Unconventional Metal-Framework Interaction in MgSi5

The silicon‐rich cage compound MgSi₅ was obtained by high‐pressure high‐temperature synthesis. Initial crystal structure determination by electron diffraction tomography provided the basis for phase analyses in the process of synthesis optimization, finally facilitating the growth of single crystals suitable for X‐ray diffraction experiments. The crystal structure of MgSi₅ (space group Cmme, Pearson notation oS24, a=4.4868(2) Å, b=10.1066(5) Å, and c=9.0753(4) Å) constitutes a new type of framework of four‐bonded silicon atoms forming Si₁₅ cages enclosing the Mg atoms. Two types of smaller Si₈ cages remain empty. The atomic interactions are characterized by two‐center two‐electron bonds within the silicon framework. In addition, there is evidence for multi‐center Mg−Si bonding in the large cavities of the framework and for lone‐pair‐like interactions in the smaller empty voids.

Indium thiospinel In1-x□xIn2S4- structural characterization and thermoelectric properties

A detailed study of polycrystalline indium-based In_1-x□_xIn₂S₄ (x = 0.16, 0.22, 0.28, and 0.33) thiospinel is presented (□- vacancy). Comprehensive investigation of synthesis conditions, phase composition and thermoelectric properties was performed by means of various diffraction, microscopic and spectroscopic methods. Single-phase α- and β-In_1-x□_xIn₂S₄ were found in samples with 0.16 ≤x≤ 0.22 and x = 0.33 (In₂S₃), respectively. In contrast, it is shown that In_0.72□_0.28In₂S₄ contains both α- and β-polymorphic modifications. Consequently, the thermoelectric characterization of well-defined α- and β-In_1-x□_xIn₂S₄ is conducted for the first time. α-In_1-x□_xIn₂S₄ (x = 0.16 and 0.22) revealed n-type semiconducting behavior, a large Seebeck coefficient (>|200|μV K^-1) and moderate charge carrier mobility on the level of ∼20 cm² V^-1 s^-1 at room temperature (RT). Decreases in charge carrier concentration (increase of electrical resistivity) and thermal conductivity (even below 0.6 W m^-1 K^-1 at 760 K) for larger In-content are observed. Although β-In_0.67□_0.33In₂S₄ (β-In₂S₃) is a distinct polymorphic modification, it followed the abovementioned trend in thermal conductivity and displayed significantly higher charge carrier mobility (∼10⁴ cm² V^-1 s^-1 at RT). These findings indicate that structural disorder in the α-modification affects both electronic and thermal properties in this thiospinel. The reduction of thermal conductivity counterbalances a lowered power factor and, thus, the thermoelectric figure of merit ZT_max = 0.2 at 760 K is nearly the same for both α- and β-In_1-x□_xIn₂S₄.

High-Pressure Synthesis and Chemical Bonding of Barium Trisilicide BaSi₃

BaSi₃ is obtained at pressures between 12(2) and 15(2) GPa and temperatures from 800(80) and 1050(105) K applied for one to five hours before quenching. The new trisilicide crystallizes in the space group I4¯2m (no. 121) and adopts a unique atomic arrangement which is a distorted variant of the CaGe₃ type. At ambient pressure and 570(5) K, the compound decomposes in an exothermal reaction into (hP3)BaSi₂ and two amorphous silicon-rich phases. Chemical bonding analysis reveals covalent bonding in the silicon partial structure and polar multicenter interactions between the silicon layers and the barium atoms. The temperature dependence of electrical resistivity and magnetic susceptibility measurements indicate metallic behavior.

Structural stability and thermoelectric performance of high quality synthetic and natural pyrites (FeS2)

Single crystalline pyrite of high quality reveals good thermal- and bad electrical conductivities resulting in poor thermoelectric performance.

Modulated vacancy ordering in SrGe6− x (x≈0.45)

The structural properties of modulated SrGe6− x (x≈0.45) were investigated by means of single-crystal and powder X-ray diffraction combined with quantum chemical calculations. The framework compound SrGe6− x adopts a defect variant of the EuGa2Ge4-type crystal structure. Samples of the binary compound with nominal compositions 0≤x≤0.5 were synthesized at pressures from 5 to 6 GPa and a temperature of typically 1400 K. The product reveals diffraction peaks of the EuGa2Ge4-type subcell plus additional reflections indicating an ordered superstructure. Detailed crystal structure analysis evidences the incommensurate nature of the superstructure and a modulation of the vacancy ordering in the germanium network. The computations have shown that the non-stoichiometric composition of the framework with its local defect organization affects the calculated charge of the strontium anions. Although the chemical composition is close to a charge-balanced situation, temperature-dependent resistivity measurements showed metal-type conductivity. At ambient pressure SrGe6− x decomposes exothermally and irreversibly at T=680(10) K into SrGe2 and germanium, indicating its metastable nature at ambient pressure.

Ca12 [Mn19 N23 ] and Ca133 [Mn216 N260 ]: Structural Complexity by 2D Intergrowth

Two new calcium nitridomanganates, Ca₁₂ [Mn₁₉ N₂₃ ] (P3, a=11.81341(3) Å, c=5.58975(2) Å, Z=1) and Ca₁₃₃ [Mn₂₁₆ N₂₆₀ ] (P3‾ , a=39.477(1) Å, c=5.5974(2) Å, Z=1), were obtained by a gas-solid reaction of Ca₃ N₂ and Mn with N₂ at 1273 K and 1223 K, respectively. The crystal structure of Ca₁₂ [Mn₁₉ N₂₃ ] was determined from high-resolution X-ray synchrotron powder diffraction data, whereas single-crystal X-ray diffraction was employed to establish the crystal structure of the Ca₁₃₃ [Mn₂₁₆ N₂₆₀ ] phase, which classifies as a complex metallic alloy (CMA). Both crystal structures have 2D nitridomanganate layers containing similar building blocks but of different levels of structural complexity. Bonding analysis as well as magnetic susceptibility and electron spin resonance measurements revealed that only a fraction of the Mn atoms in both structures carries a localized magnetic moment, while for most Mn species the magnetism is quenched as a result of metal-metal bond formation.

Lutetium Trigermanide LuGe3: High-Pressure Synthesis, Superconductivity, and Chemical Bonding

LuGe₃ was obtained under high-pressure and high-temperature conditions at pressures between 8(1) and 14(2) GPa and at temperatures in the range from 1100(150) to 1500(150) K. The high-pressure phase is isotypic to DyGe₃ and decomposes at ambient pressure and T = 690 K mainly into ( cF8)Ge and LuGe_{2- x}. Chemical bonding analysis of LuGe₃ reveals two-center electron-deficient Ge-Ge bonds, multicenter polar Lu-Ge interactions, and lone pairs on germanium. Magnetic susceptibility, specific heat, and electrical conductivity measurements indicate transition into a superconducting state below T_c = 3.3(3) K.

Cluster Formation in the Superconducting Complex Intermetallic Compound Be21Pt5

Materials 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 F4̅3m with lattice parameter a = 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 (T_c = 2.06 K).

Two-gap superconductivity in Ag1-x Mo6S8 Chevrel phase

The superconducting properties of [Formula: see text]Mo₆S₈ [[Formula: see text]] Chevrel phase [[Formula: see text] K] are studied on a sample compacted by spark plasma sintering. Both lower ([Formula: see text] mT) and the upper [[Formula: see text] T] critical magnetic fields are obtained from magnetization and electrical resistivity measurements for the first time. The analysis of the low-temperature electronic specific heat indicates [Formula: see text]Mo₆S₈ to be a two band superconductor with the energy gaps [Formula: see text] meV (95%) and [Formula: see text] meV (5%). Theoretical DFT calculations reveal a much stronger electron-phonon coupling in the studied Chevrel phase compared to earlier reports. Similar to MgB₂, the Fermi surface of studied Chevrel phase is formed by two hole-like and one electron-like bands.

Lithium alkaline earth tetrelides of the type Li2 AeTt (Ae=Ca, Ba, Tt=Si, Ge, Sn, Pb): synthesis, crystal structures and physical properties

Single crystals of the compounds Li2 AeTt (Ae=Ca, Ba, Tt=Si, Ge, Sn, Pb) were grown in reactive lithium melts in sealed tantalum ampoules from an equimolar ratio of the alkaline earth metal and the respective group 4 element. All compounds, with the exception of Li2CaSn and Li2CaPb, are isotypic and crystallize in an orthorhombic unit cell (space group Pmmn, no. 59). The crystal structure can be characterized as superimposed corrugated networks of Li2 Tt connected by calcium or barium atoms within the third dimension. Li2CaSn and Li2CaPb crystallize in the cubic space group Fm3̅m (no. 225) in a Heusler-type (MnCu2Al) structure. According to magnetic susceptibility and electric resistivity measurements, the compounds Li2BaGe, Li2BaSn, and Li2BaPb represent diamagnetic activated semiconductors.

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 C2/m, Pearson symbol mS74, a = 7.4809(1) Å, b = 12.9546(2) Å, c = 13.2479(2) Å, β = 100.879(1)°, V = 1260.82(6) ų, R_F = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an ABABB stacking of two slabs with trigonal symmetry and compositions Yb₄Ga₆ (A) and Ga₁₂Pt₆ (B). 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.

The Triply Deprotonated Acetonitrile Anion CCN3- Stabilized in a Solid

The unprecedented, fully deprotonated form of acetonitrile, the acetonitriletriide anion CCN^3- , is experimentally realized for the first time in the stabilizing bulk host framework of the Ba₅ [TaN₄ ][C₂ N] nitridometalate via a one-pot synthesis from the elements under moderate conditions (920 K). The molecular structure of this long-sought acetonitrile derivative is confirmed by X-ray diffraction, as well as NMR, IR, and Raman spectroscopy. The anion is isoelectronic to the CO₂ molecule, and, in contrast to acetonitrile (H₃ C-C≡N), the electron pairs are shifted towards two double bonds, that is, [C=C=N]^3- .

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.

A type-II clathrate with a Li-Ge framework

Na

Hierarchical and chemical space partitioning in new intermetallic borides MNi21B20 (M = In, Sn)

MNi₂₁B₂₀ (M = ln, Sn): [Ni₆@B₂₀@Ni₂₄] triple shell clusters with M atoms centering the cuboctahedra [M@Ni₁₂].

Type-I silicon clathrates containing lithium

The intermetallic phase [Li