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Andrew Shamp

Bio: Andrew Shamp is an academic researcher from University at Buffalo. The author has contributed to research in topics: Density functional theory & Phosphine. The author has an hindex of 11, co-authored 17 publications receiving 505 citations. Previous affiliations of Andrew Shamp include Lawrence Livermore National Laboratory.

Papers
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TL;DR: The results suggest that the superconductivity recently observed by Drozdov, Eremets, and Troyan when phosphine was subject to pressures of 207 GPa in a diamond anvil cell may result from these, and other, decomposition products of phosphine.
Abstract: Evolutionary algorithms (EAs) coupled with density functional theory (DFT) calculations have been used to predict the most stable hydrides of phosphorus (PHn, n = 1-6) at 100, 150, and 200 GPa. At these pressures phosphine is unstable with respect to decomposition into the elemental phases, as well as PH2 and H2. Three metallic PH2 phases were found to be dynamically stable and superconducting between 100 and 200 GPa. One of these contains five formula units in the primitive cell and has C2/m symmetry (5FU-C2/m). It comprises 1D periodic PH3-PH-PH2-PH-PH3 oligomers. Two structurally related phases consisting of phosphorus atoms that are octahedrally coordinated by four phosphorus atoms in the equatorial positions and two hydrogen atoms in the axial positions (I4/mmm and 2FU-C2/m) were the most stable phases between ∼160-200 GPa. Their superconducting critical temperatures (Tc) were computed as 70 and 76 K, respectively, via the Allen-Dynes modified McMillan formula and using a value of 0.1 for the Coulomb pseudopotential, μ*. Our results suggest that the superconductivity recently observed by Drozdov, Eremets, and Troyan when phosphine was subject to pressures of 207 GPa in a diamond anvil cell may result from these, and other, decomposition products of phosphine.

90 citations

Journal ArticleDOI
TL;DR: The most stable stoichiometries and structures of beryllium and barium polyhydrides, MHn with n > 2 and M = Be/Ba, under pressure were predicted in this article.
Abstract: Evolutionary structure searches are coupled with density functional theory calculations to predict the most stable stoichiometries and structures of beryllium and barium polyhydrides, MHn with n > 2 and M = Be/Ba, under pressure. Even though the BeHn stoichiometries we explored do not become thermodynamically stable with respect to decomposition into the classic hydride BeH2 and H2 up to 200 GPa, we find a new phase of BeH2 with R3m symmetry above 150 GPa. The barium polyhydrides become thermodynamically preferred by 20 GPa. They sport complex hydrogenic sublattices composed of H–, H3–, and H2 units. BaH6 is the first stoichiometry to emerge as stable and metallic (∼60 GPa using the Perdew–Burke–Ernzerhof functional), and the P4/mmm symmetry structure is estimated to become superconducting below 30–38 K at 100 GPa. Phases with an even greater hydrogen content lie on the convex hull at higher pressures, and an intriguing BaH10 stoichiometery becomes the global thermodynamic minimum around 150 GPa. BaH10 r...

87 citations

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TL;DR: A detailed review of the state-of-the-art EOS models for inertial confinement fusion (ICF) implosions can be found in this paper, where the authors present a detailed comparison with experiments.

65 citations

Journal ArticleDOI
TL;DR: In this article, the structure of strontium polyhydrides with n > 2 was studied using evolutionary algorithms coupled with density functional theory calculations, and a number of phases with even n were found to be thermodynamically stable below 150 GPa.
Abstract: The structures of the strontium polyhydrides, SrHn with n > 2, under pressure are studied using evolutionary algorithms coupled with density functional theory calculations. A number of phases with even n are found to be thermodynamically stable below 150 GPa. Particularly interesting is the SrH4 stoichiometry, which comprises the convex hull at 50, 100, and 150 GPa. Its hydrogenic sublattice contains H2 and H– units, and throughout the pressure range considered, it adopts one of two configurations which were previously predicted for CaH4 under pressure. At 150 GPa, the SrH6 stoichiometry has the lowest enthalpy of formation. The most stable configuration assumes P3 symmetry, and its lattice consists of one-dimensional H2···H– hydrogenic chains. Symmetrization of these chains results in the formation of ∞1[Hδ−] helices, which are reminiscent of the trigonal phase of sulfur. The R3m-SrH6 phase, which is comprised of these helices, becomes dynamically stable by 250 GPa and has a high density of states at t...

61 citations

Posted Content
TL;DR: In this article, three metallic PH2 phases were found to be dynamically stable and superconducting between 100-200 GPa, and their critical temperatures were computed as being 70 and 76 K, respectively, via the Allen-Dynes modified McMillan formula.
Abstract: Evolutionary algorithms (EA) coupled with Density Functional Theory (DFT) calculations have been used to predict the most stable hydrides of phosphorous (PHn, n = 1-6) at 100, 150 and 200 GPa. At these pressures phosphine is unstable with respect to decomposition into the elemental phases, as well as PH2 and H2. Three metallic PH2 phases were found to be dynamically stable and superconducting between 100-200 GPa. One of these contains five formula units in the primitive cell and has C2/m symmetry (5FU-C2/m). It is comprised of 1D periodic PH3-PH-PH2-PH-PH3 oligomers. Two structurally related phases consisting of phosphorous atoms that are octahedrally coordinated by four phosphorous atoms in the equatorial positions and two hydrogen atoms in the axial positions (I4/mmm and 2FU-C2/m) were the most stable phases between ~160-200 GPa. Their superconducting critical temperatures (Tc) were computed as being 70 and 76 K, respectively, via the Allen-Dynes modified McMillan formula and using a value of 0.1 for the Coulomb pseudopotential, u*. Our results suggest that the superconductivity recently observed by Drozdov, Eremets and Troyan when phosphine was subject to pressures of 207 GPa in a diamond anvil cell may result from these, and other, decomposition products of phosphine.

59 citations


Cited by
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TL;DR: Ab initio random structure searching (AIRSS) as discussed by the authors searches for stable structures of materials using first-principles electronic structure methods, such as density-functional-theory (DFT), is a rapidly growing field.
Abstract: It is essential to know the arrangement of the atoms in a material in order to compute and understand its properties. Searching for stable structures of materials using first-principles electronic structure methods, such as density-functional-theory (DFT), is a rapidly growing field. Here we describe our simple, elegant and powerful approach to searching for structures with DFT, which we call ab initio random structure searching (AIRSS). Applications to discovering the structures of solids, point defects, surfaces, and clusters are reviewed. New results for iron clusters on graphene, silicon clusters, polymeric nitrogen, hydrogen-rich lithium hydrides, and boron are presented.

890 citations

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TL;DR: The study suggests that dense hydrides consisting of lanthanum and yttrium and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.
Abstract: A systematic structure search in the La–H and Y–H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH8 a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen–Cooper–Schrieffer (BCS) arguments indicate they might be high-Tc superconductors. In particular, the superconducting transition temperature Tc calculated for LaH10 is 274–286 K at 210 GPa. Similar calculations for the Y–H system predict stability of the sodalite-like fcc YH10 and a Tc above room temperature, reaching 305–326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.

599 citations

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TL;DR: Recently developed static and dynamic high-pressure experimental techniques have led to the synthesis of many functional materials with excellent performance: for example, superconductors, superhard materials and high-energy-density materials.
Abstract: Pressure is a fundamental thermodynamic variable that can be used to control the properties of materials, because it reduces interatomic distances and profoundly modifies electronic orbitals and bonding patterns. It is thus a versatile tool for the creation of exotic materials not accessible at ambient conditions. Recently developed static and dynamic high-pressure experimental techniques have led to the synthesis of many functional materials with excellent performance: for example, superconductors, superhard materials and high-energy-density materials. Some of these advances have been aided and accelerated by first-principles crystal-structure searching simulations. In this Review, we discuss recent progress in high-pressure materials discovery, placing particular emphasis on the record high-temperature superconductivity in hydrogen sulfide and on nanotwinned cubic boron nitride and diamond, the hardest known materials. Energy materials and exotic chemical materials obtained under high pressures are also discussed. The main drawback of high-pressure materials is their destabilization after pressure release; this problem and its possible solutions are surveyed in the conclusions, which also provide an outlook on the future developments in the field. High pressure offers a unique degree of freedom for the creation of new materials, leading to new superconductors, superhard materials, high-energy-density materials and exotic chemical materials with unprecedented properties. This Review discusses these materials, along with recently developed theoretical and experimental methods for materials discovery at high pressures.

427 citations

01 Apr 2009
TL;DR: In this article, the performance of recent density functionals for the exchange-correlation energy of a nonmolecular solid, by applying accurate calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid metals and nonmetals.
Abstract: We assess the performance of recent density functionals for the exchange-correlation energy of a nonmolecular solid, by applying accurate calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid metals and nonmetals. The functionals tested are the modified Perdew-Burke-Ernzerhof generalized gradient approximation PBEsol GGA, the second-order GGA SOGGA, and the Armiento-Mattsson 2005 AM05 GGA. For completeness, we also test more standard functionals: the local density approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria meta-GGA. We find that the recent density functionals for solids reach a high accuracy for bulk properties lattice constant and bulk modulus. For the cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is actually better for the alkali metals and alkali halides. For fair comparison of calculated and experimental results, we consider the zeropoint phonon and finite-temperature effects ignored by many workers. We show how GAUSSIAN basis sets and inaccurate experimental reference data may affect the rating of the quality of the functionals. The results show that PBEsol and AM05 perform somewhat differently from each other for alkali metal, alkaline-earth metal, and alkali halide crystals where the maximum value of the reduced density gradient is about 2, but perform very similarly for most of the other solids where it is often about 1. Our explanation for this is consistent with the importance of exchange-correlation nonlocality in regions of core-valence overlap.

413 citations

Journal ArticleDOI
TL;DR: In this paper, the authors provide an up-to-date compendium of the available results on superconducting hydrides and explain how the synergy of different methodologies led to extraordinary discoveries in the field.

265 citations