Cage-like Voids Research Articles

Hydrogen transfer reactions via organic radicals in gamma-irradiated chibaite.

Chibaite, a silica-framework structure with cage-like voids occupied by gaseous molecules, was found in marine sediments. Its formation age could be evaluated using electron spin resonance (ESR) if the radicals formed by natural radiation can be assumed to accumulate over time. To investigate whether hydrogen transfer reactions, where organic radicals withdraw hydrogen atoms from other molecules in adjacent cages, occur in chibaite and affect ESR dating, gamma-irradiated chibaite was measured by ESR. Methyl, ethyl, n-propyl, isopropyl, tert-butyl radicals and hydrogen atoms were created by gamma irradiation at 77K. The amount of tert-butyl radicals increased around 240K and the similar amount of the other organic radicals decreased simultaneously, implying that hydrogen transfer reactions occur between isobutane and the organic radicals in chibaite around 240K and therefore would have no influence on ESR dating because the reactions are completed at the environmental temperature.

Bosoite, a new silica clathrate mineral from Chiba Prefecture, Japan

AbstractBosoite (IMA2014-023) is a new silica clathrate mineral containing hydrocarbon molecules in its crystal structure. Bosoite can be considered structurally as a silica analogue of the structure-H gas hydrate, where guest molecules are trapped in cage-like voids constructed of the host framework. The mineral occurs in the Miocene tuffaceous sedimentary rocks at Arakawa, Minami-boso City, Chiba Prefecture, Japan. Bosoite is hexagonal, and it crystallises as an epitaxial intergrowth on chibaite crystals, with the {0001} of bosoite parallel to octahedral {111} form of chibaite. Crystals are colourless and transparent with vitreous lustre. The calculated density is 2.04 g/cm3. The empirical formula (based on 2 O apfu and guest molecules assumed as CH4) is Na0.01(Si0.98Al0.02)Σ1.00O2⋅0.50CH4; the end-member formula is SiO2⋅nCxH2x+2. Bosoite has the space groupP6/mmm, with the unit-cell parametersa= 13.9020(3) Å,c= 11.2802(2) Å,V= 1887.99(6) Å3andZ= 34. The crystal structure of bosoite was refined by single-crystal X-ray diffraction and converged toR1= 4.26% for the average model andR1= 2.96% for the model where all oxygen sites are split.

Size effect on the electrochemical reaction path and performance of nano size phosphorus rich skutterudite nickle phosphide

Due to cage-like voids in the lattice, skutterudite NiP3 is regarded as a promising negative electrode material with high specific capacity. The fabrication of phosphide based on ceramic routes always leads to multiphase formation and serious agglomeration of particles. The resultant cycle performance is thus to be improved. In this paper, single phase skutterudite NiP3 particles (NNP) were fabricated by the reaction of NiS precursor with red phosphorus. As comparison, skutterudite NiP3 was also prepared by the direct reaction of Ni with red phosphorus (MNP). After 50 cycles, NNP retained 712 mAh g−1 at current density of 159.1 mAg−1, while MNP delivered a capacity less than that of commercial graphite within 16 cycles. According to CV analysis, the improvement in cycling performance of NNP was attributed to an insertion-dominated two-step lithiation process, while MNP undergone a straightforward conversion process. The formation of layer-structured intermediate LixNiP3 phase by an insertion reaction would compete with the direct generation of Li3P and Ni (0) composites by a typical conversion reaction. The improvement in the rating performance originates from the low-impedance of charge transfer and fast migration of Li+ in the electrode of NNP.

Control of thermal expansion in a low-density framework modification of silicon

The low-density clathrate-II modification of silicon, Si136, contains two distinct cage-like voids large enough to accommodate various types of guest atoms which influence both the host structure and its properties. Although the linear coefficient of thermal expansion of Si136 (293 K < T < 423 K) is only about 20% larger than that of the ground state α-Si (diamond structure), the coefficient of thermal expansion monotonically increases by more than 150% upon filling the framework cages with Na atoms in NaxSi136 (0 < x < 24), ranging from α = 2.6 × 10−6 K−1 (x = 0) to 6.8 × 10−6 K−1 (extrapolated to x = 24) by only varying the Na content, x. Taken together with the available heat capacity and bulk modulus data, the dramatic increase in thermal expansion can be attributed to an increase in the mode-averaged Grüneisen parameter by a factor of nearly 3 from x = 0 to x = 24. These results highlight a potential mechanism for tuning thermal expansion, whereby guest atoms are incorporated into the voids of rigid, covalently bonded inorganic frameworks to influence the lattice dynamics.

Enhanced thermoelectric properties of Ga and In Co-added CoSb3-based skutterudites with optimized chemical composition and microstructure

Skutterudite compounds such as Co antimonite (CoSb3) contain cage-like voids inside crystal structure, which can be completely or partially filled with various different atoms, including group 13 elements. The multiple filling approach is known as an effective way of reducing lattice thermal conductivity (κlat), which results in a high value of the thermoelectric dimensionless figure of merit (zT). In this work, enhanced zT was achieved for the Ga and In co-added CoSb3 samples with a preferable microstructure and the nominal composition (Ga0.8In0.2)xCo4Sb12 (x = 0.05−0.45). Although all added In atoms occupied exclusively the void sites, the Ga species filled both the void and Sb sites of CoSb3. Moreover, Ga atoms added in the quantities exceeding the solubility limit precipitated as GaSb nanoparticles. The sample with x = 0.45 was characterized by the largest filling factions of Ga and In as well as the unique microstructure, consisting of microscale grains of the skutterudite phase and corresponding amounts of the GaSb nanoparticles. The Ga and In co-added skutterudite samples with optimized chemical composition and microstructure maintained high carrier mobility and sufficiently low κlat values, resulting in zT > 1.1, one of the best values for the skutterudites filled with group 13 elements.

New silica clathrate minerals that are isostructural with natural gas hydrates

Silica clathrate compounds (clathrasils) and clathrate hydrates are structurally analogous because both materials have framework structures with cage-like voids occupied by guest species. The following three structural types of clathrate hydrates are recognized in nature: cubic structure I (sI); cubic structure II (sII); and hexagonal structure H (sH). In contrast, only one naturally occurring silica clathrate mineral, melanophlogite (sI-type framework), has been found to date. Here, we report the discovery of two new silica clathrate minerals that are isostructural with sII and sH hydrates and contain hydrocarbon gases. Geological and mineralogical observations show that these silica clathrate minerals are traces of low-temperature hydrothermal systems at convergent plate margins, which are the sources of thermogenic natural gas hydrates. Given the widespread occurrence of submarine hydrocarbon seeps, silica clathrate minerals are likely to be found in a wide range of marine sediments.

Formation of clathrasil layers by secondary growth of DOH-type nuclei for gas separation applications

Dodecasil 1H (D1H) possessing the zeolite framework type DOH is one of the all-silica zeolites and therefore offers high chemical, thermal and hydrothermal stability. D1H is a 6-ring clathrasil with small pore apertures of about 3Å and may be able to selectively transport hydrogen or helium while rejecting other gasses such as CO2. These properties make D1H an interesting microporous membrane material for precombustion applications. Moreover, it has been shown that D1H crystals can be prepared practically structure directing agent-free.Prior to the attempt to prepare complete layers of D1H crystals, the general crystallisation behaviour of D1H was investigated. To obtain optimal crystallisation conditions a large number of synthesis experiments were performed aiming, in particular, at the preparation of pure material consisting of small and very thin crystals with a reduced number of organic molecules occupying the cage-like voids of the structure. The syntheses were performed in the system aminoadamantane/silica/ethylenediamine or ammonia/water and by systematically varying synthesis parameters like temperature, gel composition, aging of the synthesis mixture, seeding and the type of agitation during the synthesis run. In some additional runs aminoadamantane was replaced by other structure directing agents.Moreover, the removal the organics filling the zeolite cages was studied by different approaches, including calcination at atmospheric pressure and by applying high pressure cycling and calcination of ball milled crystals. The obtained materials were then characterized by powder XRD, SEM, thermal analysis, chemical analysis and optical microscopy. Knowing favourable synthesis conditions, D1H layers have been prepared on a α-Al2O3 support by using secondary growth of small D1H nuclei.

Similarities between the clathrasils DOH, DDR, MEP and MTN

AbstractClathrasils are porous framework silicates with cage-like voids. Structural similarities between the clathrasilsDOH,MTN,MEP, DD3H andDDRhave been analyzed in the context of host/guest interaction and crystallization mechanism. A building unit consisting of 30 T atoms is used as common structural feature to give an alternative description of the well-known layers of [512]-cages. The [512]-layers, or Periodic Building Units (PerBU’s), are constructed from two 12-rings connected through a common 6-ring to form two half cages or a 12-ring double cup. These ‘half-cages’ are essentially the bottom of the large ‘inter-layer’ cages in the clathrasils. The rigorous derivation of all possible framework types and their cages based on these principles shows the richness of the concept and makes possible new strategies for the synthesis. The ‘half-cages’ are also possible candidates for precursors which might exist in solution in close contact with templating guest species initiating the nucleation of the crystals.

Single crystal structure analysis of nonasil(pyr), (C 4H 10N) 4[Si 84B 4O 176

Nonasil(pyr) is a member of the nonasil family (structure type code, NON). Nonasils are characterized by a unique silicate host framework capable of enclathrating various types of guest species. Nonasil(pyr) is orthorhombic (space group Cmca) with a 0 = 22.039(4)A˚, b 0 = 14.948(3), c 0 = 13.573(2)A˚and an idealized unit cell composition of (C 4H 10N) 4[Si 84B 4O 176]. The single crystal structure refinement using 1,697 nonequivalent reflexions led to a residual of R w = 0.071. Boron ions (B 3+) partially but randomly replace the silicon ions (Si 4+) in the framework. The negative charge introduced by this replacement is compensated by pyrrolidinium cations (C 4H 10N +). The framework consists of three different types of cage-like voids: the small [5 46 4]- and [4 15 8]-cages, which are empty, and the large [5 86 12]-cages (free volume ≈ 290A˚ 3), which contain the pyrrolidinium cations. The silicate framework of nonasil(pyr) has several unusually short Si O distances and high Si O Si angles, which are interpreted as due to static or dynamic disorder. Fourier syntheses showed that the guest ions are positionally disordered since the symmetry of the guest ion (point group symmetry, 2 mm) is incompatible with the symmetry of the [5 86 12]-cage (point group symmetry, 2/m).

Decasils, a new order-disorder family of microporous silicas

Decasils are a new order-disorder family of porous structures. All ordered and disordered decasil structures are built from one-dimensional chain-like basic building units that can be connected in different ways. Three simplest ordered framework structures of this family are possible: decasil types A, B, and C. All decasil types possess cage-like voids that share 8MR pore openings to give a one-dimensional channel system. Using two different templates an ordered and a disordered form of the decasil family of structures have been synthesized. RUB-3 (decasil type A) was obtained with exo-2-aminobicyclo[2.2.1.]heptan as template and is ordered; RUB-4 was obtained with 3-azabicyclo[3.2.2]nonane as template and is disordered. The structure of RUB-3, which crystallizes in space group C 2/m ( a = 14.039(2)Å, b = 13.602(2) Å, c = 7.428(1) Å, and β = 102.22(3)°), has been solved by model building and simulation of X-ray powder pattern and has been confirmed in a Rietveld analysis.

The role of non-deformable units in pressure-induced reversible amorphization of clathrasils

PRESSURE-induced amorphization of solids has been much studied since it was first observed in 19841. It was found recently2,3 that some materials can be amorphized reversibly under pressure, reverting back to the original crystalline structure and orientation when the pressure is decreased. It has been suggested4 that the presence of non-deformable units is essential for this reversibility, these units acting as templates around which the original structure is reformed. Here we investigate this idea by comparing the effect of pressure on two clathrasils—silica solids with open, microporous structures—with and without guest molecules inside the pores. We have studied pressure-induced amorphization of dodecasil-3C, which has cage-like voids, and dodecasil-3R, which has two-dimensional channels. For both materials, our experiments and simulations show that amorphization is fully or partly reversible only when guest molecules are present, suggesting that these do indeed act as rigid 'organizing centres' for the reversible transformation.

Structural links between zeolite-type and clathrate hydrate-type materials: Redetermination of the crystal structure of [N(CH 3) 4] 8[Si 8O 20]·65H 2O by single-crystal X-ray diffraction and variable-temperature MAS NMR spectroscopy

The tetramethylammonium silicate hydrate [NMe 4] 8[Si 8O 20]·65H 2O (1 hereafter) crystallizes triclinically with space group P 1 , Z = 1 and the cell constants a = 15.644(3), b = 15.756(4), c = 15.967(5) Å, α = 119.31(2), β = 108.83(2), γ = 91.99(2)° (T = 215 K). It is shown that the polyhydrate is a host—guest compound with a mixed three-dimensional host structure composed of oligomeric [Si 8O 20] 8− anions and H 2O molecules linked via hydrogen bonds OH⋯O (heteronetwork clathrate). The silicate anions possess a cube-shaped double four-ring structure. Four distinct large, irregular, cage-like voids house the guest species NMe 4 +. MAS NMR spectra measured at various temperatures between 170 and 290 K reveal that the 13C resonance of the NMe 4 + cation is shifted continuously to higher field with decreasing temperature and depends primarily on the chemical composition of the cage wall rather than on the cage size. At room temperature the 1H MAS NMR spectrum is well resolved, suggesting that both the H 2O molecules and NMe 4 + cations are highly mobile on the NMR time scale. The water dynamics slow down at temperatures below ca. 250 K which in turn affects the motions of the guest species. The polyhydrate melts incongruently at 345 K with the formation of a liquid phase and a crystalline heteronetwork clathrate, the host structure of which possesses the topology of the zeolite structure type AST.

Studies on clathrasils VIII. Nonasils-[4158], 88SiO2 � 8M8 � 8M9 � 4M20: Synthesis, thermal properties, and crystal structure

Nonasils-[4158], 88SiO2·8M8·8M9·4M20, have been synthesized with 2-methylpyrrolidine, hexamethyleneimine, 2-(aminomethyl)-tetrahydrofuran, 1,2-diaminocyclohexane, 2-methylpiperidine, 2-methylpiperazine, 1-aminobutane, 2-aminobutane, and 2-aminopentane as guest molecules, M20. The samples have been prepared from aqueous silicate solutions which were sealed in silica tubes and heated at about 200°C for several weeks. These clathrasils crystallize in space groupFmmm. For the nonasil with 2-aminopentane as the guest molecule and the unit cell dimensionsao=22.232(6) A,b0=15.058(4) A, andco=13.627(4) A, the structure has been refined using 550 non-equivalent single crystal reflexions to a reliability factorRw=0.125. The 3-dimensional 4-connected silica host framework has three types of cage-like voids, [5464], [4158], and [58612], the latter housing the structure-controlling guest molecules, M20. The non-spherical shape of the guest molecules is the most important factor for the formation of nonasils-[4158]. On heating nonasils-[4158] up to 950°C the organic guest species are driven out and the pure silica form of nonasil is obtained.

Käfigstrukturen in intermetallischen Verbindungen: zur Kenntnis von LaRuSn 3, CeRuSn 3, PrRuSn 3 UND NdRuSn 3

The new compounds LaRuSn 3, CeRuSn 3, PrRuSn 3 and NdRuSn 3 crystallize in the cubic system, space group Pm3 n (No. 223) with the lattice constants: LaRuSn 3, a = 977.3(5) pm, CeRuSn 3, a = 972.6(5) pm; PrRuSn 3, a = 970.8(5) pm; NdRuSn 3, a = 968.7(5) pm. In the structure the tin atoms form strings which are connected by ruthenium atoms to a framework with cage-like voids occupied by the rare earth atoms.

1-Aminoadamantane as guest molecule in dodecasil 1H: an X-ray crystallographic study

Dodecasil 1H, 34SiO2 · 3M12 · 2M12′ · 1M20 with M12, M12′=N2 and M20=1-aminoadamantane, is hexagonal witha=13.825(2) A,c=11.189(2) A and crystallizes in space groupP6/mmm. With 767 unique reflexions the structure has been refined to a weighted reliability factorR w =0.054. The three-dimensional four-connected host framework is built by corner sharing [SiO4] tetrahedra and shows three types of cagelike voids, [512] cages, [435663] cages and [51268] cages, the latter housing the 1-aminoadamantane guest molecule. Difference Fourier synthesis ρ(obs)-ρ(calc(Si,O)) delineate the guest molecules within the different types of cage. Residual electron densities reveal positional disorder of the 1-aminoadamantane guest molecule. Four crystallographically different preferred orientations for the 1-aminoadamantane guest molecule are found.

Carbons of high surface area. A study by adsorption and high resolution electron microscopy

Five carbons of high surface area, ~ 2000–3000 m 2g −1 are studied by adsorption of carbon dioxide at 195 and 273 K. Effective surface areas are calculated using Langmuir and Dubinin- Radushkevich equations. Structure in these carbons is assessed by phase contrast high resolution transmission electron microscopy. The lamellae or constituent layers of these carbons are resolved as fringe images. Careful examination of thin sections of these carbons shows significant differences in separation distances of lamellae which indicate differences in the size and shape of the supermicroporosity which exists as the space between the lamellae. These differences correlate closely with the effective surface areas. The supermicroporosity consists of cage-like voids 1–5 nm dia., the cages being separated by walls of 1–3 carbonaceous layers in thickness. The filling of such supermicroporosity by a mechanism of increasing adsorption potential or cooperative adsorption adequately accounts for high internal volumes of up to 1.7cm 3g −1 and of effective surface areas of about 3000 m 2g −1. The size and shape of supermicroporosity can be deduced from micrographs.