Making and Breaking Bonds in Superconducting SrAl_4–<i>x</i>Si_<i>x</i> (0 ≤ <i>x</i> ≤ 2)

We explored the role of valence electron concentration in bond formation and superconductivity of mixed silicon–aluminum networks by using high-pressure synthesis to obtain the BaAl₄-type structural pattern in solid solution samples SrAl_4–<i>x</i>Si_<i>x</i> where 0 ≤ <i>x</i> ≤ 2. Local ordering of aluminum and silicon in SrAl_4–<i>x</i>Si_<i>x</i> was evidenced by nuclear magnetic resonance experiments. Subsequent bonding analysis by quantum chemical techniques in real space demonstrated that the strong deviation of the lattice parameters in SrAl_4–<i>x</i>Si_<i>x</i> from Vegard’s law can be attributed to the strengthening of interatomic Al–Al and Al–Si bonds within the layers (perpendicular to [001]) for 0 ≤ <i>x</i> ≤ 1.5, followed by the breaking of the interlayer bonds (parallel to [001]) for 1.5 < <i>x</i> ≤ 2 and leading to the structural transition from the BaAl₄ structure type with three-dimensional anionic framework at lower <i>x</i> values to the two-dimensional anion of the BaZn₂P₂ structure type with increasing <i>x</i> values. Low-temperature measurements of the resistivity and heat capacity reveal that SrAl_2.5Si_1.5 and SrAl₂Si₂ prepared at high pressures exhibit superconductivity with critical temperatures of 2.1 and 2.6 K, respectively

Redox Route from Inorganic Precursor Li₂C₂ to Nanopatterned Carbon

We present the synthesis route to carbon with hierarchical morphology on the nanoscale. The structures are generated using crystalline orthorhombic lithium carbide (Li₂C₂) as precursor with nanolamellar organization. Careful treatment by SnI₄ oxidizes carbon at the fairly low temperature of 80 °C to the elemental state and keeps intact the initial crystallite shape, the internal lamellar texture of particles, and the lamellae stacking. The reaction product is amorphous but displays in the microstructure parallel band-like arrangements with diameters in the range of 200–500 nm. These bands exhibit internal fine structure made up by thin strips of about 60 nm width running inclined with respect to the long axis of the band. The stripes of neighboring columns sometimes meet and give rise to arrow-like arrangements in the microstructure. This is an alternative preparation method of nanostructured carbon from an inorganic precursor by a chemical redox route without applying physical methods such as ion implantation, printing, or ablation. The polymerization reaction of the triple bond of acetylide anions gives rise to a network of carbon sp² species with statistically sized and distributed pores with diameters between 2 and 6 Å resembling zeolite structures. The pores show partially paracrystal-like ordering and may indicate the possible formation of carbon species derived from graphitic foams

Cluster Formation in the Superconducting Complex Intermetallic Compound Be₂₁Pt₅

ConspectusMaterials with the crystal structure of γ-brass type (Cu₅Zn₈ type) are typical representatives of intermetallic compounds. From the electronic point of view, they are often interpreted using the valence electron concentration approach of Hume–Rothery, developed previously for transition metals. The γ-brass-type phases of the main-group elements are rather rare. The intermetallic compound Be₂₁Pt₅, a new member of this family, was synthesized, and its crystal structure, chemical bonding, and physical properties were characterized.Be₂₁Pt₅ crystallizes in the cubic space group <i>F</i>4̅3<i>m</i> with lattice parameter <i>a</i> = 15.90417(3) Å and 416 atoms per unit cell. From the crystallographic point of view, the binary substance represents a special family of intermetallic compounds called complex metallic alloys (CMA). The crystal structure was solved by a combination of synchrotron and neutron powder diffraction data. Besides the large difference in the scattering power of the components, the structure solution was hampered by the systematic presence of very weak reflections mimicking wrong symmetry. The structural motif of Be₂₁Pt₅ is described as a 2 × 2 × 2 superstructure of the γ-brass structure (Cu₅Zn₈ type) or 6 × 6 × 6 superstructure of the simple bcc structural pattern with distinct distribution of defects. The main building elements of the crystal structure are four types of nested polyhedral units (clusters) with the compositions Be₂₂Pt₄ and Be₂₀Pt₆. Each cluster contains four shells (4 + 4 + 6 + 12 atoms). Clusters with different compositions reveal various occupation of the shells by platinum and beryllium. Polyhedral nested units with the same composition differ by the distance of the shell atoms to the cluster center.Analysis of chemical bonding was made applying the electron localizability approach, a quantum chemical technique operating in real space that is proven to be especially efficient for intermetallic compounds. Evaluations of the calculated electron density and electron localizability indicator (ELI-D) revealed multicenter bonding, being in accordance with the low valence electron count per atom in Be₂₁Pt₅. A new type of atomic interactions in intermetallic compounds, cluster bonds involving 8 or even 14 atoms, is found in the clusters with shorter distances between the shell atoms and the cluster centers. In the remaining clusters, four- and five-center bonds characterize the atomic interactions. Multicluster interactions within the polyhedral nested units and three-center polar intercluster bonds result in a three-dimensional framework resembling the structural pattern of NaCl. Be₂₁Pt₅ is a diamagnetic metal and one of rather rare CMA compounds revealing superconductivity (<i>T</i>_c = 2.06 K)

Intermediate-Valence Ytterbium Compound Yb₄Ga₂₄Pt₉: Synthesis, Crystal Structure, and Physical Properties

The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 °C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 °C. The crystal structure of Yb₄Ga₂₄Pt₉ was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group <i>C</i>2<i>/m</i>, Pearson symbol <i>mS</i>74, <i>a</i> = 7.4809(1) Å, <i>b</i> = 12.9546(2) Å, <i>c</i> = 13.2479(2) Å, β = 100.879(1)°, <i>V</i> = 1260.82(6) ų, <i>R</i>_<i>F</i> = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an <i>ABABB</i> stacking of two slabs with trigonal symmetry and compositions Yb₄Ga₆ (<i>A</i>) and Ga₁₂Pt₆ (<i>B</i>). The hard X-ray photoelectron spectrum (HAXPES) of Yb₄Ga₂₄Pt₉ shows both Yb²⁺ and Yb³⁺ contributions as evidence of an intermediate valence state of ytterbium. The evaluated Yb valence of ∼2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is a bad metallic conductor