Classical and Nonclassical Germanium Environments in High-Pressure BaGe₅

A new crystalline form of BaGe₅ was obtained at a pressure of 15(2) GPa in the temperature range from 1000(100) to 1200(120) K. Single-crystal electron and powder X-ray diffraction patterns indicate a body-centered orthorhombic structure (space group <i>Imma</i>, Pearson notation <i>oI</i>24) with unit cell parameters <i>a</i> = 8.3421(8) Å, <i>b</i> = 4.8728(5) Å, and <i>c</i> = 13.7202(9) Å. The crystal structure of <i>hp</i>-BaGe₅ consists of four-bonded Ge atoms forming complex layers with Ge–Ge contacts between 2.560(6) and 2.684(3) Å; the Ba atoms are coordinated by 15 Ge neighbors in the range from 3.341(6) to 3.739(4) Å. Analysis of the chemical bonding using quantum chemical techniques in real space reveal charge transfer from the Ba cations to the anionic Ge species. Ge atoms having nearly tetrahedral environments show an electron-localizability-based oxidation number close to 0; the four-bonded Ge atoms with a Ψ-pyramidal environment adopt a value close to 1-. In agreement with the calculated electronic density of states, the compound is a metallic conductor (electrical resistivity of ca. 240 μΩ cm at 300 K), and magnetic susceptibility measurements evidence diamagnetic behavior with χ₀ = −95 × 10^–6 emu mol^–1

New Monoclinic Phase at the Composition Cu₂SnSe₃ and Its Thermoelectric Properties

A new monoclinic phase (<i>m2</i>) of ternary diamond-like compound Cu₂SnSe₃ was synthesized by reaction of the elements at 850 K. The crystal structure of <i>m2</i>-Cu₂SnSe₃ was determined through electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data (space group <i>Cc</i>, <i>a</i> = 6.9714(2) Å, <i>b</i> = 12.0787(5) Å, <i>c</i> = 13.3935(5) Å, β = 99.865(5)°, <i>Z</i> = 8). Thermal analysis and annealing experiments suggest that <i>m2</i>-Cu₂SnSe₃ is a low-temperature phase, while the high-temperature phase has a cubic crystal structure. According to quantum chemical calculations, <i>m2</i>-Cu₂SnSe₃ is a narrow-gap semiconductor. A study of the chemical bonding, applying the electron localizability approach, reveals covalent polar Cu–Se and Sn–Se interactions in the crystal structure. Thermoelectric properties were measured on a specimen consolidated using spark plasma sintering (SPS), confirming the semiconducting character. The thermoelectric figure of merit <i>ZT</i> reaches a maximum value of 0.33 at 650 K

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

Crystal Structure and Physical Properties of Ternary Phases around the Composition Cu₅Sn₂Se₇ with Tetrahedral Coordination of Atoms

A new monoclinic selenide Cu₅Sn₂Se₇ was synthesized, and its crystal and electronic structure as well as thermoelectric properties were studied. The crystal structure of Cu₅Sn₂Se₇ was determined by electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data: space group <i>C</i>2, <i>a</i> = 12.6509(3) Å, <i>b</i> = 5.6642(2) Å, <i>c</i> = 8.9319(4) Å, β = 98125(4)°, <i>Z</i> = 2; <i>T</i> = 295 K. Thermal analysis and high-temperature synchrotron X-ray diffraction indicated the decomposition of Cu₅Sn₂Se₇ at 800 K with formation of the tetragonal high-temperature phase Cu_4.90(4)Sn_2.10(4)Se₇: space group <i>I</i>4̅2<i>m</i>, <i>a</i> = 5.74738(1) Å, <i>c</i> = 11.45583(3) Å; <i>T</i> = 873 K. Both crystal structures are superstructures to the sphalerite type with tetrahedral coordination of the atoms. In agreement with chemical bonding analysis and band structure calculations, Cu₅Sn₂Se₇ exhibits metal-like electronic transport behavior

ZSM-5 Zeolite Single Crystals with <i>b</i>-Axis-Aligned Mesoporous Channels as an Efficient Catalyst for Conversion of Bulky Organic Molecules

The relatively small and sole micropores in zeolite catalysts strongly influence the mass transfer and catalytic conversion of bulky molecules. We report here aluminosilicate zeolite ZSM-5 single crystals with <i>b</i>-axis-aligned mesopores, synthesized using a designed cationic amphiphilic copolymer as a mesoscale template. This sample exhibits excellent hydrothermal stability. The orientation of the mesopores was confirmed by scanning and transmission electron microscopy. More importantly, the <i>b</i>-axis-aligned mesoporous ZSM-5 shows much higher catalytic activities for bulky substrate conversion than conventional ZSM-5 and ZSM-5 with randomly oriented mesopores. The combination of good hydrothermal stability with high activities is important for design of novel zeolite catalysts. The <i>b</i>-axis-aligned mesoporous ZSM-5 reported here shows great potential for industrial applications

Homo- and Heterovalent Substitutions in the New Clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i>: Synthesis, Crystal Structure, and Thermoelectric Properties

The new cationic clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> were synthesized by the standard ampule technique. The Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> (<i>x</i> = 0–2.3) clathrates crystallize in the cubic space group <i>Pm</i>3̅<i>n</i> with the unit cell parameter <i>a</i> ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si_30+xP_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> (<i>x</i> = 1–6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (<i>x</i> = 1–3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group <i>Pm</i>3̅<i>n</i> to <i>Pm</i>3̅. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 × 10^–6 to 1.0 × 10^–5 K^–1 in the temperature range of 298–1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of <i>p</i>-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (<i><i>ZT</i></i> = 0.1) was achieved for the Br-bearing clathrate Si_32.1(2)P_13.9(2)Te_6.6(2)Br_1.0(1) at 750 K

Homo- and Heterovalent Substitutions in the New Clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i>: Synthesis, Crystal Structure, and Thermoelectric Properties

The new cationic clathrates I Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> and Si_30+<i>x</i>P_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> were synthesized by the standard ampule technique. The Si₃₀P₁₆Te_8–<i>x</i>Se_<i>x</i> (<i>x</i> = 0–2.3) clathrates crystallize in the cubic space group <i>Pm</i>3̅<i>n</i> with the unit cell parameter <i>a</i> ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si_30+xP_{16–<i>x</i>}Te_8–<i>x</i>Br_<i>x</i> (<i>x</i> = 1–6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (<i>x</i> = 1–3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group <i>Pm</i>3̅<i>n</i> to <i>Pm</i>3̅. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 × 10^–6 to 1.0 × 10^–5 K^–1 in the temperature range of 298–1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of <i>p</i>-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (<i><i>ZT</i></i> = 0.1) was achieved for the Br-bearing clathrate Si_32.1(2)P_13.9(2)Te_6.6(2)Br_1.0(1) at 750 K