Min Ji

Iowa State University, Ames, Iowa, United States

Are you Min Ji?

Claim your profile

Publications (42)76.64 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Information about the atomic structures at solid-solid interfaces is crucial for understanding and predicting the performance of materials. Due to the complexity of the interfaces, it is very challenging to resolve their atomic structures using either experimental techniques or computer simulations. In this paper, we present an efficient first-principles computational method for interface structure prediction based on an adaptive genetic algorithm. This approach significantly reduces the computational cost, while retaining the accuracy of first-principles prediction. The method is applied to the investigation of both stoichiometric and non-stoichiometric SrTiO3 Σ3(112)[-110] grain boundaries with unit cell containing up to 200 atoms. Several novel low-energy structures are discovered, which provide fresh insights into the structure and stability of the grain boundaries.
    The Journal of Physical Chemistry C 04/2014; 118:9524-9530. · 4.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The competition between the characteristic medium-range order corresponding to amorphous alloys and that in ordered crystalline phases is central to phase selection and morphology evolution under various processing conditions. We examine the stability of a model glass system, Cu–Zr, by comparing the energetics of various medium-range structural motifs over a wide range of compositions using first-principles calculations. We focus specifically on motifs that represent possible building blocks for competing glassy and crystalline phases, and we employ a genetic algorithm to efficiently identify the energetically favored decorations of each motif for specific compositions. Our results show that a Bergman-type motif with crystallization-resisting icosahedral symmetry is energetically most favorable in the composition range 0.63 < xCu < 0.68, and is the underlying motif for one of the three optimal glass-forming ranges observed experimentally for this binary system (Li et al., 2008). This work establishes an energy-based methodology to evaluate specific medium-range structural motifs which compete with stable crystalline nuclei in deeply undercooled liquids.
    Acta Materialia. 01/2014; 81:337–344.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a genetic algorithm (GA) for structural search that combines the speed of structure exploration by classical potentials with the accuracy of density functional theory (DFT) calculations in an adaptive and iterative way. This strategy increases the efficiency of the DFT-based GA by several orders of magnitude. This gain allows a considerable increase in the size and complexity of systems that can be studied by first principles. The performance of the method is illustrated by successful structure identifications of complex binary and ternary intermetallic compounds with 36 and 54 atoms per cell, respectively. The discovery of a multi-TPa Mg-silicate phase with unit cell containing up to 56 atoms is also reported. Such a phase is likely to be an essential component of terrestrial exoplanetary mantles.
    Journal of Physics Condensed Matter 12/2013; 26(3):035402. · 2.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Understanding the behavior of MgSiO3 post-perovskite (PPV) under extreme pressures is fundamental for modeling the interiors of super-Earth type exoplanets and the cores of solar giants. Previously, MgSiO3 PPV was predicted to dissociate into MgO and MgSi2O5 and then into MgO and SiO2 (Umemoto et al., Science 311, 983 (2006); Umemoto and Wentzcovitch, EPSL 311, 225 (2011)). Using the adaptive genetic algorithm, we predict new phase transitions in MgSiO3. The phase diagram calculated using the quasi-harmonic approximation shows that some transitions can occur in some super-Earths type exoplanets.
    03/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Based on density-functional calculation and genetic algorithm structure search, we propose a series of 16-coordinate core-shell clusters: M@Li(16)(M = Ca, Sr, Ba, Ti, Zr, Hf). A tetrahedral (T(d)) structure with an outer shell of 16 lithium atoms and one enclosed heavy atom is found to be the global minimum in the structural exploration of BaLi(16) based on genetic algorithm. This structure also has lower energy compared to the other isomers we employed in all the MLi(16) clusters. In this structure, the atoms are bonded together by metallic bonds with alkali (IA) and alkaline-earth (IIA) metal atoms. Their corresponding first electronic shells are closed with significant energy gaps because their total numbers of valence electrons fulfil the 18-electron rule. Such a combination could be extended to 20-electron systems by enclosing IVB elements. With simple valence electrons and highly symmetric structures, superatomic molecular orbitals are identified in all of the T(d) clusters.
    Nanoscale 03/2012; 4(8):2567-70. · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using genetic algorithm with first-principle calculations, we searched for low-energy crystal structures of Fe1−xCox alloys. We found that Fe1−xCox alloys are highly configurationally degenerate with many additional off-stoichiometric stable structures to the well-known B2 structure. The average magnetic moment of Fe atom increases with concentration of Co in the alloy, while that of Co atom is almost constant, which are consistent with experiments and earlier studies. The magnetic moment of Fe atom is strongly dependent on the number of Co nearest neighbor and it increases with this number.
    Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The discovery of the post-perovskite (PPV) transition of MgSiO3 in 2004 invited a new question: What would be the next phase transition from the PPV phase? The importance of this question has increased, since many terrestrial exoplanets with masses of a few to 10 times Earth's (super-Earth) have been recently discovered. Here we predict the new class of phase transitions of MgSiO3 PPV under ultrahigh pressure by first-principles calculations combined with the adaptive genetic algorithm, which is a powerful tool for blind structural searches for systems with the large number of atoms. We discuss implications of these new phase transitions in modeling of interiors of terrestrial exoplanets.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Recently, experiments observed phase separation in rapidly quenched Fe1-xCox alloy in the ordered α' phase [ J. Alloys Compd. 424, 145 (2006) ]. It is also believed that this is not an equilibrium result because of phase rule violation in the published phase diagram. To clarify this situation, we calculate the phase diagram of the system using cluster expansion in combination with a genetic algorithm. We calculated free energy of the system using super-cells up to 32 atoms with compositions ranging from 35 to 100 at. % Fe. Possible explanations for the experimental observations will be discussed.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using genetic algorithm with first-principles calculations, we performed a broad global search for low-energy crystal structures of Fe-rich Fe1-xCox alloys. We found that Fe-rich Fe1-xCox alloys are highly configurationally degenerate and there are many additional off-stoichiometric stable structures to the well-known stoichiometric FeCo - B2 structure, giving a possibility for atomistic manipulation of the alloys. The Co-Co nearest-neighbor pair is strongly unfavorable in Fe-rich Fe1-xCox alloys. The magnetic moment of Fe atom is increasing with Co concentration while that of Co atom is almost constant, inducing a Slater-Pauling curve for magnetic moment per atom. The magnetic moment of Fe atom is strongly dependent on the number of Co nearest-neighbors and it increases with this number.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We developed a fast and efficient method for crystal structure prediction and materials discovery. The method is based on the cut-and-paste genetic algorithm (GA) scheme introduced by Deaven and Ho [1]. In the evaluation of energies of target structures, first-principles calculations are accurate but time-consuming. Our method performs GA searches uses auxiliary model potentials to screen the energy of candidate structures, selecting only a few for more extensive first principles evaluation. Parameters of the auxiliary potentials are adaptively adjusted to reproduce the first-principles results during the course of the GA search. Our method combines the speed of empirical potential searches with the accuracy of first principles calculations. We will present results on applications to various systems including metallic alloys and ultrahigh pressure SiO2, H2O and Mg-Si-O systems. [4pt] [1] D. M. Deaven, K. M. Ho, Phys. Rev. Lett. 75, 288 (1995).
    02/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We propose three phases of H2O under ultrahigh pressure. Our structural search was performed using an adaptive genetic algorithm which allows an extensive exploration of crystal structure at density functional theory accuracy. The sequence of pressure-induced transitions beyond ice X at 0 K should be ice X-->Pbcm-->Pbca-->Pmc21-->P21-->P21/c phases. Across the Pmc21-P21 transition, the coordination number of oxygen increases from 4 to 5 with a significant increase of density. All stable crystalline phases have nonmetallic band structures up to 7 TPa.
    Physical review. B, Condensed matter 12/2011; 84. · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ice X is the highest-pressure form of H2O "ice" identified experimentally so far. Knowledge of phases beyond ice X is fundamental for modeling the interiors of icy solar giants (Uranus and Neptune) and icy exoplanets. Theoretically, Pbcm, Pbca, and Cmcm phases were proposed [Benoit et al, Phys. Rev. Lett. 76, 2934 (1996), Caracas, Phys. Rev. Lett. 101, 085502 (2008), Militzer and Wilson, Phys. Rev. Lett. 105, 195701 (2010)] as candidates for phases beyond ice X. Here we propose three new phases of H2O under ultrahigh pressure using the adaptive genetic algorithm. Our calculations reveal that the new sequence of pressure-induced phase transitions should be: ice X -> Pbcm -> Pbca -> Pmc21 -> P21 -> P21/c phases. Contrary to previous prediction, all stable crystalline phases should have nonmetallic band structure up to 7 TPa.
    AGU Fall Meeting Abstracts. 12/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using a first-principles genetic algorithm we predict an Fe2P phase is the first post-pyrite phase of SiO2 at low temperatures. This contrasts with a recently predicted cotunnite phase. Static enthalpy differences between these two phases are small near the transition pressure (0.69 TPa). While quasiharmonic free energy calculations predict an Fe2P-→cotunnite-type transition with increasing temperature, another phase, NbCoB type, is identified as being structurally and energetically intermediate between Fe2P and cotunnite phases. This structure suggests a possible temperature-induced gradual transformation between Fe2P and cotunnite phases. This finding would change our understanding of how planet-forming silicates, for example, MgSiO3 post-perovskite and its solid solutions, dissociate into elementary oxides at thermodynamic conditions expected in the interior of solar giants and exoplanets.
    Physical review. B, Condensed matter 05/2011; 83(18). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Here we propose a new phase of SiO2 beyond the pyrite-type phase. SiO2 is one of the most important minerals in Earth and planetary sciences. So far, the pyrite-type phase has been identified experimentally as the highest-pressure form of SiO2. In solar giants and extrasolar planets whose interior pressures are considerably higher than that on Earth, a post-pyrite transition in SiO2 may occur at ˜ 1 TPa as a result of the dissociation of MgSiO3 post-perovskite into MgO and SiO2 [Umemtoto et al., Science 311, 983 (2006)]. Several dioxides considered to be low-pressure analogs of SiO2 have a phase with cotunnite-type (PbCl2-type) structure as the post-pyrite phase. However, a first-principles structural search using a genetic algorithm shows that SiO2 should undergo a post-pyrite transition to a hexagonal phase, not to the cotunnite phase. The hexagonal phase is energetically very competitive with the cotunnite-type one.
    03/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Here we propose a new phase of SiO2 beyond the pyrite-type phase. SiO2 is one of the most important minerals in Earth and planetary sciences. So far, the pyrite-type phase has been identified experimentally as the highest-pressure form of SiO2. In solar giants and extrasolar planets whose interior pressures are considerably higher than that on Earth, a post-pyrite transition in SiO2 may occur at ~ 1 TPa as a result of the dissociation of MgSiO3 post-perovskite into MgO and SiO2 [Umemtoto et al., Science 311, 983 (2006)]. Several dioxides considered to be low-pressure analogs of SiO2 have a phase with cotunnite-type (PbCl2-type) structure as the post-pyrite phase. However, a first-principles structural search using a genetic algorithm shows that SiO2 should undergo a post-pyrite transition to a hexagonal phase, not to the cotunnite phase. The hexagonal phase is energetically very competitive with the cotunnite-type one. This work was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering and NSF under ATM-0428774 (VLab), EAR-0757903, and EAR-1019853. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. The computations were performed at the National Energy Research Supercomputing Centre (NERSC) and the Minnesota Supercomputing Institute (MSI).
    AGU Fall Meeting Abstracts. 12/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: It is well known that conversion of Si(111)-(7×7) into the (√3×√3)R30° phase of adsorbed Ag requires a change in the Si density, and causes formation of islands and holes at the surface. By mass balance, the ratio of areas of islands and holes (RIH) should be approximately 1. However, we find that the ratio is significantly higher, depending on preparation conditions. A possible explanation would be that there are different types of (√3×√3)R30° structures. However, neither scanning tunneling microscopy nor density-functional theory (implemented as a genetic algorithm search) supports this explanation. We propose that the edges of the islands contain excess Ag which becomes available to expand the holes, when the island perimeter decreases. Under certain conditions, excess Ag is also made available by dissolution of small islands that are Ag rich.
    Physical review. B, Condensed matter 12/2010; 82(24). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this work several crystal structure prediction problems which have been studied by first-principles evolutionary algorithms recently are revisited. We increased the system size to see how the search efficiency changes with respect to problem size. We find that the relative performance and underlying mechanism of genetic algorithms in crystal structure searches for Al(x)Sc(1-x) strongly depend on the system composition as well as the size of the problem. Because of this strong dependence, caution should be taken in generalizing performance comparison from one problem to another even though they may appear to be similar. We also investigate the performance of the search algorithm for crystal structure prediction of boron with and without a priori knowledge of the lattice vectors. The results show that the degree of difficulty increases dramatically if the lattice vectors of the crystal are allowed to vary during the search. Comparison of the minima hopping algorithm with the genetic algorithm at small (<10 atoms) to larger problem sizes is also carried out. At the small sizes we have tested, both methods show comparable efficiency. But at large sizes the genetic algorithm becomes advantageous over minima hopping.
    Physical Chemistry Chemical Physics 10/2010; 12(37):11617-23. · 4.20 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Plastic deformation in metallic nanowires was studied by atomistic simulations using empirical interatomic potentials based on embedded atom method (EAM). Several factors affecting the results of the deformation simulation such as temperature effect and the accuracy of the EAM potentials have been investigated. Deformation structures of Cu, Al and Au 100 nanowires after their yield point were analyzed and compared with each other. We found that metals with different stacking fault (SF) energy behavior differently during the plastic deformation at nanoscale. Furthermore deformation behavior of the nanowires under compression and tension were also compared.
    03/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Finding the global minimum structure of complex condensed matter systems is a long-standing challenge in computational physics. Genetic algorithm, basin-hopping or simulated annealing are popular methods to explore the configurational space. Here we applied these methods to periodical systems such as crystalline solids and alloys, metal on semiconductor surfaces and interface structures. Novel structures can be revealed in complex and multicomponent systems. The efficiency of different search strategies is also discussed.
    03/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Vacancy and hydrogen concentrations in Al were determined by first-principles calculations and statistical-mechanics modeling, as functions of temperature and hydrogen chemical potential μH. Formation energies of Al vacancies, H interstitials, and H-Al vacancy complexes were obtained from first-principles calculations. The statistical-mechanics model incorporated these energies and included configurational entropy contributions through the grand canonical ensemble. We found that the hydrogen chemical potential under different chemical environments plays an important role in determining the relative equilibrium defect concentrations in the Al-H system. Estimates of the hydrogen chemical potential during hydrogen charging were obtained experimentally. At comparable the calculated concentrations are consistent with these values, along with previously reported measurements of hydrogen concentration.
    Physical review. B, Condensed matter 01/2010; 81(2). · 3.77 Impact Factor