A Borosilicide with Clathrate VIII Structure.

The high-pressure phase Na8BxSi46-x (3 < x < 5) is the first representative of a borosilicide crystallizing in the rarely occurring clathrate VIII type structure. Crystals with composition Na8B4Si42 (space group I43̅m; a = 9.7187(2) Å; Pearson symbol cI54) were obtained at 5-8 GPa and 1200 K. The clathrate I modification exists for the same composition at lower pressure with a larger cell volume (Pm3̅n; a = 9. 977(2) Å; cP54). Profound structural adaptions allow for a higher density of the clathrate VIII type than clathrate I, opening up the perspective of obtaining clathrate VIII type compounds as high-pressure forms of clathrate I.

Intermetallic clathrates are framework compounds established by covalently bonded p- or d-elements enclosing filler atoms in polyhedral cages.[1] Their significance is fueled by beneficial electrochemical properties,[2] superconductivity,[3,4] and an inherently low thermal conductivity, which qualifies them as potential thermoelectric energy materials.[5] The broad variability of their chemical compositions opens a wide design space for tuning physical properties, e.g., from metallic to semiconducting behavior.[6] The vast majority of clathrate representatives belong to type I (space group Pm3̅n), consisting of 20- and 24-atom cages (Figure a), or to the clathrate II structure (Fd3̅m) with 20- and 28-atom cages. The crystal structure of clathrate VIII[7] (I4̅3m) offers (20 + 3)-atom cavities and is, thus, able to adapt to filler atoms that are too small to stabilize larger cages (Figure b). Representatives of this type are rare[8−13] but of current interest, as they feature promising thermoelectric efficiency.[14,15] The clathrate VIII pattern exhibits a higher density than the clathrate I arrangement,[9] so it is reasonable to assume that type VIII is favored at high pressure. Herein, we will provide evidence that the clathrate VIII pattern is preferred at the cost of clathrate I upon enhanced compression in borosilicides Na8BSi46 (3 ≤ y ≤ 5). Single crystals of the clathrate I and VIII modifications were characterized for the same composition, which we assign to Na8B4Si42, allowing for a direct comparison of structural features. Nonetheless, the finding of a clathrate VIII was a surprise because our experiments aimed to complete the clathrate I borosilicides M8–BSi46– (M = K, Rb, Cs) with M = Na. In this series, only the representative K7B7Si39, which is remarkably stable against oxidizing environments,[16,17] was prepared at ambient pressure. The clathrate I borosilicides with large cations Rb8B8Si38[18] and Cs8B8Si38[19] only form under high-pressure conditions. Consistently, binary clathrate I silicides such as Ba8Si46[3] or Cs8–Si46[20] are high-pressure phases as well. This finding can be explained by the flexibility of the clathrate network under pressure and the high coordination number achieved for the M atoms (pressure-coordination rule[21]). For the smaller alkali metal Na, preparation at ambient pressure conditions did not result in a clathrate phase either. For experiments under high-pressure conditions, a 5:2 mixture of finely ground NaSi[22] and activated amorphous boron[23] was filled into BN crucibles. After a reaction time of 45 min at p = 5 GPa and T = 1200(100) K, XRPD data revealed the formation of a clathrate I phase and a clathrate VIII phase in similar amounts. When the pressure was raised to 6 GPa, the clathrate I phase was no longer observed in XRPD, and clathrate VIII became the majority phase, evidencing that clathrate VIII Na8–BSi46– is a high-pressure form. In addition, a hexagonal minority phase with an estimated composition of Na2Si2B6[24] is formed. The highest yield of clathrate VIII was obtained after a reaction time of only 10 min (Figure ). Without adding boron, a clathrate VIII phase was not observed in the system.

Figure 1

Modifications of Na8B4Si42. (a) Clathrate I structure with 24- (gray) and 20-atom (green) cages (b) Clathrate VIII type structure with Na@E20 polyhedra (gray) and empty E8 realgar voids (green) (E = Si, B). The E–E bonds are indicated in gray.

Figure 2

XRPD of a sample obtained at 6 GPa and 950 °C containing clathrate VIII Na8B4Si42 (82(1) mass %) and Na2B6Si2 (18(1) mass %).

For transmission electron microscopy studies, thin lamellas were prepared using the focused ion beam technique.[25] The sample was investigated by selected-area electron diffraction tomography (SA-EDT). The structure solution from precession data (922 symmetry-independent reflections) revealed the clathrate VIII structure (see the Supporting Information). A qualitative SEM/EDXS analysis of the same specimen area confirmed a ternary main phase consisting of Na, B, and Si (Na:Si ≈ 1:4). The final clathrate VIII structure model was determined from single-crystal X-ray diffraction data. The structure refinement in space group I43̅m started with a binary model Na8Si46– comprising the Wyckoff positions, Si1 (12d), Si2 (2a), Si3 (24g), Si4 (8c), and Na (8c) centering the 20 + 3-atom cage (Figures a and 4). After the atomic positions, the site occupancies, and the atomic displacement parameters (ADP) were refined. Only position Si4 showed a mixed occupancy described by 4 Si and 4 B atoms. Because the bond distances d(Si–B) ≈ 2.0 Å and d(Si–Si) ≈ 2.3 Å differ, structural disorder occurs. The refined distance values d(Si4/B–Si3) = 2.15 Å and d(Si4/B–Si2) = 2.26 Å thus represent the mean distances of different local environments. The ADPs of the Si3 atoms feature a cigar-shaped anisotropy with the long axis of the ellipsoid directed toward the center of the empty 8-atom cage (Figure e). The anisotropy can be alternatively described by closely adjacent split positions Si31 and Si32 (see Supporting Information). Position Si3 is fully occupied within experimental error, but because of the high site multiplicity, even a minute boron content would significantly change the composition. Moreover, the ADPs of Si2 are slightly enlarged because Si2 is surrounded by 4 Si4/B positions (Figure ). The presence of vacancies at Si4 instead of boron atoms cannot be ruled out from occupancy refinement. However, such Zintl defects typically cause more pronounced ADP than observed for Si2 and Si3.[26] Assuming that boron atoms only occupy position Si4, the refinement resulted in the composition Na8B4.2(1)Si41.8(1) and the residual R = 0.03.

Figure 3

(top) Six- (red) and five-membered rings (blue) in clathrates I and VIII. (a) Alternative representation of the clathrate VIII structure with empty realgar cages (green). (b) Condensed 24- (gray) and 20-atom (green) cages in the clathrate I structure. (bottom) Eclipsed (c) and staggered (d) conformation. (e) Anisotropic displacement ellipsoids of clathrate VIII Na8B4Si42.

Figure 4

Clathrate VIII arrangement with emphasis on the space diagonal and distances dNa–Si4/B (red), dNa–Si2 (blue), and dSi2–Si4/B (orange) along ⟨111⟩. Other atoms are omitted for clarity.

The preference of B atoms for the Si4 position can be rationalized by the close contact with the adjacent Na atoms. Both Na and Si4 occupy individual 8c positions (xxx) on the space diagonal, allowing for a scaling toward the optimal distance under a strong electrostatic interaction. In the 23-atom cage, Na is surrounded by four Si4 positions. Na always approaches the one located on the same space diagonal. Therefore, the Na positions are off-centered. With the short distance dNa–Si4/B = 2.988(5) Å on the space diagonal, the structure adapts to the small lattice parameter (Table , Figure ). This distance is distinctly smaller than the opposite distance dNa–Si2 = 3.171(3) Å but similar to dNa–Si = 2.876 Å in Na4Si4.[22]

Table 1

Selected Interatomic Distances in Clathrate I and VIII Na8B4.1Si41.9

clathrate I		clathrate VIII
atoms	d [Å]	atoms	d [Å]
Na1–8 Si2/B	3.216(3)	Na1–1 Si2	3.171(3)
Na1–12 Si3/B	3.259(2)	Na1–3 Si3	3.127(3)
Na2–4 Si1	3.5274(4)	Na1–6 Si3	3.221(2)
Na2–8 Si2/B	3.696(2)	Na1–1 Si4/B	2.988(5)
Na2–8 Si3/B	3.382(2)
Si1–Si3/B	2.351(2)
Si2/B–Si2/B	2.208(5)	Si1–Si3	2.3530(8)
Si2/B–Si3/B	2.300(2)	Si2–Si4/B	2.257(3)
Si3/B–Si1	2.351(2)	Si3–Si3	2.343(3)
Si3/B–Si3/B	2.371(4)	Si3–Si4/B	2.151(2)

Interestingly, the off-centering of the cage atom is a feature observed for all clathrate VIII type structures, even for the optimized hypothetical binary Ba8Si46.[27] The magnitude of the off-center displacement can be attributed to the extent of the respective ionic interaction. Depending on the synthesis pressure (5–8 GPa), the lattice parameter of clathrate VIII Na8BSi46– varies from a = 9.7579(6) to 9.6750(4) Å. In the type I clathrate K8–BSi46–,[17] the substitution of Si by B causes a reduction of the lattice parameter of Δa ≈ 0.04 Å/atom. Therefore, with a = 9.7187(2) Å for Na8B4.1Si41.9, a composition range of Na8B3Si43–Na8B5Si41 can be estimated. The electron balance (Na+)8[(4b)(B–)4.1][(4b)(Si0)38]·3.9 e– reveals ≈4 excess electrons per formula unit. Consequently, the clathrate VIII phase significantly deviates from an electron-precise composition. We assume the cages become too small to accommodate the Na atoms at higher boron contents.

Among the series of clathrate VIII crystals tested for data collection, one revealed the clathrate I type of structure, although the phase is not visible in the XRPD pattern (Figure ). We assume that the slow cooling rate of the die may allow for a partial phase transformation on cooling. Crystals of the byproduct Na2Si2B6 were not identified. For the clathrate I borosilicide, the refinement strategy has been described in detail.[8] The crystal structure in Pm3̅n features three framework positions, Si1 (6c), Si2 (16i), Si3 (24k), and two Na positions at 2a (Na1) and 6d (Na2). Boron occupancy is found for position Si2 and, to a small extent, for Si3, whereas Si and Na fully occupy Si1, Na1, and Na2. Boron atoms at the Si2 position are a peculiarity of clathrate I borosilicides, in which the boron atoms prefer positions of the small 20-atom cage. Typically, foreign atoms preferably occupy the six rings of the 24-atom cages (Figure b). The ADPs of the Na2 atoms in the larger 24-atom cage show a disc-shaped electron density. ADPs of Si2 are elongated in the direction of the Si2–Si2 bond. Because of mixed occupancy with B and Si, the Si2–Si2 distance represents the superposition of a Si–Si and a B–Si bond (Table ). Because of the low scattering power of the crystal, split positions for Si2[17] are not resolved. The refinement results in a residual value RF = 0.05 and the composition Na8B4.1(7)Si41.9(7). Consequently, the clathrate I phase is the only exception in the clathrate I series M8–BSi46– (M = Na, K, Rb, Cs) that deviates significantly from Zintl’s rule. We attribute the low boron content of the clathrate I crystal to the depletion of boron in the reaction mixture because of the formation of Na2Si2B6.

The lattice parameter a = 9.7187(2) Å of clathrate VIII Na8B4Si42 is distinctly smaller than a = 9.977(2) Å for the clathrate I modification (Table ), setting a new benchmark for the smallest lattice parameter observed for clathrate phases. (e.g., Na8Si46, a = 10.19(2) Å[28]). The higher density of clathrate VIII is caused by the bond conformation within the four-bonded framework. In the densely packed diamond-type structure of α-Si, all (Si3Si)2 units have the energetically favored staggered configuration, while the units are eclipsed in clathrate I and clathrate II (Figure c,d). The eclipsed conformation leads to the open framework structure with polyhedral cavities and flat five- and six-membered rings (Figure b). However, in the clathrate VIII structure, 52% of the bonds feature a staggered arrangement so that the structure is closer to the diamond type. The presence of both corrugated six and flat five-membered rings (Figure a) leads to a higher dispersion of bond angles in the framework of clathrate VIII (average x̅ = 109.3°, standard deviation σ = 9.5°) compared to clathrate I (x̅ = 109.4°, σ = 4.8°).

Table 2

Lattice Parameter and Average Distances d̅ in Clathrate Borosilicides

composition	a [Å]	d̅ [Å]	ref
Na8B4.2(1)Si41.8(1) (Cl–VIII)	9.7187(2)	2.302	this work
Na8B4.1(7)Si41.9(7) (Cl–I)	9.977(2)	2.320	this work
K7.12(4)B7.1(6)Si38.9(6)	9.9393(2)	2.319	(8)
K7.85(2)B7.8(1)Si38.2(1)	9.9050(2)	2.301	(8)
Rb8B7.9(1)Si38.1(1)	9.9583(1)	2.315	(9)
Cs8.0(1)B8.0(1)Si38.0(1)	10.0312(3)	2.332	(10)

The electric transport behavior has not been determined as all syntheses resulted in polycrystalline products with impurity phases. Nevertheless, metallic behavior and low thermoelectric efficiency are expected because both clathrates are not electron balanced. For the clathrate VIII sample with composition Na8B4Si42, no superconducting transition is observed down to 1.9 K (see Supporting Information).

In conclusion, two clathrates occur as high-pressure phases in the ternary system of sodium, boron, and silicon and are structurally characterized for the composition Na8B4Si42. The clathrate I modification completes the series M8–BSi46– (M = K, Rb, and Cs). The clathrate VIII modification is the first borosilicide crystallizing in this rarely observed structure type. The high content of bonds with staggered conformation leads to a more compact packing than in clathrate I, resulting in the smallest lattice parameter for a silicon clathrate. The formation of clathrate VIII at the cost of clathrate I upon pressure enhancement opens the perspective for preparing new members of this rare, nevertheless attractive, structure type.