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A new tetragonal polymorph of CaZrF6 can be prepared by high energy ball milling of a CaF2/ZrF4 mixture, followed by heat treatment at 325 °C. This polymorph is thermodynamically stable with respect to the well-known cubic form at low temperatures. However, it readily transforms to the cubic form on heating above ∼400 °C. The tetragonal (β) CaZrF6 is not isostructural with any previously known alkaline earth AZrF6 phase. Unlike the cubic form, which shows strong negative thermal expansion over a wide temperature range, the tetragonal form displays positive thermal expansion in all directions (100–400 K: αl ∼ +17 × 10−6 K−1 and +13 × 10−6 K−1 along the a- and c-axes, respectively).
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Formation of nanostructured silicas through the fluoride catalysed self-polymerization of Q-type functional silica cages
Octa(dimethylsiloxy)silica cages (Q8M8H) undergo rapid self-polymerization in the presence of a fluoride catalyst to form complex 3D porous structural network materials with specific surface areas up to 650 m2g−1. This establishes a new method to form bio-derived high inorganic content soft silicas with potential applications in filtration, carbon capture, catalysis, or hydrogen source.
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- Award ID(s):
- 2137672
- PAR ID:
- 10349868
- Date Published:
- Journal Name:
- Chemical Communications
- ISSN:
- 1359-7345
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Motivated by the recent success in using a latticed-based version of the aggregation-volume-bias Monte Carlo method to determine the thermodynamic stabilities of both bcc and fcc clusters formed by Lennard-Jones particles, this approach is extended to the calculation of the nucleation-free energies of solid clusters formed by urea at 300 K in two different polymorphs, i.e., form I and form IV. In addition to the lattice confinement, the constraint on the molecular orientation was found necessary to ensure that the clusters sampled in these simulations are in the corresponding form. A model that can reproduce the experimental properties such as density and lattice parameters of form I at ambient conditions is used in this study. From the size dependencies of the free energies obtained for a finite set of clusters studied, the free energies of clusters at other sizes, including an infinitely large cluster, were extrapolated. At the infinite size, equivalent to a bulk solid, form I was found to be more stable than form IV, which agrees with the experimental results. In addition, form I was found to be thermodynamically stable throughout the entire cluster size range investigated here, which contradicts the previous finding that small form I clusters are unstable from the crystal nucleation simulation studies.more » « less
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Abstract Let be the three‐dimensional space form of constant curvature , that is, Euclidean space , the sphere , or hyperbolic space . Let be a smooth, closed, strictly convex surface in . We define an outer billiard map on the four‐dimensional space of oriented complete geodesics of , for which the billiard table is the subset of consisting of all oriented geodesics not intersecting . We show that is a diffeomorphism when is quadratically convex. For , has a Kähler structure associated with the Killing form of . We prove that is a symplectomorphism with respect to its fundamental form and that can be obtained as an analogue to the construction of Tabachnikov of the outer billiard in defined in terms of the standard symplectic structure. We show that does not preserve the fundamental symplectic form on associated with the cross product on , for . We initiate the dynamical study of this outer billiard in the hyperbolic case by introducing and discussing a notion of holonomy for periodic points.more » « less
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A class of nonlinear, stochastic staticization control problems (including minimization problems with smooth, convex, coercive payoffs) driven by diffusion dynamics with constant diffusion coefficient is considered. A fundamental solution form is obtained where the same solution can be used for a limited variety of terminal costs without re-solution of the problem. One may convert this fundamental solution form from a stochastic control problem form to a deterministic control problem form. This yields an equivalence between certain second-order (in space) Hamilton-Jacobi partial differential equations (HJ PDEs) and associated first-order HJ PDEs. This reformulation has substantial numerical implications.more » « less
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A new metastable crystalline form of gallium has been computationally designed using density functional calculations with imposing periodic boundary conditions. The geometric and electronic structures of the predicted new allotrope were calculated on the basis of a diamond lattice in which all carbon atoms are replaced by gallium Ga4tetrahedra. This form does not have any imaginary phonons, thus it is a metastable crystalline form of gallium. The new form of gallium is a metal and shows high plasticity and low‐melting temperature. Molecular dynamics simulations show that this form of gallium will melt at about 273 K with a sharp increase in temperature in the system during the melting process from 273 to 1800 K. This melting process is very different from conventional melting, where temperature stays the same until complete melting. That unusual melting can be explained by the fact that supertetrahedral gallium is a metastable structure that has an excess of strain energy released during melting. If made this new material may find many useful applications as a new low density metal with stored internal energy. © 2019 Wiley Periodicals, Inc.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d2cc02672d
