A guest-free germanium clathrate

Guloy, Arnold M.; Ramlau, R.; Tang, Zhongjia; Schnelle, Walter; Baitinger, Michael; Grin, Yuri

doi:10.1038/nature05145

Cited by 400 publications

(421 citation statements)

References 29 publications

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“…q increases monotonically with temperature, and remains less than 1 mOhm cm in the entire temperature range. The value of 0.7 mOhm cm at 300 K is very close to that reported 17 for the stoichiometric clathrate Cs 8 Na 16 Si 136 . The values for S remain relatively low, and the magnitude also increases monotonically with temperature.…”

Section: Resultssupporting

confidence: 69%

“…6 The type II clathrate (space group Fd " 3m) crystal structure is comprised of a covalently bonded framework (E 136 in the conventional cell, E = Si, Ge, or Sn) formed by the face sharing of 20-membered dodecahedra (E 20 ) and 28-membered hexakaidecahedra (E 28 ) in an E 28 :E 20 ratio of 1:2. The framework may be empty, as in the case of E = Si 14,15 or Ge, 16 or its polyhedral cages can incorporate guest atomic species in their interiors. Large-amplitude, localized vibrations of these guests in the relatively oversized framework cages can potentially result in strong scattering of the heat-carrying phonons.…”

Section: Introductionmentioning

confidence: 99%

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Synthesis, Crystal Structure, and Transport Properties of Na22Si136

Beekman

Sebastian

Grin

et al. 2009

Journal of Elec Materi

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The type II clathrate Na 22 Si 136 is prepared by the thermal decomposition of NaSi. Thermal analysis indicates this phase is metastable yet has a relatively high decomposition temperature. Rietveld analysis indicates that Na in the larger Si 28 cage is shifted off-center, analogous to observations in some type I clathrates. Temperature-dependent electrical and thermal transport properties are reported for Na 22 Si 136 , for which the spark plasma sintering technique was found to be effective in achieving intergrain sintering in the consolidated specimen. The potential that type II clathrate materials possess for thermoelectric applications is discussed.

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Section: Resultssupporting

confidence: 69%

Section: Introductionmentioning

confidence: 99%

Synthesis, Crystal Structure, and Transport Properties of Na22Si136

Beekman

Sebastian

Grin

et al. 2009

Journal of Elec Materi

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“…While guest molecules are necessary for stabilization of the open lattice structure of 3D gas clathrates of water at ambient pressure, it is known that a guest-free clathrate of germanium can be synthesized (16), and an almost empty clathrate of Si has been described (17,18). The latter can definitely be formed in simulations with the Stillinger-Weber potential (19).…”

Section: Resultsmentioning

confidence: 89%

Guest-free monolayer clathrate and its coexistence with two-dimensional high-density ice

Bai

Angell

Zeng

2010

Proc. Natl. Acad. Sci. U.S.A.

101

127

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Three-dimensional (3D) gas clathrates are ice-like but distinguished from bulk ices by containing polyhedral nano-cages to accommodate small gas molecules. Without space filling by gas molecules, standalone 3D clathrates have not been observed to form in the laboratory, and they appear to be unstable except at negative pressure. Thus far, experimental evidence for guest-free clathrates has only been found in germanium and silicon, although guest-free hydrate clathrates have been found, in recent simulations, able to grow from cold stretched water, if first nucleated. Herein, we report simulation evidence of spontaneous formation of monolayer clathrate ice, with or without gas molecules, within hydrophobic nano-slit at low temperatures. The guest-free monolayer clathrate ice is a low-density ice (LDI) whose geometric pattern is identical to Archimedean 4 · 8 2 -truncated square tiling, i.e. a mosaic of tetragons and octagons. At large positive pressure, a second phase of 2D monolayer ice, i.e. the puckered square high-density ice (HDI) can form. The triple point of the LDI/liquid/HDI three-phase coexistence resembles that of the ice-I h ∕water∕ice-III three-phase coexistence. More interestingly, when the LDI is under a strong compression at 200 K, it transforms into the HDI via a liquid intermediate state, the first direct evidence of Ostwald's rule of stages at 2D. The tensile limit of the 2D LDI and water are close to that of bulk ice-I h and laboratory water.2D high-density ice | 2D low-density ice | 2D monolayer ice clathrate | Ostwald rule of stages | tensile limit of 2D liquid N atural gas is largely reserved in nature in the form of gas clathrates on the world's ocean floors (1-7). In the laboratory, gas clathrates can be produced by bringing a nonpolar gas in direct contact with liquid water under moderate pressure and at low temperature (1,(8)(9)(10)(11)). Since gas clathrates possess an open-cage structure to accommodate small gas molecules, gas clathrates can be characterized by two parameters: (1) the minimum gas pressure needed to stabilize the clathrates, and (2) the degree of occupancy or the fraction of cages occupied by the gas molecules. Although structural and thermodynamic properties of many gas clathrates are well characterized, the kinetic process of 3D clathrate formation is still less understood (1, 12). In an attempt to gain molecular insight into kinetics of clathrate formation, we carried out molecular dynamics (MD) simulations of clathrate formation within a slit nanopore whose width D is on the scale of 0.6 nm (see Materials and Methods). Because the slit nanopore can only accommodate one molecular layer of water, the timescale required for the formation of the quasi-2D gas clathrate is within the reach of MD simulation (typically 10-10 2 ns). Results and DiscussionsWe first considered a binary fluid mixture of water and argon (with 11.1% mole fraction of Ar) confined to the slit nanopore (D ¼ 0.62 nm) whose two opposing walls are smooth and hydrophobic (13-15). The fluid mixture was ...

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“…For boron nitride (BN), for example, the soft hexagonal layered polymorph (h-BN) is used as a lubricant, while the cubic polymorph (wurtzite structure) is extremely hard and is employed as an industrial abrasive. Germanium also exhibits more than one polymorph (or more strictly ''allotrope'' for elemental systems) having very different optical properties; with the diamond structured -germanium having an indirect band gap, and the recently synthesized lowdensity clathrate-II structure [1] a direct gap. Evidently, having access to more than one polymorph can significantly extend the range of properties and applications of a particular compound.…”

mentioning

confidence: 99%

Apparent Scarcity of Low-Density Polymorphs of Inorganic Solids

2010

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For most inorganic solids, very few dense polymorphs and no low-density polymorphs are observed. Taking a wide range of tetrahedrally-coordinated binary solids (e.g., ZnO, GaN) as a prototypical system, we show that the apparent scarcity of low-density polymorphs is not due to significant structural or energetic limitations. Using databases of periodic networks as sources of novel crystal structures, followed by ab initio energy minimization, we predict a dense spectrum of low-density low-energy polymorphs. The diverse range of materials considered indicates that this is likely to be a general phenomenon. DOI: 10.1103/PhysRevLett.104.175503 PACS numbers: 61.50.Ah, 61.66.Fn, 71.15.Nc, 82.75.Fq The structure and properties of inorganic solids are intimately linked to such an extent that even different structures of the same composition (so-called polymorphs) often have widely differing properties and applications. For boron nitride (BN), for example, the soft hexagonal layered polymorph (h-BN) is used as a lubricant, while the cubic polymorph (wurtzite structure) is extremely hard and is employed as an industrial abrasive. Germanium also exhibits more than one polymorph (or more strictly ''allotrope'' for elemental systems) having very different optical properties; with the diamond structured -germanium having an indirect band gap, and the recently synthesized lowdensity clathrate-II structure [1] a direct gap. Evidently, having access to more than one polymorph can significantly extend the range of properties and applications of a particular compound. For the vast majority of inorganic solids, however, typically only three or fewer distinct polymorphs have been prepared experimentally and a similarly small number of hypothetical polymorphs have been proposed. Moreover, where more than one polymorph is known, these extra phases very rarely have a significantly lower density than the most stable polymorph.In stark contrast to this situation, silica (SiO 2 ) has been prepared as more than 40 polymorphs [2], the majority of which are considerably less dense than -quartz; the most stable polymorph under ambient conditions. The rich polymorphism of silica, especially in its low-density forms, allows for fine-tuning of applications (e.g., gas separation membranes) by choosing the best suited polymorph. In addition, 100 000þ hypothetical silica polymorphs have been predicted theoretically as fourfold-connected networks (4CNs) [3][4][5][6][7], a large fraction of which, after accurate theoretical evaluation as silica materials, were found to have comparable energetics to experimentally prepared polymorphs [8,9]. Similarly rich polymorphism has only been further observed for a select group of other inorganic solids, for example, aluminophosphates (AlPO 4 ). A tantalizing hint that there may as yet exist a greater pool of experimentally accessible polymorphs for many other inorganic solids is given by the recent low temperature deposition synthesis of the tetrahedral wurtzite polymorph of LiBr [10] (after previous theor...

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A guest-free germanium clathrate

Cited by 400 publications

References 29 publications

Synthesis, Crystal Structure, and Transport Properties of Na22Si136

Synthesis, Crystal Structure, and Transport Properties of Na22Si136

Guest-free monolayer clathrate and its coexistence with two-dimensional high-density ice

Apparent Scarcity of Low-Density Polymorphs of Inorganic Solids

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