53 research outputs found
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
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
A zeolitic imidazolate framework with conformational variety
We show via structural considerations and DFT calculations that for a zeolitic imidazolate framework (ZIF) with sodalite (SOD) topology, [Zn(dcim)2]-SOD (dcim = 4,5-dichloroimidazolate), structural models of an infinite number of hypothetical conformational polymorphs with distinct linker orientations can be generated, which can be interconverted most likely only via reconstructive structural transitions. The relative total energies suggest that some of those polymorphs might be synthetically accessible. Efforts in that direction led to the synthesis of new trigonal 1 and previously known cubic 2 with improved crystallinity. According to structural analyses based on powder X-ray diffraction (PXRD) methods supported by NMR spectroscopy, 1 is the most stable of the theoretically predicted SOD-type framework conformers (isostructural to ZIF-7), whereas 2, at variance with a recent proposal, is a SOD-type material with a high degree of orientational disorder of the dcim linker units. The statistics of the linker orientations in 2 is close to that in 1, indicating that the disorder in 2 is not random. Rather, crystals of 2 are likely twins consisting of nanoscopic domains of trigonal 1 that are deformed to a cubic metric, with linker disorder located in the domain interfaces. As structural differences appear to be more related to characteristics of real as opposed to ideal crystal structures, we propose to not consider 1 and 2 as true conformational polymorphs. Systematic investigations of solvent mixtures led to the discovery of intermediate materials of 1 and 2. The PXRD patterns and SEM images indicate that they belong to a complete series of structural intermediates. Differences in the Ar adsorption/desorption behaviours reveal that 1, in contrast to 2, is a flexible ZIF framework.DFG/Priority Program/141
A family of 2D and 3D coordination polymers involving a trigonal tritopic linker
Five new coordination polymers, namely, [Zn2(H2O)2(BBC)](NO3)(DEF)6 (DUT-40), [Zn3(H2O)3(BBC)2] (DUT-41), [(C2H5)2NH2][Zn2(BBC)(TDC)](DEF)6(H2O)7 (DUT-42), [Zn10(BBC)5(BPDC)2(H2O)10](NO3)(DEF)28(H2O)8 (DUT-43), and [Co2(BBC)(NO3)(DEF)2(H2O)](DEF)6(H2O) (DUT-44), where BBC – 4,4′,4′′-(benzene-1,3,5-triyl-tris(benzene-4,1-diyl))tribenzoate, TDC – 2,5-thiophenedicarboxylate, BPDC – 4,4′-biphenyldicarboxylate, DEF – N,N-diethylformamide, were obtained under solvothermal conditions and structurally characterized. It has been shown that compounds DUT-40, DUT-41 and DUT-44 exhibit 2D layered structures with large hexagonal channels. Utilization of additional angular dicarboxylic TDC linker led to the formation of the DUT-42 compound with the structure consisting of three interpenetrated 3D networks. Using the linear co-linker dicarboxylic BPDC, DUT-43 was obtained which forms a complicated 3D architecture arising from the polycatenation of triple-layered 2D building units and 2D single layer units. The pore accessibility of the synthesized compounds in the liquid phase was proved by the adsorption of dye molecules.Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich
On the Origin of Crystallinity: a Lower Bound for the Regularity Radius of Delone Sets
The local theory of regular or multi-regular systems aims at finding
sufficient local conditions for a Delone set to be a regular or
multi-regular system. One of the main goals is to estimate the regularity
radius for Delone sets in terms of the radius of the
largest "empty ball" for .
The present paper establishes the lower bound for all
, which is linear in . The best previously known lower bound had been
for . The proof of the new lower bound is
accomplished through explicit constructions of Delone sets with mutually
equivalent -clusters, which are not regular systems
Taxonomical Study and Diversity of Rotifers in Chikkadevarayana Canal of Cauvery River, Karnataka, India
The study was undertaken to assess the diversity and taxonomical observations of rotifers in Chikkadevaraya canal (CDC). The investigation revealed that 27 species belonging to 15 genera and 9 families coming under 3 orders and 2 classes, were observed, photographed and also identified up to species level and are reported for the first time in this region. Among 22 genera of rotifers, the maximum of 11 species belonged to family Brachionidae followed by 5 species from family Lecanidae, 2 species of Euchlanidae, Collurellidae, Notommatidae and Philodinidae respectively. One species each from family Trichocercidae, Testudinellidae and Filinidae. The genus Brachionus, Keratella and Euchlanis was found in all the five sampling points. Euchlanis dialata was abundant while Macrotrachela quadricornifera was found to be least. Species abundance was higher at Gandehosahalli followed by Darsaguppe, Kannalu, Edmuri and lowest in Pandavapura sampling stations. Shannon�s diversity was also found to be highest at Gandehosahalli (2.63) while low in Pandavapura (1.78). The samples of water were used to conduct physico-chemical parameters like temperature, electrical conductivity, pH, dissolved oxygen, total alkalinity, total hardness, chlorides, total solids, total dissolved solids, total suspended solids, sulphate, phosphate and nitrate etc. Statistical interpretations of data were presented based on correlation analysis. The population of rotifes were positively correlated with pH, total alkalinity, hardness and negativly correlated with total suspended solids and total solids of water sample
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