Na2B6Si2: A Prototype Silico-boride with Closo (B6)2– Clusters

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.

−7 An analogous sodium phase Na 2 B 12 Si 2 is predicted to be stable, and calculations indicate that the compound is a promising hard material. 8At high pressures, however, access is granted to the more uncommon structural arrangement of Na 2 B 6 Si 2 .The compound was discovered during high-pressure syntheses of Na 8 B 4 Si 42 with a clathrate-VIII-type crystal structure. 9In the following experiments, it turned out that Na 2 B 6 Si 2 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 Na 2 B 6 Si 2 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 Xray diffraction data revealed approximately 15 at-% Na 2 B 6 Si 2 , 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 % Na 2 B 6 Si 2 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 Na 2 B 6 Si 2 , 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 Na 2 B 6 Si 2 precluding single-crystal X-ray diffraction experiments (Figure S1).Thus, selected area electron diffraction images were collected, revealing the trigonal symmetry of the Na 2 B 6 Si 2 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 R3̅ m.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 R F of 0.21. 13,14For a subsequent dynamical refinement, 1287 symmetry-independent reflections with I > 2σ(I) were used and resulted in R F = 0.097.The derived composition Na 2 B 6 Si 2 of the structure model (Table S2) is consistent with that of Na 1.8 B 5.9 Si 2.3 determined by EDXS analysis.
In addition, the starting model was refined with powder Xray diffraction data (Table S1). 15Because of the byproducts in the powder specimen and the small scattering factor of boron, refinement of the atomic positions of Na 2 B 6 Si 2 (space group R3̅ m, 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. 16Those 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.
The atomic arrangement contains four-bonded (4b) silicon atoms grouped into (Si 2 ) 0 dumbbells and closo (B 6 ) 2− clusters     with six exobonds to silicon (Figure 2a).The structure may be seen as a hierarchical variation of Heusler phase Cu 2 MnAl, with the Al atoms substituted by B 6 polyhedrons, the Mn atoms by Si 2 dumbbells, and the Cu atoms by Na atoms (Figure S6).The occurrence of closo (B 6 ) 2− clusters in silicoborides is observed for the first time.Under pressure, they replace the (B 12 ) 2− clusters, which occur in the structurally related compounds Li 2 B 12 Si 2 and MgB 12 Si 2 . 7,17The similarity between Na 2 B 6 Si 2 and Li 2 B 12 Si 2 , however, is not limited to their obviously similar electron balance.By cutting the Si 2 dumbbells, the 3D network of the Na 2 B 6 Si 2 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 Li 2 B 12 Si 2 the layers contain closo (B 12 ) 2− , and in Na 2 B 6 Si 2 closo (B 6 ) 2− anions.Such a crystallographic feature has been discussed for the structurally related gallides Na 2 Ga 7 and NaLiGa 7 . 16,18,19The sodium atoms in Na 2 B 6 Si 2 are situated in cavities of the B−Si framework (Figure 2c).−27 The FINDSYM utility was used to determine the symmetry of the relaxed structures. 28The 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 B 0 and equilibrium volume V 0 amount to 111.3 GPa and 351.90 Å 3 , respectively.−34 Thus, the calculated compressibility reveals that Na 2 B 6 Si 2 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. 35he chemical bonding in Na 2 B 6 Si 2 was analyzed applying the electron localizability approach�a quantum chemical technique in position space. 36First, the effective charges were evaluated from the calculated electron density by using the QTAIM formalism. 37The 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 electroncounting 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. 36While 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.
As a result of this study, we present a new structural motif for boron−silicon networks based on anionic B 6 octahedra and four connected silicon atoms forming dumbbells.In accordance with the calculated band gap, Na 2 B 6 Si 2 is an electron-precise valence compound with the balance (Na + ) 2 [(B 6 ) 2− ](Si 0 ) 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.

Figure 2 .
Figure 2. (a) Crystal structure of Na 2 B 6 Si 2 .(b) Layers perpendicular [001].(c) Local environment of sodium.Boron atoms are depicted in green, silicon atoms in orange, and sodium atoms in blue.

Figure 3 .
Figure 3. Electronic density of states of Na 2 B 6 Si 2 .

Table 1 .
Positional and Displacement Parameters of Na 2 B 6 Si 2 Obtained from Rietveld Refinement Using the Starting Model of the Electron Diffraction Data Improved by Quantum Chemical Optimization of the Coordinates (Space Group R3̅ m with a = 5.0735(1) Å and c = 16.0004(7)Å)