Novel metastable metallic and semiconducting germaniums

By means of ab initio metadynamics runs we explored the lower-pressure region of the phase diagram of germanium. A monoclinic germanium phase with four-membered rings, less dense than diamond and compressible into \beta-tin phase (tI4) was found. A metallic bct-5 phase, mechanically stable down to room conditions appeared between diamond and tI4. mC16 is a narrow-gap semiconductor, while bct-5 is metallic and potentially still superconducting in the very low pressure range. This finding may help resolving outstanding experimental issues.Comment: 6 figure

Isotopy classes for 3-periodic net embeddings

Entangled embedded periodic nets and crystal frameworks are defined, along with their {dimension type}, {homogeneity type}, {adjacency depth} and {periodic isotopy type}. We obtain periodic isotopy classifications for various families of embedded nets with small quotient graphs. We enumerate the 25 periodic isotopy classes of depth 1 embedded nets with a single vertex quotient graph. Additionally, we classify embeddings of n-fold copies of {pcu} with all connected components in a parallel orientation and n vertices in a repeat unit, and determine their maximal symmetry periodic isotopes. We also introduce the methodology of linear graph knots on the flat 3-torus [0, 1)^3. These graph knots, with linear edges, are spatial embeddings of the labelled quotient graphs of an embedded net which are associated with its periodicity bases

HPLC-based activity profiling for GABAA receptor modulators from the traditional Chinese herbal drug Kushen ( Sophora flavescens root)

An EtOAc extract from the roots of Sophora flavescens (Kushen) potentiated γ-aminobutyric acid (GABA)-induced chloride influx in Xenopus oocytes transiently expressing GABAA receptors with subunit composition, α 1 β 2 γ 2S. HPLC-based activity profiling of the extract led to the identification of 8-lavandulyl flavonoids, kushenol I, sophoraflavanone G, (−)-kurarinone, and kuraridine as GABAA receptor modulators. In addition, a series of inactive structurally related flavonoids were characterized. Among these, kushenol Y (4) was identified as a new natural product. The 8-lavandulyl flavonoids are first representatives of a novel scaffold for the targe

Hydrogen storage in high surface area graphene scaffolds

Using an optimized KOH activation procedure we prepared highly porous graphene scaffoldmaterials with SSA values up to 3400m² g⁻¹ and a pore volume up to 2.2 cm³ gˉ¹, which are among the highest for carbon materials. Hydrogen uptake of activated graphene samples was evaluated in a broad temperature interval (77–296 K). After additional activation by hydrogen annealing the maximal excess H2 uptake of 7.5 wt% was obtained at 77 K. A hydrogen storage value as high as 4 wt% was observed already at 193 K (120 bar H₂), a temperature of solid CO₂, which can be easily maintained using common industrial refrigeration methods

Covalent Organic Framework (COF-1) under High Pressure

COF-1 has a structure with rigid 2D layers composed of benzene and B3O3 rings and weak van der Waals bonding between the layers. The as-synthesized COF-1 structure contains pores occupied by solvent molecules. A high surface area empty-pore structure is obtained after vacuum annealing. High-pressure XRD and Raman experiments with mesitylene-filled (COF-1-M) and empty-pore COF-1 demonstrate partial amorphization and collapse of the framework structure above 12–15 GPa. The ambient pressure structure of COF-1-M can be reversibly recovered after compression up to 10–15 GPa. Remarkable stability of highly porous COF-1 structure at pressures at least up to 10 GPa is found even for the empty-pore structure. The bulk modulus of the COF-1 structure (11.2(5) GPa) and linear incompressibilities (k[100]=111(5) GPa, k[001]=15.0(5) GPa) were evaluated from the analysis of XRD data and cross-checked against first-principles calculations.\ua0\ua9 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Ge136 type-II clathrate as precursor for the synthesis of metastable germanium polymorphs: a computational study

The response to compression of the clathrate type-II structure Ge(cF136) is investigated by means of ab initio small-cell metadynamics at different temperatures and pressures. Amorphous intermediates obtained from the collapse of Ge 136 rapidly crystallize into different end products, depending on the choice of thermodynamic parameters. At lower pressure p = 2.5 GPa the metastable metallic bct-5 phase competes against β -Sn [Ge(tI4)], which forms at higher pressures. Upon lowering the temperature, amorphous intermediates are instrumental to the formation of denser structural motifs, from which metallic bct-5 can form. Therein, anisotropic box fluctuations promote phase formation. The metadynamics runs are analyzed in depth using a set of topological descriptors, including coordination sequence and ring statistics. Differences in the structural landscape history and amorphous intermediates are critical for the selective formation of particular metastable polymorphs. Their understanding opens possibilities towards turning crystal structure predictions into actual materials, via identifying viable kinetic routes

Polymorphs of lithium-boron imidazolates: energy landscape and hydrogen storage properties

The topological diversity of lithium-boron imidazolates LiB(imid)4 was studied by combining topological enumeration and ab initio DFT calculations. The structures based on zeolitic rho, gme and fau nets are shown to be stable and have high total hydrogen uptake (6.9–7.8 wt.%) comparable with that of MOF-177.Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich