Na3MgB37Si9: an icosahedral B12 cluster framework containing {Si8} units

Single crystals of a novel boride silicide, Na3MgB37Si9, containing B12 icosahedra and Si8 units were synthesized from Na, B, Si, B2O3 and magnesium vapor.


Chemical context
Boron-rich compounds composed of B 12 icosahedral clusters are attracting attention as thermoelectric materials because of their low thermal conductivity resulting from their complicated crystal structures (Cahill et al., 1977). In our previous study, a novel ternary borosilicide, Na 8 B 74.5 Si 17.5 , was synthesized, and its crystal structure (Morito et al. 2010) and electronic structure measured using soft X-ray spectrometry (Terauchi et al. 2018), have been reported. This compound has a three-dimensional framework structure with layers composed of B 12 icosahedral clusters and Si chains in the channels of the B 12 clusters. During the investigation of this compound, a new crystalline phase was synthesized in which the stacking sequence of the B 12 cluster layers differed from that of Na 8 B 74.5 Si 17.5 . The composition analysis revealed that the new phase contained a small amount of Mg derived from an impurity in the starting material of amorphous B powder. Single crystals of this phase were prepared in the present study by heating a starting mixture of Na, crystalline B, a flux of B 2 O 3 with Mg vapor, and the crystal structure was determined using single-crystal X-ray diffraction.

Structural commentary
The crystal structure of the new phase of composition Na 3 MgB 37 Si 9 is trigonal (space group R3m,No. 166), and the hexagonal lattice constants are a = 10.1630 (3) Å and c = 16.5742 (6) Å . The structure is composed of B 12 icosahedral clusters: the B-B distances of the 30 distinct bonds in the cluster are in the range of 1.791 (3)-1.843 (5) Å and the average distance is 1.811 Å (  (Salvador et al. 2003).

Table 1
Selected geometric parameters (Å , ).  (Ludwig et al. 2013). Thus, it may be seen that the lattice constants of Na 3 MgB 37 Si 9 are larger than those of related compounds and the unit-cell volume of Na 3 MgB 37 Si 9 is approximately 2% larger than the maximum unit-cell volume of 1454 Å 3 for the RE 1-x B 12 Si 3.3series with RE = Yb (Zhang et al. 2003). This increase in the lattice constants could be related to the occupancy of the Mg1 site, which is not found in other compounds. Table 2 compares the interatomic distances for Na 3 MgB 37 Si 9 , Dy 2.1 B 37 Si 9 and Mg 3 B 36 Si 9 C. The average B-B distances of B 12 icosahedra, B2-B2 distances between clusters, and Si2-B4 distances for Na 3 MgB 37 Si 9 are longer than those of other compounds. However, only the bond distance of Si3-Si3, in which Si3 only binds to Si, is specifically shorter. It is assumed that this bond became shorter because of an increase in the bond order from 1 because of a decrease in the number of electrons in the antibonding orbitals of the Si3-Si3 unit with a decrease in the electron count for the entire framework. Assuming that the main cause of the lattice expansion of Na 3 MgB 37 Si 9 is a decrease in the bonding force between B-B and B-Si atoms because of electron deficiency in the bonding orbitals of the B 37 Si 9 framework, the lattice constant can be reduced by increasing the Mg occupancy, which can be attained by increasing the Mg vapor pressure during the synthesis.

Database survey
In space group R3m, the framework structures of B 12 icosahedral clusters containing {Si 8 } units similar to Na 3 MgB 37 Si 9 have been reported for Mg 3 B 36 Si 9 C (Ludwig et al. 2013),

Synthesis and crystallization
Na metal pieces (purity 99.95%, Nippon Soda Co., Ltd.), crystalline B powder (99.9%, FUJIFILM Wako Pure Chemical Industries Co., Ltd.) and Si powder (99.999%, Kojundo Chemical Lab. Co., Ltd.) were weighed in a BN crucible (99.5%, Showa Denko K. K., outer diameter = 8.5 mm, inner diameter = 6.5 mm, depth = 18 mm), with a molar ratio of Na:B:Si = 5:4:3 (a total weight 280 mg) in a high-purity Arfilled glove box (O 2 < 1 ppm, H 2 O < 1 ppm). Then, 10 mg of B 2 O 3 powder (90%, FUJIFILM Wako Pure Chemical Industries, Ltd.) were added to the crucible, which was stacked on another BN crucible containing 30 mg of Mg powder (99.9%, rare metallic), and these crucibles were encapsulated in a stainless steel container (SUS316, outer diameter = 12.7 mm, inner diameter = 10.75 mm, length 80 mm) with Ar gas. The container was heated at 1373 K for 24 h using an electric furnace. After cooling, the crucible was taken out from the reaction container, and any Na and NaSi remaining in the crucible were reacted and removed with 2-propanol and ethanol. Then, the sample was washed with pure water to remove water-soluble compounds such as sodium borate and alkoxide produced by the reaction of Na and alcohol to leave black plates of the title compound. An electron probe microanalyzer (EPMA; JEOL Ltd., JXA-8200) was used to analyze the composition of the obtained single crystal as Na  Table 2 Cell parameters (Å ), cell volumes (Å 3 ) and selected bond lengths (Å ) of Na 3 MgB 37 Si 9 , Dy 2.1 B 37 Si 9 a and Mg 3 B 36 Si 9 C.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component inversion twin.