Douglas Galvao, Universidade Estadual de Campinas (UNICAMP)
Xingao Gong, Fudan University
Susan Sinnott, University of Florida
Frederic Soisson, CEA Saclay
ZZ2: Functional Materials
Monday PM, November 30, 2015
Sheraton, 2nd Floor, Independence West
2:30 AM - ZZ2.01
Computationally Driven Discoveries of New MAX-Phase Materials: From Magnetic MAX-Phases to 2D-ldquo;Dirac MXenesrdquo;
Igor Abrikosov 1 2 3 Martin Dahlqvist 1 Arni Sigurdur Ingason 1 Bjorn Alling 1 Johanna Rosen 1 Hossein Fashandi 1 Viktor Ivady 1 4 Mikhail Katsnelson 5
1Linkoping Univ Linkoping Sweden2NUST "MISIS" Moscow Russian Federation3Tomsk State University Tomsk Russian Federation4Hungarian Academy of Sciences Budapest Hungary5Radboud University Nijmegen NetherlandsShow Abstract
We demonstrate the power of modern first-principles theory by presenting examples of successful knowledge-based design of novel materials belonging to a family the so-called MAX phases. The MAX phases are a comparatively new class of nanolaminated compounds with a unique combination of metallic and ceramic properties. They have a common formula Mn+1AXn (n = 1-3), where M is an early transition metal,A is an A-group element, and X is carbon or nitrogen. However, no magnetic MAX phases were known prior to our work. Using first-principles calculations, we predicted a series of thermodynamically stable magnetic MAX phases, e.g. (Cr1-xMnx)2AlC , (Cr1-xMnx)2GeC , and Mn2GaC . The materials have been synthesized [2,3] and have shown a variety of intriguing magnetic behavior. Furthermore, we have investigated theoretically a series of two-dimensional materials synthesized by selective etching of the A element from the MAX phases, known as MXenes, and have discovered MXenes with Dirac electrons . The Dirac MXenes possess twelve conical crossings in the 1st Brillouin zone with giant spin-orbit splitting. Our findings indicate that the 2D band structure of MXenes is protected against external perturbations and it is preserved even in multilayer phases. These results, together with experimental advances in MXene phases, presents a novel entire class of two-dimensional materials that may exhibit Dirac fermions with corresponding properties.
. M. Dahlqvist, B. Alling, I. A. Abrikosov, J. Rosen, Phys. Rev. B 84, 220403(R) (2011).
. A. S. Ingason, A. Mockute, M. Dahlqvist, F. Magnus, S. Olafsson, U. B. Arnalds, B. Alling, I.A. Abrikosov, B. Hjörvarsson, P. O. Å. Persson, and J. Rosen, Phys. Rev. Lett. 110, 195502 (2013).
 A. S. Ingason, A. Petruhins, M. Dahlqvist, F. Magnus, A. Mockute, B. Alling, L. Hultman, I. A. Abrikosov, P. O. Å. Persson, and J. Rosen, Mater. Res. Lett. 2, 89 (2014).
 H. Fashandi, V. Ivády, P. Eklund, A. Lloyd. Spetz, M. I. Katsnelson, and I. A. Abrikosov, arXiv:1506.05398 [cond-mat.mtrl-sci]
3:00 AM - ZZ2.02
Local Probing of Ferroelectric and Ferroelastic Switching by Phase-Field Simulation
Ye Cao 1 5 David Edwards 2 Stephen Jesse 1 5 Long-Qing Chen 3 Amit Kumar 2 Nazanin Bassiri-Gharb 4 Sergei V. Kalinin 1 5
1Oak Ridge National Laboratory Oak Ridge United States2Queenrsquo;s University Belfast Belfast United Kingdom3The Pennsylvania State University University Park United States4Georgia Institute of Technology Atlanta United States5Oak Ridge National Laboratory Oak Ridge United StatesShow Abstract
Scanning probe induced mechanical switching has recently emerged as an alternative method for domain manipulation due to their highly localized and electrically erasable characteristics.  Therefore the fundamental understanding of the role of local strains to the large effective piezoelectric and ferroelectric response of ferroelectric thin films is imperative. Here, we developed a self-consistent phase-field model to investigate the tip pressure (600nN) induced domain reorganization in 50nm thick (001) oriented lead zirconate titanate (Pb(Zr1-xTix)O3) films of rhombohedral (R) (x=0.2), tetragonal (T) (x=0.8) and morphotropic phase boundary (MPB) compositions (x=0.47). We found that at T phase composition c(001) domains is switched into a1(100) and a2(010) lateral domains and their growths are blocked by a/c twin domain walls, while at R phase composition the initial upward r1+ (111) and r4+ (1-11) domains are vertically switched into downward r1-(11-1) and r4-(1-1-1) domains respectively. Both switchings occur in limited 10nm deep regions from the top surface where the tip is located. At MPB composition R-T switching occurs, confirming that a transition between the R and T phase is ferroelastically active and can be directly induced by externally applied pressure. The domain switching at MPB is found to penetrate the entire film thickness, highlighting that the MPB composition is indeed softer than either pure R and T compositions. Our simulation results substantiate the experimental observation of change in hysteretic piezoresponse and domain structure caused by externally applied pressure. The phase-field model, combined with strain-mediated spectroscopy approach shows capability of exploring switching behaviors in a spatially resolved manner which would be otherwise difficult to resolve.
This research was sponsored by the Division of Materials Sciences and Engineering, Basic Energy Sciences, Department of Energy (YC, SJ, SVK). Research was conducted at the Center for Nanophase Materials Sciences, which also provided support and which is a DOE Office of Science User Facility. NBG gratefully acknowledges funding from the US National Science Foundation under grant number DMR-1255379. LQC gratefully acknowledges funding from DOE under Award No. DE-FG02-07ER46417.
 H. Lu, C.-W. Bark, D. Esque de los Ojos, J. Alcala, C. B. Eom, G. Catalan and A. Gruverman, Science 336, 59 (2012)
3:15 AM - *ZZ2.03
Functional Properties of Superlattices from First-Principles Models
Karin M. Rabe 1
1Rutgers Univ Piscataway United StatesShow Abstract
The physics of perovskite superlattices is well-known to be rich and varied. A feature of particular interest is that the properties of a superlattice can be enhanced over or even distinct from those of the constituent compounds, depending furthermore on the choice of layer thicknesses, termination and epitaxial strain. In this talk I describe recent work on first-principles-based modeling of polarization, dielectric response and piezoelectric response in ferroelectric-ferroelectric BaTiO3/PbTiO3 superlattices and of a first-order coupled magnetic nonpolar-polar metal-insulator transition in epitaxially strained SrCrO3/SrTiO3. The use of the first-principles models to interpret and interpolate first-principles results forms an essential part of a guided-sampling first-principles high-throughput approach allowing the identification of superlattice materials with optimal functional behavior targeted to specific technological applications and potentially the discovery of further novel physical phenomena in these systems.
4:15 AM - ZZ2.04
First Principles Computational Screening of Earth-Abundant Nitride Semiconductors
Yoyo Hinuma 1 Taisuke Hatakeyama 3 Yu Kumagai 2 Yoshinori Muraba 3 Hikaru Sato 3 Hidenori Hiramatsu 2 3 Isao Tanaka 1 Hideo Hosono 2 3 Fumiyasu Oba 1 2 3
1Kyoto University Kyoto Japan2Tokyo Institute of Technology Yokohama Japan3Tokyo Institute of Technology Yokohama JapanShow Abstract
Semiconductors obviously cover very wide applications in electronics, optoelectronics, and photovoltaics, and suggestions for novel useful semiconducting materials are always welcome. In particular, semiconductors that consist of earth-abundant and non-toxic elements only are especially attractive; stable production is possible because short supply of key elements is less likely to happen and these materials are likely to be environmentally friendly. Nitride semiconductors are appealing because the anion is nitrogen, a very common element, but those currently commercialized are mostly limited to GaN and related alloys. This motivates us to investigate ternary nitride compounds using high-throughput first principles calculations to identify promising semiconductors. Our algorithm involves looking at thermodynamic and lattice dynamic stability as well as electronic properties. The algorithm identifies previously reported nitride ternaries. ZnSnN2 is a relatively new material that is recently gaining interest because this compound consists of earth-abundant elements only . Its theoretical direct band gap is around 1.4 eV, which makes it an excellent candidate for potential applications in thin film photovoltaics. Other examples of previously reported ternary nitrides include LiZnN , ZnSiN2  and ZnGeN2 . Unreported and promising compounds are also found through calculations.
 L. Lahourcade et al., Adv. Mater. 25, 2562 (2013).  K. Kuriyama et al., Phys. Rev. B 49, 4511 (1994).  T. Endo et al., J. Mater. Sci. Lett. 11, 424 (1992).
4:30 AM - ZZ2.05
The Role of Low-Lying Optical Phonons in Lattice Thermal Conductance of Rare-Earth (RE) Pyrochlores: A First-Principle Study
Guoqiang Lan 1 Jun Song 1
1McGill Univ Montreal CanadaShow Abstract
Rare-earth pyrochlores, commonly exhibiting anomalously low lattice thermal conductivities, are considered as promising topcoat materials for thermal barrier coatings. However the structural origin underlying their low thermal conductivities remain unclear. In the present study, we investigated the phonon properties of two groups of RE pyrochlores, Ln2Zr2O7 (Ln = La, Nd, Sm, Gd) and Gd2T2O7 (T = Zr, Hf, Sn, Pb) employing density functional theory and quasi harmonic approximation. Through the relaxation time approximation (RTA) with Debye model, the thermal conductivities of those RE pyrochlores were predicted, showing good agreement with experimental measurements. The low thermal conductivities of RE pyrochlores were shown to largely come from the interference between the low-lying optical branches and acoustic branches. The structural origin underlying the low-lying optical branches was then clarified and the competition between scattering processes in transverse and longitude acoustic branches was discussed.
4:45 AM - *ZZ2.06
Ab Initio Screening of Photovoltaic Semiconductors with Benign Defect Properties
Shiyou Chen 1
1East China Normal University Shanghai ChinaShow Abstract
The number of component elements increased steadily in the 60-year development of photovoltaic semiconductors, i.e., from silicon in 1950s, to GaAs and CdTe in 1960s, CuInSe2 in 1970s, Cu(In,Ga)Se2 in 1980s, Cu2ZnSnS4 in 1990s and more recently Cu2ZnSn(S,Se)4 and CH3NH3PbI3. The increased number of elements makes the material properties more flexible, however, it also causes the dramatic increase of possible point defects in the lattice, which can significantly influence the optical and electrical properties and thus the photovoltaic performance of these multinary compound semiconductors. Whether they can work as ideal solar cell absorber material depends on the behavior of their intrinsic defects. Through ab initio calculations, we can predict the dominant defects in new semiconductors and determine whether there are high concentration of deep-level defects that may act as electron-hole recombination centers. Then the semiconductors free of detrimental defects can be identified, which will accelerate the discovery of new photovoltaic semiconductors with high energy-conversion efficiency. I will discuss such ab initio screening in four classes of semiconductor systems, including the quaternary I2-II-IV-VI4 (Cu2ZnSnS4, Cu2ZnSnSe4), the ternary I-V-VI2 (CuSbS2 and CuSbSe2), the binary Sb2Se3 as well as the halide perovskites (CsSnI3, CH3NH3SnI3 and CH3NH3PbI3), which were all proposed as the candidate photovoltaic materials with high efficiency. Based on the calculated formation energies (concentration) and transition energy levels of possible defects, I will discuss the influence of the chemical component and growth conditions on the defect formation/ionization and thus the electrical and optical properties of the samples, which will help us understand the related experiments and also judge whether the defects impose any intrinsic limit to the efficiency of these photovoltaic semiconductors.
(1) S. Chen, A. Walsh, X.-G. Gong, S.-H. Wei, Adv. Mater. 25, 1522 (2013)
(2) P. Xu, S. Chen, H.-J. Xiang, X.-G. Gong, S.-H. Wei, Chem. Mater. 26, 6068 (2014)
(3) B. Yang, L. Wang, J. Han, Y. Zhou, H. Song, S. Chen, J. Zhong, L. Lv, D. Niu, J. Tang, Chem. Mater. 26, 3135 (2014)
(4) A. Walsh, D. Scanlon, S. Chen, S.-H. Wei, X.-G. Gong, Angew. Chem. Int. Ed. 54, 1791 (2015)
(5) Y. Zhou, L. Wang, S. Chen, S. Qin, X. Liu, J. Chen, D. Xue, M. Luo, Y. Cao, Y. Cheng, E. Sargent, J. Tang, Nature Photonics 9, 409 (2015)
5:15 AM - ZZ2.08
First-Principles Study of Microporous Magnets M-MOF-74 (M = Ni,Co, Fe, Mn): The Role of Metal Centers
Zhang Qiu Ju 1
1Ningbo Institute of Material technology and engineering, CAS Ningbo ChinaShow Abstract
It is anticipated that coupling the intrinsic magnetism and porosity within the same material will offer an ideal medium for applications in magnetic separation, magnetic sensing, or low-density magnets. Beyond the adsorption and separation applications, some MOFs also exhibit some unique magnetic properties. Therefore, a clear understanding of the origin of magnetism in metalminus;organic frameworks (MOFs) would provide useful insight for tuning the electromagnetic properties of MOFs and finding new applications. Motivated by the manipulation of magnetic ordering with metal-doping methods, we chose M-MOF-74 (M=Mn, Fe, Co, Ni) to systematically investigate the electromagnetic behavior in MOFs by means of a density functional theory (DFT+U) method because isostructural MOF-74 containing various paramagnetic metal centers have been successfully synthesized.
Our calculated results show that the open paramagnetic metal sites in three-dimensional porous magnets M-MOF-74 (M = Ni, Co, Fe, Mn) favor high-spin electronic arrangement. Fe- and Co-MOF-74 exhibit ferromagnetic (FM) features and significantly distinct energy gaps between spin-up and spin-down channels in metastable states. After replacement of the Co center with a Ni ion, the FM feature is exhibited for the stable state since the “extra” valence electron is filled in the spin-down 3d bands to shift the Fermi level to higher energy. In contrast, after removal of one valence electron (i.e., replacement of the Fe center with Mn atoms), the energy gap is significantly enlarged and an antiferromagnetic (AFM) feature will be discerned. Our studies may enhance the understanding of the origin of magnetism in MOFs and provide useful insight for tuning the electromagnetic properties of MOFs and designing low-magnetic materials.
ZZ1: First Principles Calculations of Electronic Materials and Alloys
Monday AM, November 30, 2015
Sheraton, 2nd Floor, Independence West
9:00 AM - *ZZ1.01
Hierarchical Structure and Mechanics of Plants
Lorna J. Gibson 1
1MIT Cambridge United StatesShow Abstract
The cell walls in plant tissues are made up of just four basic building blocks: cellulose, the main structural fiber of the plant kingdom, hemicellulose, lignin and pectin. Although the microstructure of plant cell walls varies in different types of plants, broadly speaking, cellulose fibers reinforce a matrix of hemicellulose and either pectin or lignin. The cellular structure of plants varies from the honeycomb-like cells of wood to the closed-cell, liquid-filled foam-like parenchyma cells of apples and potatoes. The arrangement of the four basic building blocks plant cell walls and the variations in cellular structure give rise to a remarkably wide range of mechanical properties: the Young&’s moduli span 4 orders of magnitude while the compressive strengths span nearly 3 orders of magnitude. Here, we review the microstructure of both the cell wall and the cellular structure in four plant materials (wood, arborescent palm stems, bamboo and parenchyma) to explain the wide range in mechanical properties in plants.
9:30 AM - ZZ1.02
Electronic Structure of Si Nano Sheets under the Strain Induced by the Surface Oxidation
Mina Park 1 Seungchul Kim 1 Kwang-Ryeol Lee 1
1KIST Seoul Korea (the Republic of)Show Abstract
Nano structure of Si has drawn much attention owing to their unique electronic and optical properties. Recently, two dimensional silicon nano sheets (Si-NSs) were successfully synthesized [1,2] and extensively investigated on the possible applications to novel devices. It is common for the nanostructured materials to be under high level of stressed state due to the significant surface or interfacial effect. In the present work, we focused on the effect of lattice strain induced by surface oxidation of Si-NSs on their electronic band structures. We employed the density functional theory for the electronic structure calculation of the oxidized Si-NSs simulated by reactive molecular dynamics simulation. Si-NSs exhibit either direct or indirect band gap depending on the growth orientation while the bulk Si shows only indirect band gap. Moreover, applying bilateral strain to the Si-NSs induces the change in the conduction band minimum of the electronic band structure and leads to the band gap transition. For the (111) surface orientation, Si-NSs undergoes the indirect-to-direct transition by tensile strain. Meanwhile, (001)/(110) orientation Si-NSs undergoes the direct-to-indirect transition by compression. We suppose that the structural deformation due to the strain leads to the adjustment of the conduction band minimum energy according to its bonding or antibonding character. The present work revealed a general tendency that increasing the bond length along the lateral dimension enhanced the Si-NSs to possess the direct band gap characteristics. This finding would be very important for the strain engineering of the Si-NSs, because a slight change in the surface state such as surface oxidation can affect significantly the physical properties of the materials.
 U. Kim et al, “Synthesis of Si Nanosheets by a Chemical Vapor Deposition Process and Their Blue Emissions”, ACS Nano, 5, 2176 (2011).
 S. Kim et al, “Two-dimensionally Grown Single-Crystal Silicon Nanosheets with Tunable Visible-Light Emissions”, ACS Nano, 8, 6556-6562 (2014).
9:45 AM - ZZ1.03
Electronic Transport in VO2: A DFT - Boltzmann Transport Approach
Alper Kinaci 1 Motohisa Kado 2 Daniel Rosenmann 1 Chen Ling 2 Gaohua Zhu 2 Debasish Banerjee 2 Maria K Chan 1
1Argonne National Laboratory Lemont United States2Toyota Motor Eng amp; Mfg NA Ann Arbor United StatesShow Abstract
Materials that undergo metal-insulator transitions (MITs) are under intense study because the transition is scientifically fascinating and technologically promising for various applications. Among these materials, VO2 has served as a prototype due to its favorable transition temperature. While the physical underpinnings of the transition have been heavily investigated experimentally and computationally, quantitative modeling of electronic transport in the two phases has yet to be undertaken. In this work, we establish a density-functional-theory-based (DFT) approach to model electronic transport properties in VO2 in the semiconducting and metallic regimes, focusing on band transport using Boltzmann transport equation. Free parameters in the model are calibrated using experimentally measured transport quantities. We find that the approach can efficiently model the metallic and semiconducting phases. Using this methodology, we performed high throughput DFT calculations to investigate effects of doping on VO2 from mechanical, thermodynamic and electronic transport aspects.
10:00 AM - *ZZ1.04
Electronic Structure and Charge Transport in Nanosystems with Ab Initio Calculations
Lin-Wang Wang 1
1Lawrence Berkeley National Lab Berkeley United StatesShow Abstract
In this talk, I will present our recent simulations of nanostructure systems using linear scaling three dimensional fragment method (LS3DF) and nonadiabatic molecular dynamics (NMD). The LS3DF method is used to study the effects of Moire's pattern of a double layer MoS2 and MoSe2, and it is found that the structural Moire's pattern will localize the electronic states in such systems. The LS3DF method is also used to study the effect of electrostatic potential fluctuations in hybrid perovskite (CH3NH3)PbI3, and found that the random orientation of the organic molecule will cause carrier localization in the system, which will have significant impact on the carrier transport in the system. NMD is used to study the carrier transport in a monolayer of organic molecule, and it is found that the dynamics disorder play an important role in the carrier transport of such systems. The same method is also used to study carrier transport in the hybrid perovskite system.
11:00 AM - *ZZ1.05
Understanding Complex Materials at Finite Temperatures by Ab Inito Methods
Tilmann Hickel 1 Biswanath Dutta 1 Albert Glensk 1 Fritz Koermann 1 Blazej Grabowski 1 Joerg U. Neugebauer 1
1Max-Planck-Institut fuuml;r Eisenforschung GmbH Duesseldorf GermanyShow Abstract
Fully parameter-free ab initio methods based on density functional theory are steadily gaining popularity. Their atomistic view on physical processes and the access to chemical trends are attractive for a knowledge-driven development of novel electronic, biological or engineering materials. However, the apparent restriction of the method to ground state properties is a severe challenge for the understanding and prediction of many materials properties, as, e.g., heat capacities and phase diagrams, for which finite temperature effects are decisive. Over the last years we have developed a large range of multi-physics tools to go beyond this limitation. A key challenge was and is the accurate determination of free energies including all relevant excitation mechanisms individually as well as non-adiabatic coupling effects stabilizing certain phases. Within this talk we will discuss some of our recent methodological developments that foster a large-scale screening of material properties even at temperatures where anharmonic lattice vibrations and magnetic disorder become important. The finite-temperature ab initio methods will finally be applied to the calculation of martensitic and intermartensitic phase transitions in Ni-Mn-X Heusler alloys. The predicted chemical trends are important for a tailored design of their magnetocaloric and shape-memory properties.
11:30 AM - ZZ1.06
Computational Screening of Half-Heusler Compounds for Thermoelectric Applications Using Electron-Phonon-Averaged Approximation
Georgy Samsonidze 1 Boris Kozinsky 1
1Robert Bosch LLC Cambridge United StatesShow Abstract
We present a simple and efficient approximation to the quantum mechanical formulation of the electron relaxation time induced by electron-phonon interaction. The approximation is suited for the high temperature regime providing a practical procedure for high-throughput screening of thermoelectric materials. The method is applied to calculate the electronic transport coefficients and to determine the optimal carrier concentration which maximizes the thermoelectric figure of merit ZT at a target temperature. Computational screening of optimized ZT values is performed in the compositional space of half-Heusler compounds selected from materials databases and consisting of cheap earth-abundant elements. The method is validated by demonstrating agreement with the results of the exact electron-phonon calculations and with the experimental data for the currently used compounds. Several new compounds promising for thermoelectric applications are identified, one of which was successfully synthesized by our experimental collaborators . The computational results are examined for the presence of correlations between the ZT values and the material properties of half-Heusler compounds. In particular, we find deviations from the Wiedemann-Franz law in these compounds at low carrier concentrations and high temperatures by directly computing electrical and the electronic part of the thermal conductivities.
 G. Joshi et al., Energy Environ. Sci. 7, 4070 (2014)
11:45 AM - ZZ1.07
Solubility of Hydrogen and Vacancy Concentration in Nickel from First Principles Calculations
Arnaud Metsue 1 Abdelali Oudriss 1 Xavier Feaugas 1
1University of La Rochelle La Rochelle FranceShow Abstract
The incorporation of hydrogen in metals can strongly affect the physical properties of the host matrix and can lead to irreversible damages. Therefore, the solubility of hydrogen in metals is a fundamental data to design new protections preventing hydrogen embrittlement and safety materials. However, the apparent solubility can be influenced by the presence of crystalline defects such as point defects, dislocations or grain boundaries. In particular, it has been suggested that hydrogen promotes the formation of new vacancies, which participates to the mechanisms of degradation of metals (Fukai, 2003, J. Alloys Comp.). In this study, we determine the vacancy and hydrogen concentrations in nickel at thermodynamic equilibrium up to the melting point from a combination of first principles calculations and statistical physic.
First, we determine the solution enthalpy and entropy of the incorporation of H in a perfect crystal. We extend the calculations of these thermodynamic quantities to the formation of H-vacancy clusters where H atom is located in the displacement field generated by the vacancy. We limit the study to the interstitial sites inside and tangent to the vacancy core where the displacement field leads to a significant deformation of the sites (Metsue et al., 2014, Phil. Mag.). The calculations are performed up to the melting point for a wide range of H2 chemical potential where the Gibbs free energy is expressed as a sum of vibration and electronic contributions from the computation of the phonon dispersion curve and the electronic density of states. The latter contribution is required to get a realistic vacancy concentration (Metsue et al., 2014, J. Chem. Phys.) The solution enthalpies and entropies calculated for a H2 fugacity of 1 bar are in good agreement with previous experimental data (Eichenuaer et al., 1965, Z. Metall., Stafford et al., 1974, Acta Met.). In addition, we found that H-rich octahedral sites inside the vacancy core are destabilized at high H2 chemical potential, similarly to fcc Fe (Nazarov et al., 2010, Phys. Rev. B).
Then, we calculate the solubility and the vacancy concentration from the solution and H-vacancy clusters formation Gibbs free energy. We take into account an additional configuration part to the total entropy related to the distribution of H in the interstitial sites and the H-vacancy clusters. A close agreement is found between experiments and the hydrogen concentration calculated for a H2 fugacity of 1 bar. In particular, we show that the electronic excitations lead to a positive deviation in the Arrhenius plot of the solubility at temperatures close to the melting point. The vacancy concentration is calculated for a wide range of H2 chemical potential and we discuss its implications on the deformation of the lattice.
12:15 PM - ZZ1.09
Magnetic Exchange Interactions and Critical Temperature of the Nanolaminate Mn2GaC from First-Principles Supercell Methods
Andreas Thore 1 Bjorn Alling 1 Johanna Rosen 1
1Linkouml;ping University Linkouml;ping SwedenShow Abstract
The magnetic critical order-disorder temperature Tc of a material is a parameter that sets an upper operational limit for magnetic devices. First-principles based methods hold the promise of significantly speeding up the search for new magnetic materials by predicting Tc and thus guide experiments.
In this work, we employ and critically evaluate a first-principles approach based on supercell calculations for predicting Tc. Supercell calculations have the benefit of allowing for straightforward incorporation of structural or vibrational disorder effects on the magnetic interactions. As a model material we use the recently discovered nanolaminate Mn2GaC,1 which belongs to the family of so-called MAX phases (where M is an early transition metal, A an A-group element, and X is C and/or N).
First, we derive the exchange interaction parameters Jij between pairs of atoms i and j of the bilinear Heisenberg Hamiltonian using the novel magnetic direct cluster averaging method (MDCA).2 In this method, the J&’s are calculated through an averaging procedure over a large number of supercells in which the magnetic state has been randomly generated with respect to all magnetic atoms except for those of atoms i and j, for which Jijis sought. We compare the J&’s from the MDCA calculations to the same parameters calculated using the Connolly-Williams structure inversion method, and show that the two methods yield closely matching results.
Secondly, we use the MDCA-derived J's as input parameters in Monte Carlo simulations to calculate the magnetic energy, total magnetic moment, specific heat, magnetic susceptibility as well as Tc. For Mn2GaC, we find Tc = 660 K. The uncertainty in the calculated value of Tc caused by uncertainties in the J&’s is discussed and exemplified in our case by a calculation of the standard deviation, which is 133 K.
1 A. S. Ingason, A. Petruhins, M. Dahlqvist, F. Magnus, A. Mockute, B. Alling, L. Hultman, I. A. Abrikosov, P. O. Å. Persson, and J. Rosen, Materials Research Letters 2, 89 (2014).
2 A. Lindmaa, R. Lizárraga, E. Holmström, I. A. Abrikosov, and B. Alling, Physical Review B 88, 054414 (2013).
Douglas Galvao, Universidade Estadual de Campinas (UNICAMP)
Xingao Gong, Fudan University
Susan Sinnott, University of Florida
Frederic Soisson, CEA Saclay
ZZ4: Modeling Material Damage
Tuesday PM, December 01, 2015
Sheraton, 2nd Floor, Independence West
2:30 AM - ZZ4.01
Atomistic Modeling of Xe Adsorption on UO2 Surfaces
Jack Arayro 1 Fabienne Ribeiro 1 Guy Treglia 2
1Institute of Radioprotection and Nuclear Safety (IRSN) Marseille France2Interdisciplinary Nanoscience Center of Marseille (CINaM) Marseille FranceShow Abstract
Uranium dioxide UO2 is used as a standard fuel in pressurized water reactors (PWR). During fission reactions of uranium intragranular bubbles of xenon are generated. The presence of these bubbles modifies the thermomechanical properties of the fuel. Challenges in terms of safety due to the presence of these bubbles led to extensive work both from experimental and theoretical points of view, in order to better understand the changing of the physical properties and behavior of the fuel.
It is known from the literature that these bubbles are microfaceted, with (111) and (100) surfaces. We then study here simplified systems of xenon on semi infinite UO2 surfaces. In a first step, we assess the relative stability of UO2 surfaces according to their orientation and then to their polarity, by mixing thermostatistical relaxation and analytic formulations within a simple electrostatic model. The main result is that, whereas the (111) surface appears stable by construction and does not involved major reorganization, the polar (100) one is only stabilized through drastic rearrangement of the surface region.
In a second step, we proceed to xenon adsorption on these relaxed surfaces through Monte Carlo simulations in the grand canonical ensemble within semi-empirical potentials. The most striking feature revealed by the simulation is the existence of a phase transition from a dilute xenon phase towards a dense one, in which coexist the FCC, BCC and HCP structures, by increasing the chemical potential. Otherwise, the xenon density is found to increase with the temperature for a given chemical potential.
In a third step, the pressure inside the xenon bubble and in the UO2 matrix has been investigated. In the former case, we show that whatever the xenon structure may be, the pressure increases with the xenon density, but not with the temperature for a fixed density. In what concern the matrix, we will present the pressure profile before and after xenon adsorption. The next step will be to introduce these so obtained results in a micromechanical model, which will allow us to propose a thermomechanical behavior law for the porous UO2.
2:45 AM - ZZ4.02
Cluster Dynamics in Irradiated UO2: Off-Stoichiometric Considerations for Voids, Loops and the Oxide Matrix
Sarah Khalil 1 Todd Allen 3 Anter El-Azab 2 Ahmed Hamed 2
1UW - Madison Madison United States2Purdue University West Lafayette United States3Idaho National Lab Idaho Falls United StatesShow Abstract
A novel Cluster Dynamics (CD) model that describes the nucleation and evolution of defect clusters in oxides systems has been developed. The model has been used to predict clustering of vacancies and interstitials into voids and dislocation loops, respectively, in irradiated UO2. The model reproduces well a range of experimental data on nucleation and growth behavior and its temperature dependence. A very important feature of this model is its ability to predict the off-stoichiometry (or composition) of defect clusters, allowing, in turn, for the tracking of off-stoichiometry of the matrix. The effect of migration energy of point defects on the concentration and average size of voids has been studied. Also, the effect of irradiation conditions such as irradiation temperature, irradiation dose and dose rate on clusters concentration and composition has been investigated. The preliminary results show that Frenkel defects, as opposed to Schottky defects, dominate the nucleation process in irradiated UO2. Vacancy clusters tend to grow mainly by absorbing oxygen vacancies and the migration energy of uranium vacancies is the rate limiting energy in nucleation and growth of voids. The results also show that, in a stoichiometric UO2 under irradiation, vacancy clusters (voids) tend to have both hypo- and hyper-stoichiometric composition with a higher fraction of hyper-stoichiometric composition clusters. Hyper-stoichiometric cluster compositions indicate that the matrix would become oxygen rich even if the initial state is perfectly stoichiometric.
3:00 AM - *ZZ4.03
The Role of Chemistry and Disorder on Ionic Conductivity in Pyrochlore
Blas P. Uberuaga 1 Romain Perriot 1
1Los Alamos Nat'l Lab Los Alamos United StatesShow Abstract
Complex oxides are being extensively studied as superionic materials (in which ionic conductivity is large) for application as fuel cell and battery materials. Experimental studies have revealed that the ionic conductivity in A2B2O7 pyrochlores is sensitive to the chemical composition of the material, with, in particular, B=Ti pyrochlores exhibiting relatively low ionic conductivities and B=Zr pyrochlores having much higher conductivities. This difference in conductivity has been linked to the tendency of the material to exhibit cation disorder (the swapping of A and B cations). The disordering tendency of the material depends on its chemical composition, such that a change in chemistry is often accompanied by a concurrent change in the level of disorder, leaving some doubt as to the true origin (chemistry or disorder) of the enhancement in conductivity. Meanwhile, cation disorder can be controllably introduced into these materials via irradiation and much work has been done examining the disordering that occurs in pyrochlores as a function of the irradiation dose, as these materials are also proposed for nuclear waste forms. However, the two properties - ionic conductivity and radiation-induced disordering - have not been explicitly connected.
Here, in support of ongoing experimental studies, we examine the independent contributions of chemistry and disorder on ionic conductivity in two pyrochlores, Gd2Ti2O7 and Gd2Zr2O7. Using molecular dynamics and accelerated molecular dynamics, we isolate the role that the B-site chemistry and the A-B disorder have on oxygen ionic conductivity. We examine both structural defects, i.e. vacant oxygen sites relative to the basic fluorite structure, as well as intrinsic defects (interstitials and vacancies) that might be present after irradiation. We find contrasting effects of both disorder and chemistry on the mobility of each type of defect. In particular, disorder always increases the conductivity associated with the structural defects, but low levels of disorder can inhibit the motion of intrinsic defects. We contrast this behavior with that found in AB2O4 spinels, where disorder decreases the mobility of oxygen defects. These results highlight the non-trivial and contrasting effects associated with ionic conductivity in these complex materials.
4:00 AM - ZZ4.04
Non-Adiabatic Aspects of the Initial Stages of Radiation Damage
Magdalena Caro 1 Alfred Correa 2 Alfredo Caro 1
1Los Alamos National Laboratory Los Alamos United States2Lawrence Livermore National Laboratory Livermore United StatesShow Abstract
We report on non-adiabatic aspects of the initial stages of radiation damage, when ions and electrons get out of mutual equilibrium, and energy is exchanged between them, determining the ways radiation energy is dissipated. The hypothesis behind this work is that by understanding the mechanisms of energy dissipation by electrons and ions, we provide a means to manipulate them to reduce the defect production, and quench the damage. Our work describes quantitatively the energy transfer process from electrons to atoms via ionization, a process known as Coulomb explosion.
Recently, we reported on a correlation between nuclear and electronic stopping power that revealed the effects of ionization on the ions dynamics [Correa et al. Phys. Rev. Lett. 108 (2012)069901]. In this work, we use the same TD-DFT methodology to determine the time dependent forces experienced by Ni target atoms as a Ni projectile moves along its trajectory. Similar to our previous observation, we find large differences in the magnitude of momentum transfer as compared to the value given by the adiabatic approximation. This effect is the quantitative signature of the Coulomb explosion (CE). With these results in mind, we discuss the impact CE has on the modification of the energy dissipation pathways in the early stage of defect formation.
Implications of these results in more accurate computer simulations of radiation damage are expected, as both the nuclear and electronic aspects of the problem are both included in the atomic-scale simulations of radiation damage.
Work supported by the Energy Dissipation to Defect Evolution Center (EDDE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
4:15 AM - ZZ4.05
Acceleration of Cr Precipitation in alpha;-Fe by Irradiation
Frederic Soisson 1
1CEA Saclay Gif-sur-Yvette FranceShow Abstract
Iron-chromium alloys are the basis for ferritic and ferritic-martensitic steels that will be used in future fission (generation IV) and fusion nuclear reactors. With Cr contents between typically 8 to 12%, or even 14% in the matrix of some oxide dispersion-strengthened steels, one expects a precipitation of a Cr-rich phase (α&’), leading to the hardening and embritshy;tlement of the materials. For such compositions and at temperatures of interest, the kinetics of precipitation during a thermal ageing is usually very slow, but it can be strongly accelerated under irradiation, due to point defect supersaturation. We present a modeling of the α&’ precipitation under irradiation, based on atomistic kinetic Monte Carlo simulations, with parameters fitted on ab initio calculations. The Monte Carlo simulations are combined with a cluster dynamics model to get an accurate description of the evolution of sink densities, which control the point defect supersaturation. The simulations are used to predict the acceleration factor, depending on the alloy composition, irradiation flux and temperature. They are compared to available experimental results. The possible effects of ballistic mixing (which occurs in displacement cascades) and of the strong interaction between carbon atoms and vacancies, are also discussed.
4:30 AM - ZZ4.06
Dislocation Core Reconstruction Induced by Solute Atom Segregation in bcc Metals
Berengere Luthi 1 Lisa Ventelon 1 David Rodney 2 Francois Willaime 1
1CEA Gif-Sur-Yvette France2Universiteacute; Lyon I Villeurbanne FranceShow Abstract
In order to better understand plasticity in alloys, it is important to describe the deformation mechanisms with accuracy, and in particular to understand how dislocations, responsible for the plastic deformation, interact with solute atoms. Long-range interactions are well described with continuum elasticity theory, whereas short-range interactions depend on atomistic mechanisms. Previous ab initio studies of the ½<111> screw dislocation in body centered cubic (bcc) metals showed that in pure metals, the dislocation core adopts a symmetrical configuration, the easy core configuration, centered on a triangle of first-neighbor <111> atomic columns where helicity is reversed with respect to the bulk. The other core configurations - the asymmetric core, the hard core where the three <111> core atomic columns are at the same altitude, and the split core centered near an atomic column - are all unstable in pure bcc metals .
In this work, we investigate the effect of interstitials solute atoms on the ½<111> screw dislocations in bcc metals using ab initio calculat