Cage Adaption by High-Pressure Synthesis: The Clathrate-I Borosilicide Rb8B8Si38

Rb8B8Si38 forms under high-pressure, high-temperature conditions at p = 8 GPa and T = 1273 K. The new compound (space group Pm3̅n, a = 9.9583(1) Å) is the second example for a clathrate-I borosilicide. The phase is inert against strong acids and bases and thermally stable up to 1300 K at ambient pressure. (Rb+)8(B–)8(Si0)38 is electronically balanced, diamagnetic, and shows semiconducting behavior with moderate Seebeck coefficient below 300 K. Chemical bonding analysis by the electron localizability approach confirms the description of Rb8B8Si38 as Zintl phase.


INTRODUCTION
Intermetallic boron compounds are often characterized by high thermal stability 1,2 and are thus frequently studied as candidate materials in application oriented research, e.g., in the context of thermoelectrics. 3−8 The often complex crystal structures provide a favorable basis for the required low heat conductivity, and a number of extensive studies focused on low-density binary and ternary borides. 4 More recent interest was directed on framework compounds of abundant elements such as boron-rich chalcogenides B 6 X (X = S, Se), boron suboxide B 6 O, or boron carbide. 5−8 Borosilicides with high boron content typically form framework structures comprising [B 12 ] 2− dodecahedra. 1 In boron-rich compounds like Li 2 B 12 Si 2 9 or Tb 0.68 B 12 Si 3.04 , 10 the dodecahedra are interconnected via exo-bonds or Si 2 dumbbells. In Na 8 B 74.5 Si 17.5 , multicenter bonds in closo-clusters go along with four-bonded silicon atoms interconnecting the clusters. 11 By comparison, silicon-rich borosilicides are rare. One of the few examples is clathrate-I type K 7 B 7 Si 39 , comprising fourbonded boron in the polyanion. 12 In contrast, most of the alkali metal silicon clathrates of the heavier homologues Al and Ga A 8−x Z 8−y Si 38+y (A = Na, K, Rb, Cs, Z = Al, Ga with x < 0.55, y < 0.9) have already been prepared, 13 38 , and Rb 8 Ga 8 Si 38 adopt nonmetal-deficient and electron-precise compositions in agreement with the Zintl rule. Nevertheless, subsequent attempts to prepare further boron-containing clathrate-I phases remained unsuccessful at ambient pressure. At this stage, the synthesis strategy was reconsidered by taking into account the beneficial effect of elevated pressures as evidenced by the recent preparation of the borosilicide LiBSi 2 comprising a [Si 2 B − ] n framework of four-bonded atoms 18 and the theoretical prediction of quenchable sodalite-type RbB 3 Si 3 . 19 In the scope of the present work, the influence of highpressure conditions on the formation of a clathrate-I borosilicide is investigated. We find that high-pressure synthesis grants access to the clathrate-I Rb 8 B 8 Si 38 showing remarkable thermal stability. The adaption of the crystal structure to cage filling and boron substitution is discussed, and the chemical bonding is studied by quantum chemical methods in direct space. Finally, thermal and electronic transport properties are reported.

EXPERIMENTAL SECTION
Synthesis. Sample handling, except for high-pressure synthesis and washing procedure, was performed in argon-filled glove boxes (MBraun, H 2 O and O 2 < 0.1 ppm). Rubidium (Chempur, 99.95%) and silicon (Chempur, 99.9999%) were used to synthesize the precursor compound Rb 12 Si 17 in a closed tantalum tube by annealing at 750°C for 7 h and slow cooling to room temperature within 8 h. Amorphous boron (Alfa Aesar) was cleaned and activated in a streaming hydrogen plasma. 20 High-pressure, high-temperature preparation started from educt mixtures with ratio Rb:B:Si = 3.25:2:5, which were thoroughly ground in agate mortars. The powders were filled into boron nitride crucibles before being placed in MgO octahedra with an edge length of 18 mm. The high-pressure, high-temperature syntheses were conducted using a multianvil press comprising a Walker-type module. 21 Calibration of pressure and temperature had been realized prior to the experiments by recording the resistance changes of bismuth and thermocouple-calibrated runs, respectively. A pressure of p = 8 ± 1 GPa was applied, and the samples were heated to T = 1273 ± 127 K within 15 min. After annealing for 300 min, the samples were quenched under load. The reaction products were washed with ethanol and deionized water to remove traces of highly reactive Rb 4 Si 4 , followed by washing with ethanol and acetone and drying at room temperature. The compound is air stable and inert against strong acids and bases.
Metallography. Specimens were embedded in paraffin and polished with a suspension of diamond powders (grain sizes 6, 3, and 0.25 μm). Wavelength-dispersive X-ray spectroscopy (WDXS) was carried out with a Cameca SX100 electron microprobe equipped with a tungsten cathode. Ni 3 B, Mg 2 Si, and RbI were used as standards. The analysis comprehended intensity measurements of the B−Kα, Si−Kα, and Rb-Lα lines. The X-ray emission lines were excited at an electron beam of 7 keV and a beam current of 100.00(1) nA for B, 15 keV and 8.00(1) nA for Si, and 15 keV and 40.00(1) nA for Rb, respectively. The WDX spectrometer was equipped with LPC3, TAP, or LPET monochromator crystals.
NMR. Nuclear magnetic resonance (NMR) experiments were performed on a Bruker Avance 500 spectrometer with a magnetic field of B 0 = 11.74 T. The standard Bruker MAS probe for 2.5 mm ZrO 2 rotors was used for 29 Si experiments, whereas the static probe (NMR Service GmbH, Erfurt, Germany) was used for 11 B experiments. The 29 Si and 11 B signals were referenced to 1 vol % tetramethylsilane (TMS) and BF 3 × Et 2 O with the reference frequencies of 99.3596 and 160.4588 MHz, respectively. In the case of 29 Si, the Hahn-echo sequence (90°−τ−180°−τ−acquisition) with a 90°pulse of 1.8 μs, interpulse delay of 100 μs and the recovery time of 5 s was applied. The MAS rotation rate was 30 or 29 kHz. For the 11 B spectra, the signal acquisition was achieved after a single pulse of 2.5 μs and recovery times of 30 s.
Electronic Structure Calculations and Chemical Bonding Analysis. For quantum chemical computations on Rb 8 B 8 Si 38 , the mixed occupation of the 16i position in space group Pm3̅ n was described by an ordered structure model in space group P4̅ 3n with boron and silicon each located on an 8e position. Moreover, the coordinate of the boron atoms 1/2 − x was optimized to a value of x = 0.1812 by total energy calculations being markedly different to x = 0.1912(2) for the averaged Si2/B2 position from X-ray structure refinements.
The electronic structure was calculated by means of the allelectron, full-potential local orbital (FPLO) method. 23 All results were obtained within the local density approximation (LDA) to the density functional theory using the Perdew−Wang parametrization for the exchange-correlation effects. 24 A mesh of 12 × 12 × 12 k points was used for calculations.
Chemical bonding analysis in position space was performed within the approach of combined topological analysis of electron density (ED) and electron localizability indicator (ELI). The analysis of the ED was made on the basis of the quantum theory of atoms in molecules (QTAIM). 25 ELI 26,27 was calculated in the ELI-D representation by a module implemented in FPLO. 28 The topological analysis of ED and ELI-D was carried out by the program DGRID. 29 Transport Properties. Electrical resistivity, thermal conductivity, and Seebeck coefficient were measured simultaneously on a physical property measurement system with thermal transport option (PPMS, Quantum Design). Rb 8 B 8 Si 38 powder was cold pressed to a plate (2.6 × 1.4 × 0.25 mm 3 ) and contacted in a four-terminal configuration using flat gold-plated copper leads and silver epoxy (Epotek H20e). The uncertainty of resistivity and conductivity was estimated to amount to ±50% and that of the Seebeck coefficient to 25% because of uncertainties in sample geometry.
Magnetic Susceptibility. Powder samples were measured in open quartz tubes with a squid magnetometer (MPMS XL-7, Quantum Design) from 1.8 to 300 K in external fields between 0.2 mT and 7 T.

■ 3. RESULTS AND DISCUSSION
The X-ray powder diffraction pattern of Rb 8 B 8 Si 38 (a = 9.9583(1) Å) reveals the clathrate-I-type crystal structure 30 with space group Pm3̅ n ( Figure 1 and Table S1). Rietveld refinements using binary Rb 8 Si 46−x as a structure model show that the smaller 20-atom dodecahedral and the larger 24-atom tetrakaidecahedral cages are fully occupied by Rb1 (Wyckoff position 2a) and Rb2 (6d), respectively (Table 1 and Table S2 and Figure S1). In the framework, position Si1 (6c) is fully occupied by silicon atoms, whereas enlarged displacement parameters for Si2 on position 16i and Si3 on site 24k point to mixed B/Si occupancy. Indeed, position 16i shows a distinctly reduced electron density, which is compatible with the occupancy of 9.36(6) Si2 and 6.64 B atoms. The slightly decreased electron density at position 24k is assigned to a    (Table S3). A peculiar feature of Rb 8 B 8 Si 38 (and also K 7 B 7 Si 39 ) is the predominant substitution of silicon atoms on position 16i (Si2). The site typically shows the shortest interatomic framework distances, e.g., in the related binary silicon clathrates K 8−x Si 46 31 or Rb 6.15 Si 46 32,33 and is thus best suited for accommodating the small boron atoms. Position 6c (Si1), 34,35 which is preferred by larger substitution atoms like transition metals, is avoided by boron as the resulting local configuration and interatomic distances would be unfavorable for sp 3 -hybridized boron atoms. The substitution of Si by the heavier homologues Al or Ga in Rb 8 Al 8 Si 38 and Rb 8 Ga 8 Si 38 affects all Si sites, although it appears predominantly at 6c. 14,16 Comparison of lattice parameters ( Figure 2, Table S3) reveals reduced values for ternary clathrate-I borosilicides 12 in relation to their binary analogs, 32 As boron substitution and rubidium defects mainly affect positions, which are located on the body diagonal of the unit cell, the shortening of the framework distances d(16i−16i) in the ternary phase (Table S3) goes along with a pronounced contraction of the lattice.
On the other hand, the value for the lattice parameter of Rb 8 B 8 Si 38 (a = 9.9583(1) Å) is strikingly similar to that of K 7 B 7 Si 39 (a = 9.952(1) Å) 12 in which half of the dodecahedral cages are empty. For describing the framework adaption to the larger rubidium atoms, a model is applied in which the metalcentered dodecahedra (yellow in Figure 4) are surrounded by Rb2 and Si1 atoms adopting Zeolite A topology. 41 The atoms of this 24-atom sodalite cage (gray in Figure 4) remain unaffected by boron substitution and alkali metal deficit. The edge length of the polyhedron, l = 1/4 a √2, directly scales with the lattice parameter because the metal (6d) and Si1 atoms (6c) occupy special positions without variable parameters. Because the lattice parameters of K 7 B 7 Si 39 and Rb 8 B 8 Si 38 are nearly identical, the sodalite cage adopts practically the same size in both crystal structures. Nevertheless, the size of the inner dodecahedral cage can still adapt to the radius of the metal atom. Replacement of potassium by the larger rubidium atoms goes along with longer distances between metal and framework and between Si3/B−Si3/B in the smaller 20-atom polyhedron ( Figure 4). Simultaneously, distances Si2/B−Si2/B and, to a lesser extent, Si1−Si3/B become shorter (Table S3).
The local arrangement of boron and silicon atoms is characterized by solid-state NMR spectroscopy. The 11 B NMR spectrum shows a strong, slightly asymmetric signal centered at −25 ppm, and a weak signal at 60 ppm, agreeing with two boron positions as evidenced by the X-ray diffraction experiment ( Figure 5). Boron pairs do not occur as the presence of [BSi 4 ] and [BBSi 3 ] entities in the tetrahedral framework would result in a more complex 11 B NMR spectrum. Therefore, only one out of two neighboring Si2 positions (site 16i) is substituted by boron atoms. The slight asymmetry of the stronger signal is attributed to the axial symmetry of the Si2 position and positional disorder. The 29 Si NMR spectrum shows a broad signal extending from −100 ppm to 600 ppm ( Figure 5, inset), which is assigned to the superposition of different local configurations. Similar spectra have also been observed for other substituted silicon clathrates. 42 Absence of an NMR Knight shift indicates a low density of states at the Fermi level, which is in line with the electron-balanced composition obtained from structure refinement.
The topological features of the partially substituted and compressed clathrate-I framework motivate investigation of chemical bonding. For quantum chemical calculations, an ordered structure representation in space group P4̅ 3n 43 Figure S1) are omitted in this description of the clathrate-I structure.   Inorganic Chemistry pubs.acs.org/IC Article 100 and 400 K ( Figure 10). The paramagnetic upturn below 100 K is typical for minor paramagnetic impurity phases. The sum of the diamagnetic increments for Rb 1+ , B 3+ and α-Si 47,48 amounts to χ = − 4.04 × 10 −4 emu mol −1 , which is only slightly smaller than the measured value. Measurements at 0.2 mT did not show any transition into the superconducting state.     The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
We thank Susann Leipe for supporting high-pressure syntheses. We express our gratitude to Ulrich Burkhardt, Sylvia Kostmann, and Petra Scheppan for metallographic analyses.