Stanimir Bonev

Dalhousie University, Halifax, Nova Scotia, Canada

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Publications (58)203.58 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: The high temperature phase boundaries of CO2 in the proximity of the Earth's adiabat are determined using first-principles molecular dynamics simulations based on density functional theory. The melting curve, predicted here up to 71 GPa, and the molecular to polymeric solid phase transition are computed through a phase coexistence approach from free energy calculations. The resulting CO2 phase IV–phase V-liquid triple point is at 31.8 GPa and 1636 K, in excellent agreement with the available experimental data. The Earth's geotherm crosses into the non-molecular phase V near 40 GPa and 2160 K, indicating that free deposits of carbon dioxide in the lower mantle would exist as a polymeric solid. We have also examined the thermodynamic stability of phase V and find no indication of transformations into a dissociated diamond and oxygen phase at mantle conditions.
    Earth and Planetary Science Letters 07/2013; 373:228–232. · 4.35 Impact Factor
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    ABSTRACT: We report results from high pressure and temperature experiments that provide evidence for the reactivity of xenon with water ice at pressures above 50 GPa and a temperature of 1500 K-conditions that are found in the interiors of Uranus and Neptune. The x-ray data are sufficient to determine a hexagonal lattice with four Xe atoms per unit cell and several possible distributions of O atoms. The measurements are supplemented with ab initio calculations, on the basis of which a crystallographic structure with a Xe_{4}O_{12}H_{12} primitive cell is proposed. The newly discovered compound is formed in the stability fields of superionic ice and η-O_{2}, and has the same oxygen subnetwork as the latter. Furthermore, it has a weakly metallic character and likely undergoes sublattice melting of the H subsystem. Our findings indicate that Xe is expected to be depleted in the atmospheres of the giant planets as a result of sequestration at depth.
    Physical Review Letters 06/2013; 110(26):265501. · 7.94 Impact Factor
  • Brian Boates, Stanimir A Bonev
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    ABSTRACT: The phase diagrams of MgSiO_{3} and MgO are studied from first-principles theory for pressures and temperatures up to 600 GPa and 20 000 K. Through the evaluation of finite-temperature Gibbs free energies, using density-functional theory within the generalized gradient approximation as well as with hybrid exchange-correlation functionals, we find evidence for a vast pressure-temperature regime where molten MgSiO_{3} decomposes into liquid SiO_{2} and solid MgO, with a volume change of approximately 1.2%. The demixing transition is driven by the crystallization of MgO-the reaction only occurs below the high-pressure MgO melting curve. The predicted transition pressure at 10 000 K is in close proximity to an anomaly reported in recent laser-driven shock experiments of MgSiO_{3}. We also present new results for the high-pressure melting curve of MgO and its B1-B2 solid phase transition, with a triple point at 364 GPa and 12 000 K.
    Physical Review Letters 03/2013; 110(13):135504. · 7.94 Impact Factor
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    ABSTRACT: We present ab initio calculations of the phase diagram of liquid CO(2) and its melting curve over a wide range of pressure and temperature conditions, including those relevant to the Earth. Several distinct liquid phases are predicted up to 200 GPa and 10,000 K based on their structural and electronic characteristics. We provide evidence for a first-order liquid-liquid phase transition with a critical point near 48 GPa and 3,200 K that intersects the mantle geotherm; a liquid-liquid-solid triple point is predicted near 45 GPa and 1,850 K. Unlike known first-order transitions between thermodynamically stable liquids, the coexistence of molecular and polymeric CO(2) phases predicted here is not accompanied by metallization. The absence of an electrical anomaly would be unique among known liquid-liquid transitions. Furthermore, the previously suggested phase separation of CO(2) into its constituent elements at lower mantle conditions is examined by evaluating their Gibbs free energies. We find that liquid CO(2) does not decompose into carbon and oxygen up to at least 200 GPa and 10,000 K.
    Proceedings of the National Academy of Sciences 08/2012; 109(37):14808-12. · 9.74 Impact Factor
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    ABSTRACT: The thermodynamic stability of the Pnma structure of CaTiO3 has been studied using hybrid density functional theory and Gibbs free energy calculations, including anharmonic effects. The use of the screened Heyd-Scuseria-Ernzerhof (HSE06) functional shifts the room temperature transition pressure from perovskite to post-perovskite structure by around 18 GPa, or 37%, compared with GGA-PBE. Both the lattice dynamics and choice of exchange functional play a significant role in stabilizing the Pnma structure at finite temperature. The Pnma structure is found to be stable up to 65 GPa at 300 K, consistent with the recent experimental observation of stable Pnma up to 60 GPa.
    Physical review. B, Condensed matter 08/2012; 86(6).
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    ABSTRACT: We report low-frequency high-resolution Raman spectroscopy and ab-initio calculations on dense lithium from 40 to 200 GPa at low temperatures. Our experimental results reveal rich first-order Raman activity in the metallic and semiconducting phases of lithium. The computed Raman frequencies are in excellent agreement with the measurements. Free energy calculations provide a quantitative description and physical explanation of the experimental phase diagram only when vibrational effect are correctly treated. The study underlines the importance of zero-point energy in determining the phase stability of compressed lithium.
    Physical Review Letters 02/2012; 108(5):055501. · 7.94 Impact Factor
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    ABSTRACT: A computationally efficient method is proposed to compute the free energy of liquids with accuracy comparable to ab initio thermodynamic integration. The method has been applied to predict melting curves of CO2 and N over a wide range of pressure using the solid-liquid phase coexistence approach. The calculated melting lines are compared with available experimental data and the crossing of the geotherm and melting line of CO2 is determined.
    02/2012;
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    ABSTRACT: Using density functional theory and many-body perturbation theory within a GW approximation, we calculate the electronic structure of a metal-molecule interface consisting of benzene diamine (BDA) adsorbed on Au(111). Through direct comparison with photoemission data, we show that a conventional G$_0$W$_0$ approach can underestimate the energy of the adsorbed molecular resonance relative to the Au Fermi level by up to 0.8 eV. The source of this discrepancy is twofold: a 0.7 eV underestimate of the gas phase ionization energy (IE), and a 0.2 eV overestimate of the Au work function. Refinements to self-energy calculations within the GW framework that account for deviations in both the Au work function and BDA gas-phase IE can result in an interfacial electronic level alignment in quantitative agreement with experiment.
    Physical review. B, Condensed matter 11/2011; 84.
  • Brian Boates, Stanimir Bonev
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    ABSTRACT: The high-pressure phase diagrams of liquid nitrogen and carbon dioxide have been investigated using first-principles theory. Both liquids undergo rare first-order molecular-polymerization phase transitions at pressures comparable to their solid counterparts. Furthermore, both materials dissociate into metallic atomic fluids at high temperatures. The liquid regimes of their phase diagrams have been divided into several regions based on detailed analyses of changes in both structural and electronic properties for pressures and temperatures up to 200 GPa and 10,000 K, respectively. A comparison of the two liquid phase diagrams will be discussed to illustrate similarities and differences. Calculations of the shock Hugoniot are in excellent agreement with available experimental data.
    11/2011;
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    ABSTRACT: a b s t r a c t We describe ab initio electronic structure calculations (density functional theory molecular dynamics and coupled electron-ion quantum Monte Carlo) of the equation of state (EOS) of hydrogen in a pressureetem-perature regime relevant for simulating the initial phase of an inertial connement fusion capsule implosion. We nd the computed EOS to be quite close to that of the most recent SESAME table (constructed by G. Kerley, 2003). A simple density-dependent but temperature-independent correction brings the 2003-Kerley EOS into excellent agreement with ours in the chosen region of the hydrogen phase diagram. Simulations of fusion ignition experiments on the National Ignition Facility (NIF) with this modied 2003-Kerley table are shown to produce results nearly indistinguishable from those of the 2003-Kerley EOS, which was used to design the capsule. In this sense, we do not expect that further improvements to the hydrogen EOS in this particular regime will impact the capsule design.
    High Energy Density Physics 10/2011; · 1.60 Impact Factor
  • Brian Boates, Stanimir Bonev
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    ABSTRACT: The phase diagrams of liquid CO2 and N2 have been investigated using first- principles theory. Both materials exhibit transitions to conducting liquids at high temperatures (T) and relatively modest pressures (P). Furthermore, both liquids undergo polymerization phase transitions at pressures comparable to their solid counterparts. The liquid phase diagrams have been divided into several regimes through a detailed analysis of changes in bonding, as well as structural and electronic properties for pressures and temperatures up to 200 GPa and 10 000 K, respectively. Similarities and differences between the high-P and T behavior of these fluids will be discussed. Calculations of the Hugoniot are in excellent agreement with available experimental data.
    06/2011;
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    ABSTRACT: We describe ab initio electronic structure calculations (DFT molecular dynamics and quantum Monte Carlo) of the equation of state of hydrogen in a regime relevant for ICF applications. We find the computed EOS to be quite close to that of the most recent SESAME table (constructed by G. Kerley, 2004). A simple density-dependent correction brings the recent SESAME EOS into nearly perfect agreement with ours in the chosen region. Simulations of ICF applications with this corrected SESAME table are discussed.
    06/2011;
  • A. M. Teweldeberhan, S. A. Bonev
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    ABSTRACT: The thermodynamic, electronic, and structural properties of liquid Na-Li and Ca-Li alloys at high pressure have been studied using ab initio molecular dynamics simulations. Gibbs free energies of pure Na, Ca, Li, and their mixtures (Na-Li and Ca-Li) are computed from vibrational density of states to determine the mixing-demixing behavior of the alloys. The computed electronic and structural properties of the mixtures with different concentrations are compared with the pure liquids up to around 265 GPa and 2000 K.
    Physical review. B, Condensed matter 04/2011; 83(13).
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    ABSTRACT: Accurate calculations of orbital energies for molecules chemisorbed on metal surfaces are important for understanding energy conversion, molecular scale transport, and charge transfer events at metal electrodes. Here, using density functional theory (DFT) and many-body perturbation theory within the GW approximation (GWA), we report the orbital energies of a well-studied molecule, benzene diamine (and derivatives), covalently bonded to aluminum and gold surfaces. For chemisorbed derivatives on Al surfaces, we predict a shift in the highest occupied molecular orbital resonance energy greater than 1 eV relative to the DFT result. We discuss our GWA results in the context of a model self-energy approach based on prior work [1], which can be applied to larger systems at greatly reduced computational cost. [4pt] [1] J. B. Neaton, M.S. Hybertsen, and S.G. Louie, PRL, 97, 216405 (2006)
    03/2011;
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    ABSTRACT: The properties of liquid CO2 have been studied through first-principles molecular dynamics simulations in the pressure-temperature range of 0-1 TPa and 200-100,000 K. The resulting equation of state data is used to predict shock Hugoniots for several initial conditions. Comparison with available experimental data up to 70 GPa is excellent. We find a gradual phase transition characterized by the destabilization of CO2 molecules and the formation of other molecular compounds. The liquid phase diagram is divided into several regimes based on a thorough analysis on changes in bonding, structural properties, and chemical composition. Calculations of optical properties such as conductivity and reflectivity will also be discussed.
    03/2011;
  • Brian Boates, Stanimir A. Bonev
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    ABSTRACT: We present first-principles calculations of the structural and electronic properties of liquid nitrogen in the pressure-temperature range of 0–200 GPa and 2000–6000 K. Upon polymerization, the liquid becomes metallic, similar to what has been reported for the higher temperature atomic fluid. An explanation of the electronic properties of the transformed liquids and the differences with the electronic properties of insulating solid cubic-gauche nitrogen is given based on the structure and bonding character of these phases. The mechanism responsible for charge transport in polymeric nitrogen systems is examined in order to understand the semiconducting nature of low-temperature amorphous nitrogen.
    Physical review. B, Condensed matter 02/2011; 83(17).
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    ABSTRACT: We report on the use of first-principles molecular dynamics calculations to examine properties of liquid carbon dioxide in the pressure-temperature range of 0-1 TPa and 200-100 000 K. The computed equations of state points are used to predict a series of shock Hugoniots with initial starting conditions that are relevant to existing and ongoing shock-wave experiments. A comparison with published measurements up to 70 GPa shows excellent agreement. We find that the liquid undergoes a gradual phase transition along the Hugoniot and have characterized this transition based on changes in bonding and structural properties as well as the conductivity and reflectivity of the fluid.
    The Journal of Chemical Physics 02/2011; 134(6):064504. · 3.16 Impact Factor
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    ABSTRACT: The phase diagram of Ca is examined using a combination of density-functional theory (DFT) and diffusion quantum Monte Carlo (DMC) calculations. Gibbs free energies of several competing structures are computed at pressures near 50 GPa. Existing disagreements for the stability of Ca both at low and room temperature are resolved with input from DMC. Furthermore, DMC calculations are performed on 0 K crystalline structures up to 150 GPa and it is demonstrated that the widely used generalized gradient approximation of DFT is insufficient to accurately account for the relative stability of the high-pressure phases of Ca. The results indicate that the theoretical phase diagram of Ca needs a revision.
    Physical Review Letters 12/2010; 105(23):235503. · 7.94 Impact Factor
  • A M Teweldeberhan, S A Bonev
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    ABSTRACT: Comment on the Letter by Yansun Yao et al, Phys. Rev. Lett. 103, 055503 (2009). The authors of the Letter offer a Reply.
    Physical Review Letters 05/2010; 104(20):209601; author reply 209602. · 7.94 Impact Factor
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    Isaac Tamblyn, Stanimir A Bonev
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    ABSTRACT: We examine the molecular-atomic transition in liquid hydrogen as it relates to metallization. Pair potentials are obtained from first principles molecular dynamics and compared with potentials derived from quadratic response. The results provide insight into the nature of covalent bonding under extreme conditions. Based on this analysis, we construct a schematic dissociation-metallization phase diagram and suggest experimental approaches that should significantly reduce the pressures necessary for the realization of the elusive metallic phase of hydrogen.
    The Journal of Chemical Physics 04/2010; 132(13):134503. · 3.16 Impact Factor

Publication Stats

363 Citations
311 Downloads
2k Views
203.58 Total Impact Points

Institutions

  • 2006–2013
    • Dalhousie University
      • Department of Physics and Atmospheric Science
      Halifax, Nova Scotia, Canada
    • University of California, Berkeley
      • Department of Physics
      Berkeley, MO, United States
  • 2003–2013
    • Lawrence Livermore National Laboratory
      • Condensed Matter and Materials Division
      Livermore, California, United States
  • 2007
    • University of Liège
      Luik, Walloon Region, Belgium
  • 2001–2003
    • Cornell University
      • Laboratory of Atomic and Solid State Physics
      Ithaca, NY, United States