Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P40: Matter in Extreme Environments V: Novel ChemistryFocus
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Sponsoring Units: DCOMP DMP Chair: Koichiro Umemoto Room: 705 |
Wednesday, March 4, 2020 2:30PM - 3:06PM |
P40.00001: Matter in Extreme Environments: Novel Chemistry under Pressure Invited Speaker: Maosheng Miao Thanks to the developments of high-pressure techniques and quantum mechanics based crystal structure prediction methods, numerous novel compounds with atypical compositions have been obtained or predicted in the past decade. Differing from conventional solid materials, many of these new compounds consist of various homonuclear chemical species such as dimers, trimers, pentagonal and heptagonal rings, polymer chains, atomic layers, and three-dimensional networks, unexpectedly telling a story of rich chemistry under pressure. More strikingly, pressure can alter the chemical characteristics of elements by activating the core electrons, the unoccupied orbitals and the quantum orbitals at the interstitial sites, leading to many new surprising phenomena. In this talk, I will outline the novel compounds and the new chemical phenomena within one conceptual framework based on the change of quantum states of electrons under high pressure. In contrast to the conventional view and chemical intuition, the quantum mechanics features of electrons such as directional bonds, inhomogeneous distribution, lower symmetry etc. are actually magnified by the increasing pressure, giving rise to rich moieties and variations in novel inorganic compounds. One striking example is that the core electrons can be activated to form bonds, violating a primary principle of chemistry. Other examples include electrons detaching from all atoms to play the role of anions at the interstitial sites (electrides), noble gases behaving as anions because their outer-shell d orbitals gain electrons, and noble gases reacting with ionic compounds without forming any chemical bonds. The influence of this new picture on future studies that is destined to higher pressures, more complex compositions and applicable materials is discussed. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P40.00002: Theoretical phase diagram of boron carbide BxC from ambient to high pressure (P) and high temperature (T): the “single phase” regime in the light of the density functional theory (DFT) results Antoine JAY, Olivier Hardouin Duparc, Jelena Sjakste, Nathalie Vast The phase diagram of boron carbide is calculated within the DFT as a function of T and P up to 80 GPa, accounting for icosahedral, graphite- and diamond-like atomic structures [1]. Only some icosahedral phases turn out to be thermodynamically stable with atomic carbon concentrations (c) of resp. 8.7% (B10.5C), 13.0% (B6.7C), 20% (B4C) and 28.6% (B2.5C). |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P40.00003: Novel tunable band gap BC8/ST12 SixGex−1 alloys: insights from first-principles calculations Johannes Wagner, Maribel Nunez Valdez The cubic Ia-3 (BC8) and tetragonal P43212 (ST12) modifications of Si and Ge are promising candidates for applications in optoelectronic, thermoelectric or plasmonic devices. However the indirect and narrow band gaps are a limiting factor of the pure phases. Si-Ge alloys in these modifications could overcome this drawback and enable tailoring for specific use-cases. To that end, high pressure BC/ST12 SixGex−1 solid solutions for 0 ≤ x ≤ 1 have been synthesized and reported to be stable at ambient conditions[1]. Here we employ ab initio calculations to further investigate the electronic properties of these alloys as a function of x. We show how atomic site occupancy affects the band gap and which atomic arrangements stabilize intermediate compositions. We find that the ST12 phase is energetically favorable up to x≈0.75 and that the indirect band gap of the ST12 Ge end-member can be tuned to become direct for 0.05 ≤ x ≤ 0.2. Furthermore, we obtain the effective band structure of intermediate random alloys by calculating special quasi random structures (SQS) for selected compositions. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P40.00004: First-Principles Simulation of Strain Effects on Lanthanum Monopnictides Chia-Min Lin, Wei-Chih Chen, Cheng-Chien Chen Rare-earth monopnictides have attracted significant attention due to their exotic electronic and topological properties with potential thermoelectric and spintronic applications. Here, we theoretically investigate strain effects on lanthanum monopnictides LaX (X = N, P, As, Sb, and Bi) by first-principles simulations using hybrid density functionals with spin-orbit coupling. In particular, the phonon properties, electronic bandstructures, and topological natures of these materials under compressive and tensile strains are computed. It is shown that strain engineering is an effective approach to manipulating the properties of LaX for improved performance and device applications. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P40.00005: A First-Principles Exploration of NaxSy Binary Phases at 1 atm and Under Pressure Nisha Geng, Tiange Bi, Niloofar Zarifi, Yan Yan, Eva Zurek Interest in Na-S compounds stems from their use in battery materials at 1 atm, as well as the potential for superconductivity under pressure. Evolutionary structure searches coupled with Density Functional Theory calculations were employed to predict stable and low-lying metastable phases of sodium poor and sodium rich sulfides at 1 atm and within 100–200 GPa. At ambient pressures, four new stable or metastable phases with unbranched sulfur motifs were predicted: Na2S3 with C2/c and Imm2 symmetry, C2 -Na2S5 and C2 -Na2S8. Van der Waals interactions were shown to affect the energy ordering of various polymorphs. At high pressure, several novel phases that contained a wide variety of zero-, one-, and two-dimensional sulfur motifs were predicted, and their electronic structures and bonding were analyzed. At 200 GPa, P4/mmm -Na2S8 was predicted to become superconducting below 15.5 K, which is close to results previously obtained for the β -Po phase of elemental sulfur. The structures of the most stable M3S and M4S, M = Na, phases differed from those previously reported for compounds with M = H, Li, K. |
Wednesday, March 4, 2020 3:54PM - 4:30PM |
P40.00006: Tuning Magnetic and Electronic Properties in Exotic Silver(II) Fluorides Using Pressure Invited Speaker: Wojciech Grochala Silver(II) fluorides exhibit unique structural, electronic and magnetic features which render them similar to parent compounds of high-TC oxocuprate superconductors. These encompass the reduced structural and magnetic dimensionality, the persistence of the Jahn-Teller effect, substantial covalence of chemical bonding between metal and nonmetal (d-p hybridization), potent magnetic superexchange via nonmetal atom, insulating charge-transfer character within the Zaanen-Sawatzky-Allen clasification, small energy band gap at the Fermi level, comparable Debye frequencies, etc. Here I will discuss the behaviour of some of these materials (including doped systems) at elevated pressure conditions as obtained from combined experimental and theoretical studies. Influence of high pressure on bonding, as well as electronic and magnetic properties, will be discussed. Possibility to achieve metallization and superconductivity will also be addressed. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P40.00007: The change of basic chemical behavior of elements under high pressure Yuanhui Sun, Maosheng Miao The chemistry at ambient condition has implicit boundaries rooted in the atomic shell structure: The inner-shell electrons and the unoccupied outer-shell orbitals do not involve as major component in chemical reactions and in chemical bonds. The chemical properties of atoms are determined by the electrons in the outermost shell; hence, these electrons are called valence electrons. These general rules govern our understanding of chemical structures and reactions. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P40.00008: Ultrahigh-pressure induced decomposition of silicon disulfide into high coordination number silicon-sulfur compounds Yuanzheng Chen, Xiaolei Feng, Simon A. T. Redern, Hanyu Liu SiS2 is thought to occur in inter-stellar dust and is of interest more generally among the silicon chalcogenides as a comparator to SiO2, an important component of terrestrial planets. However, the high-pressure behaviors of silicon sulfides are unclear. Here, using an efficient structure search method, we systematic explore the structural evolution of different Si-S stoichiometries up to 250 GPa. We find that SiS2 is only stable below 155 GPa, which then decomposes into two previously-unreported compounds, SiS and SiS3, at higher pressures. SiS adopts a high symmetry Pm-3m structure consisting of 8-fold coordinated silicon in face-sharing SiS8 polyhedra, while SiS3 crystallizes in R3m structure containing 9-fold coordinated SiS9 polyhedra. Analysis suggests the new Si 8-fold coordination environment could be a common feature for group IV-VI compounds under high pressure. Our findings provide key insights on the nature of the Si-S compounds under ultrahigh pressure. |
Wednesday, March 4, 2020 4:54PM - 5:06PM |
P40.00009: A novel all-nitrogen molecular crystal as promising high-energy density material Lei Zhao, Shijie Liu, Yuanzheng Chen, Wencai Yi, Fenglong Gu, Yonghao Zheng, Bingbing Liu, Maosheng Miao In the decades-long search for energy-dense materials capable of greater energy output, many studies have focused on nitrogen rich compounds, which have the advantages of relative stability at ambient conditions, high-energy output when broken down, and a clean gas product (N2) that is inert, non-toxic, and a non-contributor to the greenhouse effect. The most stable form of nitrogen under ambient condition is N2 molecular crystal, because of its exceedingly stable triple bond in nature. Besides N2, there is not any report of other all nitrogen molecular crystals either at ambient condition or under high pressure, except a recently reported N8 crystal. Here, using ab initio calculations combined with an intelligence structure-searching, we report the discovery of a thermodynamically stable nitrogen molecular crystal, which has perfect packing and more energetic stability at lower pressure range, compared with the N8 crystal. While decompose at ambient pressure, this all nitrogen molecular crystal can release great amount of energy (~2.90 kJ/g). |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P40.00010: Structure and Stability of Oxyfluorides and Oxychlorides and Their Relations to Catalytic Activities in Oil Refining Reactions Dalar Khodagholian Metal oxides have long been used as catalysts or supporters of catalysts in petroleum refining processes. Mixing anions such as fluorine into the structures of these compounds can greatly enhance their structural stability. Therefore, metal oxyfluorides are potential candidates for such catalysts due to their stability, atomic and electronic properties. The stability of several compositions of these compounds has been investigated, combining different anions or replacing them partially or fully. Since synthesizing mixed anion compounds is challenging, automatic crystal structure search together with density functional theory, particle swarm optimization algorithm and geometry relaxation calculations have been performed to obtain information regarding the stability and properties of these compounds. Plotting convex hull has revealed that FeOF, Fe2OF4, and AlOF have low formation energies, therefore can be used in synthesizing the catalyst materials. Based on structures obtained, electronic properties of these structures have been studied. Also, the effects of pressure on these compounds have been explored. Under pressures such as 10 GPa, fluorine rich compound Fe2OF4 and AlOF become highly destabilized, leaving FeOF more likely to be a candidate compound under higher pressures. |
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P40.00011: Physical property modulation in 2D TMDs homo- and hetero-structures via pressure engineering Juan Xia 2D TMDs and their van der Waals heterostructures (vdWs HSs), exhibit attractive optical and optoelectronic properties thanks to the different band alignments and interlayer interactions. Their sensitivity to interlayer distance allows effective tuning of material properties through external modulation of lattice parameters. Therefore, it is of both fundamental and practical importance to explore interlayer excitons in vdWs HSs, especially their dynamic response and underlying mechanisms to different tuning techniques. So far, only limited changes in lattice parameters have been achieved, hampering effective tuning of physical properties in vdWs HSs. |
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