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J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

39186750
10.1021/jacs.4c08745
Communication
Na2B6Si2: A Prototype Silico-boride with Closo (B6)2– Clusters
Carrillo-Cabrera Wilder †
https://orcid.org/0000-0003-2048-6629
Hübner Julia-Maria †‡§
https://orcid.org/0000-0003-4273-1218
Freccero Riccardo ∥
Jung Walter †
https://orcid.org/0000-0003-1242-2959
Baitinger Michael †
Grin Juri †
https://orcid.org/0000-0002-7301-8629
Schwarz Ulrich *†
† Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany
‡ Earth and Planets Laboratory, Carnegie Institution for Science, Washington, District of Columbia 20015, United States
§ Faculty of Chemistry and Food Chemistry, TUD Dresden University of Technology, 01062 Dresden, Germany
∥ Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, I-16146 Genova, Italy
* Email: ulrich.schwarz@cpfs.mpg.de
26 08 2024
11 09 2024
146 36 2475924763
01 07 2024
20 08 2024
16 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

The compound Na2B6Si2 was synthesized under high-pressure, high-temperature conditions at pressures ranging from 6 to 9.5 GPa and temperatures from 1070 to 1270 K before quenching to room temperature followed by slow decompression. The crystal structure was determined from microcrystals using precession-assisted electron diffraction tomography, validated by dynamical refinement and full-profile refinements using optimized coordinates from quantum chemical calculations (space group R3̅m, Pearson symbol hR30, a = 5.0735(1) Å and c = 16.0004(7) Å). The atomic arrangement consists of a unique framework formed by electron-precise octahedral closo (B6)2– clusters connected via ethane-like (Si2)0 dumbbells. The Na+ cations occupy cavities in the hierarchical variation of a Heusler-type framework. The balance (Na+)2([B6]2–)(Si0)2 reveals an electron precise Zintl-Wade phase, which is in line with electronic band structure calculations predicting semiconducting behavior.

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pmcThe crystal chemistry of boron-rich intermetallic compounds is characterized by deltahedral boron clusters with specific electron counts.1−4 In silico-borides of high boron content, boron–silicon networks typically consist of icosahedral (B12)2– closo clusters and tetrahedrally coordinated Si0 atoms, as observed, e.g., in the crystal structures of Na8B74.5Si17.5, YB17.6Si4.6, and Li2B12Si2.5−7 An analogous sodium phase Na2B12Si2 is predicted to be stable, and calculations indicate that the compound is a promising hard material.8 At high pressures, however, access is granted to the more uncommon structural arrangement of Na2B6Si2. The compound was discovered during high-pressure syntheses of Na8B4Si42 with a clathrate-VIII-type crystal structure.9 In the following experiments, it turned out that Na2B6Si2 could not be made from stoichiometric amounts of elemental components. The BN crucibles used for the synthesis proved to be not inert against the reactive mixture under high-pressure high-temperature conditions, and the sodium content in the product decreased with reaction time. Therefore, an excess of sodium was required and the reactants NaSi and amorphous boron were used in a ratio of 5:2.10−12 Na2B6Si2 forms in samples prepared at pressures ranging from 6 to 9.5 GPa. After a short 10 min reaction at p = 6 GPa and T = 1220(100) K, powder X-ray diffraction data revealed approximately 15 at-% Na2B6Si2, and 85 at-% clathrate-VIII phase. After prolonging the annealing time to 180 min, the sodium had reacted with the crucible material, and only (cF8)Si reflections were detected. The highest yield of about 30 atom % Na2B6Si2 was obtained after 1 h of reaction time at p = 8 GPa and T = 1270(100) K before quenching to room temperature and subsequent slow decompression. Powder X-ray diffraction patterns of the product showed reflections of BN and silicon, indicating crucible decomposition. In the inert atmosphere of a glovebox, the product transforms within several months into an amorphous product pointing at its metastable nature. When exposed to air and moisture, the target product remains stable for at least weeks.

To elucidate the crystal structure of Na2B6Si2, thin lamellar samples were cut from grains of the 6 GPa specimen by using the focused ion beam method and isolated by the lift-out technique. Scanning electron microscopy images revealed tiny grains of Na2B6Si2 precluding single-crystal X-ray diffraction experiments (Figure S1). Thus, selected area electron diffraction images were collected, revealing the trigonal symmetry of the Na2B6Si2 crystal structure, along with its approximate unit cell parameters (Figures 1 and S2). The reflection conditions −h + k + l = 3n for hkl and l = 3n for 00l were compatible with space groups R3, R3̅, R32, R3m, and R3̅m. Precession electron diffraction and tomography data collection resulted in 474 symmetry-independent reflections with I > 2σ(I) (Figures 1 and S3, Table S1). The crystal structure was first refined with a kinematical software using the precession electron diffraction data to a residual value RF of 0.21.13,14 For a subsequent dynamical refinement, 1287 symmetry-independent reflections with I > 2σ(I) were used and resulted in RF = 0.097. The derived composition Na2B6Si2 of the structure model (Table S2) is consistent with that of Na1.8B5.9Si2.3 determined by EDXS analysis.

Figure 1 (a) Indexed SAED diffraction images of Na2B6Si2 taken along [100]*. (b) Projection of the electron diffraction volume along [001]*.

In addition, the starting model was refined with powder X-ray diffraction data (Table S1).15 Because of the byproducts in the powder specimen and the small scattering factor of boron, refinement of the atomic positions of Na2B6Si2 (space group R3̅m, a = 5.0735(1) Å, c = 16.0004(7) Å) yielded similar residuals for different parameter value combinations. Thus, the atomic positions were optimized using quantum chemical calculations with the experimental lattice parameters (Table S3) in analogy to an earlier established procedure.16 Those values were used as input for a final refinement using full diffraction profiles (Figures S4 and S5). The crystal structure determined (Table 1) represents a new structure type.

Table 1 Positional and Displacement Parameters of Na2B6Si2 Obtained from Rietveld Refinement Using the Starting Model of the Electron Diffraction Data Improved by Quantum Chemical Optimization of the Coordinates (Space Group R3̅m with a = 5.0735(1) Å and c = 16.0004(7) Å)

Atom	Site	a/x	b/y	c/z	Uiso/pm2	
Na	6c	0	0	0.2794(2)	0.054(2)	
B	18h	0.2178(4)	2x	0.1224(3)	0.032(2)	
Si	6c	0	0	0.0745(1)	0.036(1)	

The atomic arrangement contains four-bonded (4b) silicon atoms grouped into (Si2)0 dumbbells and closo (B6)2– clusters with six exobonds to silicon (Figure 2a). The structure may be seen as a hierarchical variation of Heusler phase Cu2MnAl, with the Al atoms substituted by B6 polyhedrons, the Mn atoms by Si2 dumbbells, and the Cu atoms by Na atoms (Figure S6). The occurrence of closo (B6)2– clusters in silico-borides is observed for the first time. Under pressure, they replace the (B12)2– clusters, which occur in the structurally related compounds Li2B12Si2 and MgB12Si2.7,17 The similarity between Na2B6Si2 and Li2B12Si2, however, is not limited to their obviously similar electron balance. By cutting the Si2 dumbbells, the 3D network of the Na2B6Si2 structure can be formally divided into layers perpendicular to the [001] direction (Figure 2b). In both structures, the layers are characterized by puckered six-membered rings embedding closo Wade clusters (Figure S7). In Li2B12Si2 the layers contain closo (B12)2–, and in Na2B6Si2 closo (B6)2– anions. Such a crystallographic feature has been discussed for the structurally related gallides Na2Ga7 and NaLiGa7.16,18,19 The sodium atoms in Na2B6Si2 are situated in cavities of the B–Si framework (Figure 2c). The contacts d(Si–Si) of 2.384 Å are similar to that in (cF8)Si, and the endohedral distances of 1.742 and 1.758 Å (Table S4) fall in the normal range of (B6)2– octahedra in binary MB6 compounds.20−23

Figure 2 (a) Crystal structure of Na2B6Si2. (b) Layers perpendicular [001]. (c) Local environment of sodium. Boron atoms are depicted in green, silicon atoms in orange, and sodium atoms in blue.

For calculations of the bulk modulus, full crystal structure relaxations as a function of pressure were performed with the all-electron FHI-aims DFT-software package using the PBEsol exchange-correlation functional.24−27 The FINDSYM utility was used to determine the symmetry of the relaxed structures.28 The selected Brillouin zone sampling of 12 × 12 × 4 was selected after systematic testing for total energy convergence with respect to the number of k-points (Table S5). Excellent fitting of the calculated total energies and volumes were obtained with a third-order Birch–Murnaghan isothermal equation of state (Figure S8). The resulting bulk modulus B0 and equilibrium volume V0 amount to 111.3 GPa and 351.90 Å3, respectively. For comparison, the corresponding B0 values of NaB6 and KB6 with closo-clusters (B6)1– connected by B–B bonds amount to about 133 GPa, while those of the alkaline-earth hexaborides with connected (B6)2– units range from 144 to 159 GPa.29−34 Thus, the calculated compressibility reveals that Na2B6Si2 is expected to be a moderately hard material.

Concerning electronic properties, the density of states shows a band gap of approximately 2 eV indicating semiconducting behavior (Figure 3) and an electron-balanced composition as computed with the FPLO code.35

Figure 3 Electronic density of states of Na2B6Si2.

The chemical bonding in Na2B6Si2 was analyzed applying the electron localizability approach—a quantum chemical technique in position space.36 First, the effective charges were evaluated from the calculated electron density by using the QTAIM formalism.37 The zero–flux surfaces of the gradient vector field are defined as boundaries of basins, representing atomic regions (Figure S9). The obtained charge values of +0.83 for Na, +0.81 for Si, and −0.55 for B reveal a notable positive charge for silicon, in agreement with the Pauling scale (EN(Na) = 0.93, EN(B) = 2.04, EN(Si) = 1.90), and conceptual values from combined Zintl and Wade electron-counting schemes. Further information about the interactions between atoms is obtained from the common analysis of electron density and electron localizability indicator in its ELI-D representation.36 While the distribution of ELI-D in the regions of the inner shells is virtually spherical, the valence region is clearly structured, signaling the participation of these electrons in the bonding events. Only four types of ELI-D attractors (maxima) are found here, which visualize different bonds (Figure 4a and 4b). Three kinds of homoatomic B–B and Si–Si bonds are effectively two-atomic, although they have some minor contributions of sodium (or boron) ligands (less than 5% of the bond populations). In accordance with the conceptual picture, the population of the Si–Si bond (1.97 e) is close to 2, and the B–B bonds within the octahedral boron cluster are electron depleted (0.84 and 1.22 e, respectively, Figure 4). The B–Si bond (2.13 electrons) is rather polar: boron contributes 1.42 electrons, and silicon only 0.71 electrons. Such electron redistribution is the reason for the large QTAIM charge of silicon.

Figure 4 (a) ELI-D bonding basins. (b) ELI-D isosurfaces indicating covalent bonds in Na2B6Si2.

As a result of this study, we present a new structural motif for boron–silicon networks based on anionic B6 octahedra and four connected silicon atoms forming dumbbells. In accordance with the calculated band gap, Na2B6Si2 is an electron-precise valence compound with the balance (Na+)2[(B6)2–](Si0)2. While this structural motif can currently only be realized under extreme conditions, it offers a promising perspective for the development of new silico-borides that hold potential as lightweight materials with high stability.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c08745.Additional information on preparation, crystal structure solution by electron diffraction data, Rietveld refinements with optimized atomic positions, structural relationships of Na2B6Si2 as well as crystallographic data are supplied. (PDF)

Supplementary Material

ja4c08745_si_001.pdf

Open access funded by Max Planck Society.

The authors declare no competing financial interest.
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