RESUMEN
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.
RESUMEN
Single crystals of a novel sodium-magnesium boride silicide, Na3MgB37Si9 [a = 10.1630â (3)â Å, c = 16.5742â (6)â Å, space group R m (No. 166)], were synthesized by heating a mixture of Na, Si and crystalline B with B2O3 flux in Mg vapor at 1373â K. The Mg atoms in the title compound are located at an inter-stitial site of the Dy2.1B37Si9-type structure with an occupancy of 0.5. The (001) layers of B12 icosa-hedra 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.
RESUMEN
Na-free Si clathrates consisting only of Si cages are an allotrope of diamond-structured Si. This material is promising for various device applications, such as next-generation photovoltaics. The probable technique for synthesizing Na-free Si clathrates is to extract Na+ from the Si cages of Na24 Si136 . Vacuum annealing is presently a well-known conventional and effective approach for extracting Na. However, this study demonstrates that Na+ cannot be extracted from the surface of a single-crystalline type-II metallic Si clathrate (Na24 Si136 ) in areas deeper than 150 µm. Therefore, a novel method is developed to control anisotropic ion diffusion: this is effective for various compounds with a large difference in the bonding strength between their constituent elements, such as Na24 Si136 composed of covalent Si cages and weakly trapped Na+ . By skillfully exploiting the difference in the chemical potentials as a driving force, Na+ is homogeneously extracted regardless of the size of the single crystal while maintaining high crystallinity. Additionally, the proposed point defect model is evaluated via density functional theory, and the migration of Na+ between the Si cages is explained. It is expected that the developed experimental and computational techniques would significantly advance material design for synthesizing thermodynamically metastable materials.
RESUMEN
Single crystals of (Na/Sr)-(Ga/Si) quaternary type-I clathrates, Na8-y Sr y Ga x Si46-x , were synthesized by evaporating Na from a mixture of Na-Sr-Ga-Si-Sn in a 6 : 0.5 : 1 : 2 : 1 molar ratio at 773 K for 12 h in an Ar atmosphere. Electron-probe microanalysis and single-crystal X-ray diffraction revealed that three crystals from the same product were Na8-y Sr y Ga x Si46-x with x and y values of 7.6, 2.96; 8.4, 3.80; and 9.1, 4.08. It was also shown that increasing the Sr and Ga contents increased the electrical resistivity of the crystal from 0.34 to 1.05 mΩ cm at 300 K.
RESUMEN
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 ß-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.
RESUMEN
Single crystals of a Na-Ga-Si clathrate, Na8Ga5.70Si40.30, of size 2.9 mm were grown via the evaporation of Na from a Na-Ga-Si melt with the molar ratio of Na : Ga : Si = 4 : 1 : 2 at 773 K for 21 h under an Ar atmosphere. The crystal structure was analyzed using X-ray diffraction with the model of the type-I clathrate (cubic, a = 10.3266(2) Å, space group Pm3Ìn, no. 223). By adding Sn to a Na-Ga-Si melt (Na : Ga : Si : Sn = 6 : 1 : 2 : 1), single crystals of Na8Ga x Si46-x (x = 4.94-5.52, a = 10.3020(2)-10.3210(3) Å), with the maximum size of 3.7 mm, were obtained via Na evaporation at 723-873 K. The electrical resistivities of Na8Ga5.70Si40.30 and Na8Ga4.94Si41.06 were 1.40 and 0.72 mΩ cm, respectively, at 300 K, and metallic temperature dependences of the resistivities were observed. In the Si L2,3 soft X-ray emission spectrum of Na8Ga5.70Si40.30, a weak peak originating from the lowest conduction band in the undoped Si46 was observed at an emission energy of 98 eV.
RESUMEN
Lithium-ion batteries (LIBs) are generally constructed by lithium-including positive electrode materials, such as LiCoO2, and lithium-free negative electrode materials, such as graphite. Recently, lithium-free positive electrode materials, such as sulfur, are gathering great attention from their very high capacities, thereby significantly increasing the energy density of LIBs. Though the lithium-free materials need to be combined with lithium-containing negative electrode materials, the latter has not been well developed yet. In this work, the feasibility of Li-rich Li-Si alloy is examined as a lithium-containing negative electrode material. Li-rich Li-Si alloy is prepared by the melt-solidification of Li and Si metals with the composition of Li21Si5. By repeating delithiation/lithiation cycles, Li-Si particles turn into porous structure, whereas the original particle size remains unchanged. Since Li-Si is free from severe constriction/expansion upon delithiation/lithiation, it shows much better cyclability than Si. The feasibility of the Li-Si alloy is further examined by constructing a full-cell together with a lithium-free positive electrode. Though Li-Si alloy is too active to be mixed with binder polymers, the coating with carbon-black powder by physical mixing is found to prevent the undesirable reactions of Li-Si alloy with binder polymers, and thus enables the construction of a more practical electrochemical cell.
RESUMEN
Single crystals of Ca4SiN4 were found in the product prepared by heating Ba, Ca, Si, NaN3, and Na at 900 °C. Ca4SiN4 [space group P2(1)/c (No. 14), Z = 4, a = 9.1905(4) Å, b = 5.9775(3) Å, c = 11.0138(7) Å, ß = 116.4054(17)°] is isotypic with Ca4GeN4 and K4SiO4. Isolated [SiN4](8-) tetrahedra were identified in the structure by single-crystal X-ray diffraction. After reheating the product at 900 °C, a new polymorph of Ca5Si2N6 crystallized. The space group of the polymorph [C2/m (No. 12), Z = 4, a = 6.2712(5) Å, b = 10.0175(8) Å, c = 12.0287(8) Å, ß = 99.303(2)°] is different from C2/c previously reported for Ca5Si2N6, while both polymorphs are composed of Ca(2+) and edge-sharing double tetrahedra [Si2N6](10-).
RESUMEN
Ternary sodium borosilicide, Na(8)B(74.5)Si(17.5), was newly synthesized by heating a mixture of sodium, silicon and amorphous or crystalline boron at 1073-1273 K. The crystal structure of the black hexagonal prismatic single crystal obtained at 1273 K was analyzed. The X-ray diffraction reflections of the crystal were indexed with hexagonal cell parameters a = 10.2392(3) Å and c = 10.9215(4) Å (space group P6(3)/mmc, No. 194). The structural formula could be represented as Na(8)(B(12))(6)Si(16)[BSi](1.5)[B(2)](0.5). B(12) icosahedra form a three-dimensional framework having small triangular and large hexagonal channels along the c axis. Chains of [-Si-(Si-Si)(3)-Si-] surrounded by Na atoms are located in the large channels. Trigonal prism cages for Na atoms and for atom pairs of Si-B or B-B are alternately arrayed in the small channels. The ratio of Si-B and B-B pairs in the cage is around 3:1.