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The cages, dynamics, and structuring of incipient methane clathrate hydrates
Walsh, Matthew R.,Rainey, J. Daniel,Lafond, Patrick G.,Park, Da-Hye,Beckham, Gregg T.,Jones, Michael D.,Lee, Kun-Hong,Koh, Carolyn A.,Sloan, E. Dendy,Wu, David T.,Sum, Amadeu K. Royal Society of Chemistry 2011 Physical chemistry chemical physics Vol.13 No.44
<P>Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5<SUP>12</SUP> cage, the 5<SUP>12</SUP>6<SUP><I>n</I></SUP> subset (where <I>n</I> ranges from 2–4), and the 4<SUP>1</SUP>5<SUP>10</SUP>6<SUP><I>n</I></SUP> subset (where <I>n</I> also ranges from 2–4). Transformations among these cages occur <I>via</I> water pair insertions/removals and rotations, and may elucidate the mechanisms of solid–solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5<SUP>12</SUP> cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5<SUP>12</SUP>6<SUP><I>n</I></SUP> cages outnumber their 4<SUP>1</SUP>5<SUP>10</SUP>6<SUP><I>n</I></SUP> counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5<SUP>12</SUP>6<SUP>2</SUP>/5<SUP>12</SUP> and 5<SUP>12</SUP>6<SUP>3</SUP>/4<SUP>1</SUP>5<SUP>10</SUP>6<SUP>2</SUP> cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates.</P> <P>Graphic Abstract</P><P>The dominant cages of clathrates are classified and the formation of multiple sI unit cells is reported from large-scale simulations. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1cp21899a'> </P>
Aging Effects and Working Memory in Garden-Path Recovery
Hyunsoo Yoo,Michael Walsh Dickey 한국언어재활사협회 2017 Clinical Archives of Communication Disorders Vol.2 No.2
Purpose: The relationship between working memory (WM) and sentence comprehension (SC) has drawn many researchers’ attention. The current study examined whether healthy older adults have difficulty comprehending potentially WM-demanding garden-path sentences involving syntactic ambiguities, and whether their comprehension is predicted by their working memory capacity or inhibitory control. Methods: Older adults (n=35, ages 60–89) and younger adults (n=36, ages 19–33) completed two self-paced reading experiments and a battery of cognitive measures (working memory and inhibition tasks). Participants read the sentences containing a Minimal Attachment (MA) ambiguity in Experiment 1 and a Late Closure (LC) ambiguity in Experiment 2. Results: The older adults’ garden-path effect was larger than younger adults’ in on-line measures. However, older adults exhibited higher off-line acceptability judgments for garden-path sentences than younger adults, for both LC and MA sentences. Working memory predicted off-line performance of both younger and older groups in Experiment 2 (LC). Conclusions: The results showed age-related differences in the comprehension of sentences with temporary syntactic ambiguities. Successful recovery from garden paths (revealed by off-line measures) was predicted by working memory. However, the source of age-related differences in real-time processing remains unclear. These results indicate that healthy aging can affect comprehension of challenging, syntactically ambiguous material.
Park, Sarah S.,Hendon, Christopher H.,Fielding, Alistair J.,Walsh, Aron,O’Keeffe, Michael,Dincă,, Mircea American Chemical Society 2017 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.139 No.10
<P>The structure-directing role of the inorganic secondary building unit (SBU) is key for determining the topology of metal organic frameworks (MOFs). Here we show that organic building units relying on strong Ir interactions that are energetically competitive with the formation of common inorganic SBUs can also play a role in defining the topology. We demonstrate the importance of the organic SBU in the formation of Mg2H6(H3O)(TTFTB)(3) (MIT-25), a mesoporous MOF with the new ssp topology. A delocalized electronic hole is critical in the stabilization of the TTF triad organic SBUs and exemplifies a design principle for future MOF synthesis.</P>
Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide
Gold-Parker, Aryeh,Gehring, Peter M.,Skelton, Jonathan M.,Smith, Ian C.,Parshall, Dan,Frost, Jarvist M.,Karunadasa, Hemamala I.,Walsh, Aron,Toney, Michael F. National Academy of Sciences 2018 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.115 No.47
<▼1><P><B>Significance</B></P><P>Hybrid organic–inorganic perovskites are a promising class of materials for efficient and low-cost solar cells. Unlike conventional inorganic semiconductors such as silicon and gallium arsenide, hybrid perovskites feature significant dynamic disorder in their crystal structure. This dynamic disorder can be broadly classified into lattice vibrations (phonons) and molecular rotations. Phonons interact with charge carriers through electron–phonon coupling, which has substantial impacts on the operation of solar cells. Our study shows that acoustic phonons, the type responsible for transmitting heat in conventional semiconductors, have extraordinarily short lifetimes in the archetypal hybrid perovskite methylammonium lead iodide. These short lifetimes have direct implications on the cooling and transport of electrons and reflect a key difference between hybrid perovskites and conventional photovoltaic semiconductors.</P></▼1><▼2><P>Hybrid organic–inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron–phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of picoseconds, corresponding to nanometer mean free paths and demonstrating that acoustic phonons are unable to dissipate heat efficiently. Lattice-dynamics calculations using ab initio third-order perturbation theory indicate that the short lifetimes stem from strong three-phonon interactions and a high density of low-energy optical phonon modes related to the degrees of freedom of the organic cation. Such short lifetimes have significant implications for electron–phonon coupling in MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the cooling of hot carriers and in the transport and recombination of band edge carriers. These findings illustrate a fundamental difference between HOIPs and conventional photovoltaic semiconductors and demonstrate the importance of understanding lattice dynamics in the effort to develop metal halide perovskite optoelectronic devices.</P></▼2>