Electric Transport Properties of NaAlB14 with Covalent Frameworks

A synthetic protocol was developed for obtaining a single phase of polycrystalline NaAlB₁₄ with strongly connected intergrain boundaries. NaAlB₁₄ has a unique crystal structure with a tunnel-like covalent framework of B that traps monovalent Na and trivalent Al ions. Owing to the atmospheric instability and volatility of Na, the synthesis of polycrystalline NaAlB₁₄ and its physical properties have not been reported yet. This study employed a two-step process to achieve single-phase polycrystalline NaAlB₁₄. As a first step, a mixture of Al and B with excess Al was sintered in the Na vapor atmosphere followed by HCl treatment to remove excess Al as a second step. For obtaining bulk samples with strong grain connection, vacuum or high-pressure (HP) annealing was employed. HP annealing promoted bandgap shrinkage due to the crystal strain and defect levels and suppressed intergranular resistance. As a result, the HP-annealed sample achieved superior transport properties (0.1 kΩ cm at 300 K) to the vacuum-annealed sample (260 kΩ cm). Furthermore, from the viewpoint of its crystal structure and DFT calculations, the most probable site for the defect was suggested to be the Na site.

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.

Single crystals of a novel sodium–magnesium boride silicide, Na₃MgB₃₇Si₉ [a = 10.1630 (3) Å, c = 16.5742 (6) Å, space group Rm (No. 166)], were synthesized by heating a mixture of Na, Si and crystalline B with B₂O₃ flux in Mg vapor at 1373 K. The Mg atoms in the title compound are located at an inter­stitial site of the Dy_2.1B₃₇Si₉-type structure with an occupancy of 0.5. The (001) layers of B₁₂ icosa­hedra stack along the c-axis direction with shifting in the [–a/3, b/3, c/3] direction. A three-dimensional framework structure of the layers is formed via B—Si bonds and {Si₈} units of [Si]₃—Si—Si—[Si]₃.

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.

Soft X-ray emission spectroscopy study of electronic structure of sodium borosilicide Na8B74.5Si17.5

Chemical bonding state of sodium borosilicide Na8B74.5Si17.5, which is a new member of B12-cluster materials, is investigated by soft X-ray emission spectroscopy. The material is composed of B12 cluster network and characteristic silicon chains of [-Si-(Si-Si)3-Si-] connected by sp3 bonding, in which bonding distances and bonding angles are close to those in cubic Si crystal. B K-emission spectrum of the material showed a similar but a broader intensity distribution with those of B12 cluster materials of α-r-B, B4C and β-r-B. The broader intensity distribution can be due to a variation of B-B bond length in B12 cluster. The density of states (DOS) of silicon chains of [-Si-(Si-Si)3-Si-] was experimentally derived. It shows a similar energy width, and peak or shoulder structures in intensity distribution with those of L-emission spectrum of cubic Si. From comparisons between experimental spectra and corresponding calculated DOS, covalent bonding between Si chain and B12 cluster network is suggested. Those are discussed by using a theoretically calculated density of state of Na8B74.5Si17.5 by using WIEN2k code.