Session R23: Materials in Extremes: Novel Materials and Phenomena

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Transition metal boride Os₂B₃ has excellent incompressibility and thermal stability for potential applications in extreme high-pressure high-temperature environments. In this study, we use density functional theory (DFT) to simulate the electronic and mechanical properties of Os₂B₃. We also perform phonon calculations with the quasiharmonic approximation (QHA) to study finite-temperature and finite-pressure thermodynamic quantities of Os₂B₃, including its P-V-T curves, phonon spectra, bulk modulus, specific heat, thermal expansion, and the Grüneisen parameter. The comparison of our simulation to experimental P-V-T data underscores the utility of the Armiento-Mattsson 2005 (AM05) DFT functional and QHA for describing thermodynamic properties of solid-state systems.   

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The phase diagram of boron carbide is calculated within 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% (B_10.5C), 13.0% (B_6.7C), 20% (B₄C) and 28.6% (B2.5C).
Their respective ranges of stability under pressure and temperature are calculated, and the theoretical T-P-c phase diagram boundaries are discussed. At ambient conditions, the introduction in the phase diagram of the new phase B_10.5C, with an ordered crystalline motif of 414 atoms, is shown to bring the theoretical solubility range of carbon in boron close to the experimental one. The effect of configurational entropy is studied at finite temperature, and the convex hull modified. We discuss the occurrence of the “single phase regime” in the light of these new results.
[1] A. Jay, O. Hardouin Duparc, J. Sjakste and N. Vast, J. Appl. Phys. 125, 185902 (2019). https://doi.org/10.1063/1.5091000   

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The application of extreme environments (including variable pressure, temperature and irradiation) can induce dramatic changes in materials and give us a much broader field to discover new phases and explore novel phenomena. Improving our understanding of the modifications that occur can also provide guidance for designing improved materials with desirable properties that can be utilized for energy-related applications. I will presenting some examples which demonstrate the range of new materials that can be formed and properties which can be altered under extreme environments. In particular, I will focus on experimental work using high pressure as promising variable for tuning materials behavior.   

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The high-pressure chemistry of nitrogen and nitrogen-rich compounds have been in a focus of many studies in the recent years due to both fundamental and practical interest and due to the improvement of high-pressure synthetic and characterization techniques. Polynitrogen compounds are usually considered as potential high energy density materials due to the remarkable difference in the energies of single, double and triple N-N bonds. In the present work we have systematically studied reactions between transition metals and nitrogen at pressures up to 130 GPa in laser-heated diamond anvil cells. Hf, W, Re and Os react with nitrogen forming porous metal-inorganic frameworks Hf₂N₁₁, WN₁₀, ReN₁₀ and Os₅N₃₄ of various topology [1,2]. The frameworks are built of metals interconnected via polymeric nitrogen linkers, while the pores are filled with dinitrogen molecules N₂. Lighter transition metals form non-porous polynitrides featuring polymeric nitrogen chains. In this contribution we will discuss novel synthetic routes to energetic polynitrogen compounds and crystal-chemical rules governing their formation.

[1] M. Bykov, et al. Angew. Chemie Int. Ed. 59, 10321 (2020)
[2] M. Bykov, et al. Angew. Chemie Int. Ed. 57, 9048 (2018)   

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ABSTRACT
In this talk, I will briefly introduce the methods developed in my group, including a machine learning accelerated crystal structure prediction method [1], a machine learning force field using descriptor optimization [2], and a method determining the dimensionality and multiplicity of crystal nets using quotient graph [3]. In addition, I will show some of our recent progress on the applications of these methods, for instance, the predictions on the compounds and their new states under planetary conditions (superionic state, plastic state, and coexistence) [4-6], as well as the superionic-like cooperative diffusion in an elemental system [7].

REFERENCE
1. Kang Xia, Hao Gao, Cong Liu, Jianan Yuan, Jian Sun*, Hui-Tian Wang*, Dingyu Xing, "A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search", Sci. Bull. 63, 817 (2018).
2. Hao Gao, Junjie Wang, and Jian Sun*, "Improve the performance of machine-learning potentials by optimizing descriptors", J. Chem. Phys. 150, 244110 (2019).
3. Hao Gao, Junjie Wang, Zhaopeng Guo, Jian Sun*, "Determining dimensionalities and multiplicities of crystal nets" npj Comput. Mater. 6, 143 (2020).
4. Cong Liu, Hao Gao, Yong Wang, Richard J. Needs, Chris J. Pickard, Jian Sun*, Hui-Tian Wang*, Dingyu Xing, "Multiple superionic states in helium-water compounds", Nature Physics 15, 1065 (2019).
5. Cong Liu, Hao Gao, Andreas Hermann, Yong Wang, Maosheng Miao, Chris J. Pickard, Richard J. Needs, Hui-Tian Wang, Dingyu Xing, and Jian Sun*, "Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature", Phys. Rev. X 10, 021007 (2020).
6. Hao Gao, Cong Liu, Andreas Hermann, Richard J. Needs, Chris J. Pickard, Hui-Tian Wang, Dingyu Xing, and Jian Sun*, "Coexistence of plastic and partially diffusive phases in a helium-methane compound", Natl. Sci. Rev. 7, 1540 (2020).
7. Y. Wang et al., "Electronically driven 1D cooperative diffusion in a simple cubic crystal", (submitted).   

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With experimental data it has been demonstrated that under high-pressure conditions (above 5 GPa and 1300 K) thermodynamically stable Mg-C compounds are indeed possible. The new compounds synthesized, Mg₂C and β-Mg₂C₃, crystallizes in the monoclinic C2/m space group, and contains rare allene-derrived C₃^4- anions that are isoelectronic with CO₂.
Graphite intercalation compounds (GIC) with metals such as Li, Na and K form different compositions (and crystal structures) produced by stacking along c-axis of metal (Me) and n carbon (A, B or C) layers. The n number indicate the stage of intercalation. Typically ordered compounds are obtained for n = 1 to 6 with various stacking sequences depending on metals: n = 1 for MeAMeB, n = 2 for MeABMeBAMeCA, n = 3 for MeABCMeBCAMeCAB, etc. Both high pressure and high temperature leads to increasing n. Here we will discuss the structural features of GIC, the XRD, Raman and other structurally related data, as well as corresponding “diamond intercalation compounds” (DIC) structurally related to GIC .   

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Ag-doped ZnO nanorods from 5% to 20% were synthesized on a seeded substrate using chemical bath deposition (CBD) method. Structural properties were studied by Scanning Electron Microscopy (SEM), and X-ray Diffraction Spectroscopy (XRD). SEM images showed that the diameter of nanorods increased from 68 nm to 87 nm. EDX and XRD confirmed the presence of Ag in ZnO crystal lattice. Optical properties were studied using Photoluminescence (PL) and UV-Visible spectroscopy. Band gap of Ag-doped ZnO nanrods were calculated from optical measurements and found to decrease from 3.25 eV to 2.99 eV with increasing doping concentration. Electrical transport properties of nanorods were analyzed by impedance spectroscopy using the Cole-Cole plot technique and conductivity was measured using the Vander Pauw method. Impedance analysis showed that the resistance of Ag-ZnO nanorods decreased with increasing Ag doping from 5% to 20%. Electrical results indicated that the resistivity of Ag-ZnO decreased with increasing doping concentration. Electrochemical properties, such as conduction band and valence band of Ag-doped samples were extracted from Cyclic Voltammetry. The optimized Ag-ZnO electrodes based on electrochemical studies will be developed for dye-sensitized solar cells.   

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We compute the tungsten–nitrogen (W–N) pressure–temperature phase diagram through a combination of density functional theory and thermodynamic calculations. Augmenting standard enthalpy-pressure calculations by the chemical potential change of nitrogen at high pressure and high temperature conditions, we estimate Gibbs energies under nitrogen-rich conditions. The approach allows to predict temperature and pressure conditions necessary to synthesize W–N polymorphs and to locate optimum pressure/temperature conditions with maximum driving force for successful syntheses. Our investigations includeW₂N₃, W₃N₅, W₂N₂(N₂), WN₆, and recently synthesized WN₈-N₂. Further studies address ternary Li–W–N compounds with W in high oxidation state, namely Li₆WN₄, Li₆WN₆, and Li₆WN₈.   

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New structures of phosphorous-nitrogen (P-N) crystals under pressure are predicted using an evolutionary algorithm based upon density functional theory (DFT) calculations. The new predicted structures PN₄, PN₂, and P₃N₄ at 50 GPa are stable according to convex-hull construction and phonon dispersion calculations. PN₄ has the highest concentration of nitrogen out of all known structures of P-N solids. The predicted structures PN₄ and P₃N₄ at 50 GPa are metallic, and the band gap is 0.89 eV for PN₂ according to hybrid functional DFT calculations. The symmetry group of PN₄ is P2/m, as well as that of PN₂; P₃N₄ crystal has a C2/m symmetry group. Calculations of partial density-of-states indicate that the main contribution to the density of states at the Fermi level is related with p-electrons of N for PN₄, and for P₃N₄ p-electrons of both N and P atoms define the density-of-states at the Fermi level. The high pressure structures of P-N solids are compared with available experimental data.   

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Optical and X-ray transition energies are generally reduced with increasing temperature and pressure in plasmas. This phenomenon, called ‘continuum lowering,’ is important in the high-energy density (HED) regime. To understand the relative contributions of pressure and temperature, we used density-functional theory to study diamond silicon at low temperature but under extreme pressure, up to 150 Mbar and 12× density. When the structure is compressed to 1.36× density, the system turns metallic. We used a semicore pseudopotential because we found that the 2s and 2p wavefunctions of adjacent atoms overlap under extreme pressure, altering the bandstructure. As a benchmark, we calculated deformation potentials from low pressure in good agreement with literature results. We investigated soft X-ray transitions by calculating the absorption spectrum with the random phase approximation in the BerkeleyGW code. From these results and the Kohn-Sham potential, we found opposite behavior to the expectation from continuum lowering in plasmas. We compare to various models of continuum lowering. This investigation gives insight into the mechanisms of continuum lowering, and can help in the interpretation of experimental spectra of matter under HED conditions.   

On Demand

High-melting structural diborides such as ZrB₂, TiB₂ and HfB₂ and their entropy-stabilized variants MB₂ (M= transition metals) are materials candidates for high temperature structural applications because of their high melting point, excellent mechanical strength, high electrical and thermal conductivity and thermodynamic stability. In the current study, we employed the Density functional theory (DFT) calculations to model the electron-phonon interactions and related properties including the relaxation time, electrical conductivity, and transport properties. Both low temperature behavior of the thermal conductivity, which is mainly due to the phonon contribution, and high temperature ones, governed by the electron contributions, were calculated. Additionally, we have systematically evaluated the effect of alloying elements such as Ta and Cr on the thermal properties of the diborides and compared them with the experimental results. The support from the Advanced Manufacturing program from the CMMI Division of NSF (Award No. 1902069) is gratefully acknowledged. The computational support from NERSC is highly acknowledged.