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

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

Dumbbells of Five-Connected Ge Atoms and Superconductivity in CaGe₃

CaGe₃ has been synthesized at high-pressure, high-temperature conditions. The atomic pattern comprises intricate germanium layers of condensed moleculelike dimers. Below <i>T</i>_c = 6.8 K, type II superconductivity with moderately strong electron–phonon coupling is observed

Ternary Metastable Nitrides ε‑Fe₂<i>TM</i>N (<i>TM</i> = Co, Ni): High-Pressure, High-Temperature Synthesis, Crystal Structure, Thermal Stability, and Magnetic Properties

High-pressure, high-temperature synthesis gives access to ternary metastable nitrides ε-Fe₂<i>TM</i>N (<i>TM</i> = Co, Ni) as bulk materials for the first time. Both ε-Fe₂CoN and ε-Fe₂NiN crystallize isostructural to ε-Fe₃N as evidenced by X-ray powder diffraction data. The lattice parameters of the new compounds are slightly smaller than those of ε-Fe₃N owing to the reduced atomic radii of the metal atoms. Energy-dispersive X-ray spectroscopy of metallographic samples show homogeneous metal ratios corresponding to compositions Fe_1.99(6)Co_1.01(6)N and Fe_1.97(2)Ni_1.03(2)N. Extended X-ray absorption fine spectra indicate that cobalt and nickel occupy iron positions. Thermal analysis measurements reveal decomposition of both ternary nitrides above 920 K. ε-Fe₂CoN disintegrates into N₂ and iron–cobalt alloy, while ε-Fe₂NiN decays into N₂, iron–nickel alloy as well as α-Fe. The replacement of iron by cobalt or nickel essentially lowers the saturation magnetization from roughly 6.0 μ_B/f.u. for ε-Fe₃N to nearly 4.3 μ_B/f.u. for ε-Fe₂CoN and 3.1 μ_B/f.u. for ε-Fe₂NiN. In parallel, the Curie temperature decreases from 575(3) K for ε-Fe₃N to 488(5) K for ε-Fe₂CoN and 234(3) K for ε-Fe₂NiN. Calculations of the formation enthalpies illustrate that the phases ε-Fe₂<i>TM</i>N (<i>TM</i> = Co, Ni) are thermodynamically unfavorable at ambient conditions which is consistent with our experimental observations. The substitution of one Fe by Co (Ni) yields one (two) more electrons per formula unit which reduces the magnetic interactions. First-principles analysis indicate that the replacement has a negligible influence on the electron occupation numbers and spin moments of the N and unsubstituted Fe sites, but decreases the local magnetic moments on the substituted Fe positions because the extra electrons occupy the minority-spin channel formed by states of the <i>TM</i> atoms

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

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

Dumbbells of Five-Connected Silicon Atoms and Superconductivity in the Binary Silicides MSi₃ (M = Ca, Y, Lu)

The new metastable binary silicides MSi₃ (M = Ca, Y, Lu) have been synthesized by high-pressure, high-temperature reactions at pressures between 12(2) and 15(2) GPa and temperatures from 900(100) to 1400(150) K. The atomic patterns comprise intricate silicon layers of condensed molecule-like Si₂ dimers. The alkaline-earth element adopts the oxidation state +2, while the rare-earth and transition metals realize +3. All of the compounds exhibit BCS-type superconductivity with weak electron–phonon coupling below critical temperatures of up to 7 K

Two New Arsenides, Eu₇Cu₄₄As₂₃ and Sr₇Cu₄₄As₂₃, With a New Filled Variety of the BaHg₁₁ Structure

Two new ternary arsenides, namely, Eu₇Cu₄₄As₂₃ and Sr₇Cu₄₄As₂₃, were synthesized from elements at 800 °C. Their crystal structure represents a new filled version of the BaHg₁₁ motif with cubic voids alternately occupied by Eu­(Sr) and As atoms, resulting in a 2 × 2 × 2 superstructure of the aristotype: space group <i>Fm</i>3̅<i>m</i>, <i>a</i> = 16.6707(2) Å and 16.7467(2) Å, respectively. The Eu derivative exhibits ferromagnetic ordering below 17.5 K. In agreement with band structure calculations both compounds are metals, exhibiting relatively low thermopower, but high electrical and low thermal conductivity

Two New Arsenides, Eu₇Cu₄₄As₂₃ and Sr₇Cu₄₄As₂₃, With a New Filled Variety of the BaHg₁₁ Structure

Two new ternary arsenides, namely, Eu₇Cu₄₄As₂₃ and Sr₇Cu₄₄As₂₃, were synthesized from elements at 800 °C. Their crystal structure represents a new filled version of the BaHg₁₁ motif with cubic voids alternately occupied by Eu­(Sr) and As atoms, resulting in a 2 × 2 × 2 superstructure of the aristotype: space group <i>Fm</i>3̅<i>m</i>, <i>a</i> = 16.6707(2) Å and 16.7467(2) Å, respectively. The Eu derivative exhibits ferromagnetic ordering below 17.5 K. In agreement with band structure calculations both compounds are metals, exhibiting relatively low thermopower, but high electrical and low thermal conductivity