Computational Thermodynamics and Kinetics: Phonons, Magnons and Other Excitations
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Chemistry and Physics of Materials Committee, TMS: Computational Materials Science and Engineering Committee
Program Organizers: Nana Ofori-Opoku, Canadian Nuclear Laboratories; Eva Zarkadoula, Oak Ridge National Laboratory; Enrique Martinez Saez, Clemson University; Vahid Attari, Texas A&M University; Jorge Munoz, University of Texas at El Paso

Thursday 2:00 PM
March 18, 2021
Room: RM 54
Location: TMS2021 Virtual

Session Chair: Chen Li, University of California Riverside; Huajing (Wilson) Song, Los Alamos National Laboratory; Sara Kadkhodaei, University Of Illinois At Chicago; Jorge Munoz, University of Texas El Paso


2:00 PM  Invited
Phonons and Transition-induced Plasticity of bcc Refractory High-entropy Alloys from First Principles: Yuji Ikeda1; Prashanth Srinivasan2; Blazej Grabowski1; Fritz Körmann3; 1University of Stuttgart; 2TU Delft; 3Max-Planck-Institut für Eisenforschung GmbH; TU Delft
     Recent experimental studies report that some body-centered cubic (bcc) refractory high-entropy alloys (HEAs) exhibit transition-induced plasticity (TRIP) by transforming to the hexagonal closepacked (hcp) structure. Better understanding of phase transitions of such HEAs is therefore crucial to further improve the mechanical properties. We study the thermodynamic and dynamic stability of refractory HEAs based on first-principles calculations. Phonon dispersions are simulated based on the band-unfolding technique and they reveal, e.g., softening behavior of ZrNbHf, an alloy featuring also large atomic displacements. A careful treatment of atomic relaxations of bcc HEAs found to be critical to understand the phase transition of these alloys. Phase stabilities of these HEAs also show a close connection to the valence-electronconcentration (VEC). Thermodynamic integration in combination with machine-learning interatomic potentials enables us to accurately compute the vibrational free energies of HEAs even near the melting temperature.

2:30 PM  
Contributions of Atom Vibrations to the Heat of Fusion of Germanium: Camille Bernal-Choban1; Claire Saunders1; Stefan H. Lohaus1; Douglas Abernathy2; Brent Fultz1; 1California Institute of Technology; 2Oak Ridge National Laboratory
    The latent heat of melting of germanium, like silicon, is anomalously high compared to most elements. We measured inelastic neutron scattering from germanium, obtaining atomic vibrational spectra from 300-1373 K. Across the melting temperature, there was a collapse of the higher frequency vibrational modes to significantly lower energies. From the frequency spectra of atom motions, we found that most of the latent heat of melting (4.4 kB/atom) originates from the lower frequencies of atomic motion in the liquid phase. Density functional theory calculations within both a quasi-harmonic (QH) and a temperature-dependent effective potential (TDEP) model were also performed to elucidate the behavior of each mode as Tm was approached. The cubic and quartic anharmonic contributions, in comparison to the quasi-harmonic, are discussed.

2:50 PM  
A Computational and Experimental Study of Phonon Anharmonicity and Thermal Expansion of Cuprous Oxide: Claire Saunders1; Dennis Kim2; Hillary Smith3; Brent Fultz1; 1California Institute of Technology; 2Massachusetts Institute of Technology; 3Swarthmore College
    We examine how the quasiharmonic (QH) approximation, which models volume-dependent thermal effects, predicts the thermal expansion of low negative thermal expansion materials like cuprite, Cu2O, but fails to capture the phonon behavior. Phonon dispersions of a single crystal of cuprite were measured on the ARCS spectrometer at ORNL at 10 K and 300 K. Experimental data was folded into an irreducible wedge in the first Brillouin zone, and was subsequently multiphonon scattering and thermal factor corrected. QH calculations and temperature dependent calculations using the stochastic Temperature Dependent Effective Potential (sTDEP) method (QH phonons in an anharmonic potential) were performed at 10, 150, 300, 550 K. Single crystal data will be presented and compared to results from sTDEP calculations. Both computational methods capture the reported experimental thermal expansion, but differ in phonon energy shifts. Broader implications for small negative thermal expansion materials will be discussed.

3:10 PM  Invited
Development of New Ab-initio Non-adiabatic Excited-state Molecular Dynamics Method in NWChem: Huajing Song1; Sean Fischer2; Victor Freixas3; Niranjan Govind4; Sergei Tretiak1; 1Physics and Chemistry of Materials, Los Alamos National Lab; 2U.S. Naval Research Laboratory; 3Universidad Nacional de Quilmes; 4Pacific Northwest National Laboratory
    Computational simulation of non-adiabatic molecular dynamics (NAMD) is an indispensable tool for understanding the complex material excitation. We report recent implementations of two NAMD schemes, the multiconfigurational Ehrenfest ab-initio multiple cloning (MCE-AIMC) and fewest-switches surface hopping (FSSH) algorithm in NWChem. Both methods combined with linear-response time-dependent density functional theory (LR-TDDFT) calculations of adiabatic excited-state potential energy, which provided the capability to simulate the molecular system in the high excited energy state. We demonstrated these methods by simulating the ultrafast decay of photoexcited in conjugated molecules or transition metal oxides, including a detailed analysis of the potential energy surface, equilibrium time scales, and electron-phonon coupling to the excitation dynamics. The development in this work provided the critical tool to study the role/interactions of phonons, magnons, and other excitations due to photo or radiation-induced molecular processes, which help to understand the thermodynamics and kinetic properties of materials in the high energy environment.

3:40 PM  
First Principle Studies of Charged Point Defect in Phosphorene: Biswas Rijal1; Anne Marie Tan1; Christoph Freysoldt2; Richard Hennig1; 1University of Florida; 2Max Planck Institute
    Vacancies, adatoms, dopants, and interstitials change the electronic and optical properties of materials. These defects can be charge neutral; however, some defects can have a net charge or change their charge state. The computational modeling of the formation and combination of charged defects enables us to study the doping of materials in the newly discovered family of two-dimensional materials for use in devices such as solar cells, light-emitting diodes, or transistors. However, the theoretical study of charged defects presents challenges and introduces electrostatics artifacts. In this study, we calculate the formation energies of charged point defects in phosphorene and determine their charge transition levels using a correction method based on a surrogate model for the electrostatics. The results guide the selection of possible dopants in phosphorene.

4:00 PM  
Negative Grüneisen Parameters in Nonmagnetic bcc-based Intermetallic FeTi at High Pressure: Bethuel Khamala1; Jorge Munoz2; 1University of Texas El Paso; 2University of Texas El Paso
    FeTi is a brittle intermetallic with the CsCl structure that is stable to 1600K. We investigated its band structure and phonons at different volumes using density functional theory and uncovered a range in which the phonon modes decrease in energy with decreasing volume. This behavior is observed in invar materials, but FeTi is nonmagnetic and there is negligible change in the Fermi surface with pressure. It occurs more generally in materials with negative thermal expansion, but the crystal structure of FeTi is not open and it is stable at high pressure. We show experimental measurements of the phonon density-of-states curves performed via nuclear-resonant inelastic x-ray scattering at pressures up to 55 GPa and x-ray diffraction up to 25 GPa that are consistent with the calculated negative Gruneisen parameters, and an analysis of calculated force constants and band structures that preliminarily point towards orbital hybridization as the origin of the effect.

4:20 PM  Invited
Anomalous Magnon-phonon Dynamics in Antiferromagnets: Chen Li1; 1University of California Riverside
    Antiferromagnetic materials attract much interests due to their vanishing total magnetization, which allows robustness again disturbance and possible long mean-free-path for applications in spintronics and controlling thermal transport via magnetism. Temperature, magnetic field, and pressure dependent inelastic neutron scattering is used to investigate the magnon and phonon dynamics and their interactions in some important antiferromagnetic materials. First principles calculations are used to interpret the compilated interactions between phonon lattice dynamics, spin wave, and spin dynamics, and their implications on the thermal transport properties.