Li2B12Si2: The First Ternary Compound in the System Li/B/Si: Synthesis, Crystal Structure, Hardness, Spectroscopic Investigations, and Electronic Structure

Ordering by cation replacement in the system Na2-xLixGa7

Samples of the pseudo-binary system Na2-xLixGa7 (x ≤ 1) were synthesized from the elements at 300 °C in sealed Ta ampoules or by the reaction of Na2Ga7 with LiCl. The peritectic formation temperature decreases with increasing Li content from 501(2) °C (x = 0) to 489(2) °C (x = 1). The boundary compositions Na2Ga7 and Na1Li1Ga7 crystallize with different structure types related by a group-subgroup relation. While the Na-rich compositions (x ≤ 0.5) represent a substitutional solid solution (space group Pnma), the Li-rich compositions feature an unconventional replacement mechanism in which Li atoms occupying interstitial positions induce vancancies at the Na positions (space group Cmce). The crystal structure of Na1Li1Ga7 (a = 8.562(1) Å, b = 14.822(2) Å, c = 11.454(2) Å; Z = 8) was determined from X-ray single-crystal diffraction data, and reveals an anionic framework comprising 12-bonded Ga12 icosahedra and 4-bonded Ga atoms, with alkali-metal atoms occupying channels and cavities. The arrangement of cations makes NaLiGa7 a new structure type within the MgB12Si2 structure family. Band structure calculations for the composition NaLiGa7 predict semiconducting behavior consistent with the balance [Na+]2[Li+]2[(Ga12)2-][Ga-]2, considering closo Wade clusters [(12b)Ga12]2- and Zintl anions [(4b)Ga]-. Susceptibility measurements indicate temperature-independent diamagnetic behavior.

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.

Synthesis and crystal structures of the new ternary borides Fe3Al2B2 and Ru9Al3B8 and the confirmation of Ru4Al3B2 and Ru9Al5B8−x (x≈2)

Single crystals of the new ternary borides Fe3Al2B2 and Ru9Al3B8 were obtained from the elements at 1900°C. Both compounds represent new structure types which combine well-known features of binary and ternary borides of transition metals in combination with aluminum. The crystal structure of Fe3Al2B2 (P2/m, Z=2, a=5.724, b=2.857, c=8.723 Å, β=98.57°) contains tetramers of face-sharing trigonal prisms BFe6 with a B4 unit in trans-configuration. The tetrameric units are separated by Al-atoms which occupy all remaining rectangular sites of the trigonal prisms. The structure can be derived from Fe2AlB2 by insertion of additional FeAl fragments in a bcc arrangement. The crystal structure of Ru9Al3B8 (P6̅2m, Z=1, a=9.078, c=2.913 Å) combines zig-zag chains of boron atoms made of face-sharing trigonal prisms BFe6 and isolated BFe6 units. Three of these chains are connected by common corners to rods running in direction [001]. The rods are linked to a three-dimensional framework by isolated prisms via common edges. Again, Al occupies the capping positions of the trigonal prisms. Ru9Al3B8 is the second representative for the combination of boron zig-zag chains and isolated B atoms. The existence of Ru4Al3B2 (P4/mmm, Z=2, a=8.515, c=2.924 Å) and Ru9Al5B8−x (P4/m, Z=1, a=8.741, c=2.923 Å) were confirmed and the crystal structures refined. High quality data reveal a stoichiometric composition for Ru4Al3B2, while in Ru9Al5B8−x there is a significant underoccupation (i.e. x≈2) of the central boron site within the B4 units. The crystal structures of all four compounds represent examples for the combination of CsCl and AlB2 fragments as they were frequently found for ternary borides of transition metals.

Synthesis and properties of new multinary silicides R5Mg5Fe4Al(x)Si(18-x) (R = Gd, Dy, Y, x ≈ 12) grown in Mg/Al flux

Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R(5)Mg(5)Fe(4)Al(x)Si(18-x) (x ≈ 12). These phases have a new structure type in tetragonal space group P4/mmm (a = 11.655(2) Å, c = 4.0668(8) Å, Z = 1 and R(1) = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and (27)Al and (29)Si MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at T(N) = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field.

Mechanical and electronic properties of B12-based ternary crystals of orthorhombic phase

Using first-principles calculations, the structural, mechanical and electronic properties of the experimentally synthesized B(12)-based ternary crystals (AlMgB(14), AlNaB(14), AlLiB(14), Mg(2)B(14), MgSi(2)B(12), MgC(2)B(12), Li(2)Si(2)B(12) and Li(2)C(2)B(12)) have been investigated. The theoretical equilibrium lattice constants of these crystals agree with the experimental values. The Vickers hardness (H(v)) estimated from the theoretical Young's moduli ranges from 20 to 30 GPa, and the MgC(2)B(12) compound (H(v) = 31.4 GPa) is harder than α-boron. Based on the electron density of states and Mulliken population analysis, the origination of hardness and interaction between the interstitial atoms and the B(12) framework were discussed. Scaled bond order of the B-B bonds was used to interpret the hardness of these B(12)-based ternary compounds. The crystal hardness is primarily determined by the B(12) icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms. We also calculated the ideal tensile strength of AlMgB(14) and MgC(2)B(12) and found that the <001> and <010> directions are their cleavage directions under tensile strains, respectively.