28
Published in Print: 2023-05-25

© 2023 the author(s), published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/znb-2023-0008
Keywords for this article
alkaline earth; borate; crystal structure; high-pressure synthesis; phase transition
Creative Commons
BY 4.0

Articles in the same Issue

  1. Frontmatter
  2. In this issue
  3. Research Articles
  4. Photoredox-catalyzed three-component difluorobenzylation of quinoxalin-2(1H)-ones with unactivated vinylarenes and BrCF2CO2Et/HCF2CO2H
  5. Catalyst-free direct synthesis of indeno[1,2-b]indol-5(4bH)-yl benzimidamides
  6. Crystal structure of three chloridocuprate(I, II) complexes with methylene blue (MB) counterions
  7. Crystal structure, photophysical properties, and DFT calculations of a boron difluoride curcumin complex
  8. The double cluster compound [Nb6Cl14(MeCN)4][Nb6Cl14(pyz)4]·6MeCN (Me: methyl, pyz: pyrazine) with a layered structure resulting from weak intermolecular interactions
  9. Expanding transition metal borate chemistry to include main group elements: high-pressure synthesis and structural relation of β-MgB4O7
  10. Eu2Ru3Si5 and Eu2Ir3Ga5 – first europium compounds with U2Mn3Si5-type structure but different europium valence
  11. Mössbauer-spectroscopic characterization of the stannides Sr2Pd2Sn and Eu2Pd2Sn
  12. Orthoamide und Iminiumsalze, CVIII. Umsetzungen von Orthoamiden der Alkincarbonsäuren mit CH/NH-aciden, tautomeriefähigen Verbindungen
  13. Book Review
  14. Thomas J. Meade (Guest Editor), Astrid Sigel, Helmut Sigel, Eva Freisinger, Roland K. O. Sigel (Series Editors): Molecular Bio-Sensors and the Role of Metal Ions
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