B2 Phases and their Defect Structures: Part I.I. Ab initio Vibrational and Electronic Free Energy in the Al-Ni-Pt-Ru System

2004 ◽  
Vol 842 ◽  
Author(s):  
Raymundo Arroyave ◽  
Sara Prins ◽  
Zi-Kui Liu
Keyword(s):  
Free Energy ◽  
Phonon Dispersion ◽  
Harmonic Approximation ◽  
Total Free Energy ◽  
Vibrational Entropy ◽  
Defect Structures ◽  
Phonon Dispersion Curves ◽  
Formation Enthalpies ◽  
Atomic Vibrations ◽  
Harmonic Correction

ABSTRACTIn this work, we calculate the finite temperature thermodynamic properties of the binary B2 phases in the Al-Ni-Pt-Ru system, particularly the B2 RuAl phase in the Pt-Al-Ru ternary, through the incorporation of the vibrational and electronic contributions to the total free energy. The harmonic approximation is used to consider the atomic vibrations, with the quasi-harmonic correction to account for volume expansion effects on the vibrational entropy as the temperature increases. The vibrational entropy calculations are incorporated through the supercell approach. The calculated phonon dispersion curves show that the B2 PtRu structure is mechanically unstable at low temperatures, while B2 PtAl is marginally stable. The thermal electronic contribution is added to the total free energy. Finally, the formation enthalpies and entropies of B2 RuAl are calculated as a function of temperature.

Spatial Redistribution of Niobium Additives Near Nickel Surfaces

1995 ◽  
Vol 408 ◽  
Author(s):  
Leonid S. Muratov ◽  
Bernard R. Cooper
Keyword(s):  
Molecular Dynamics ◽  
Free Energy ◽  
Harmonic Approximation ◽  
Lattice Relaxation ◽  
Pure Nickel ◽  
Quantum Mechanical ◽  
Concentration Limit ◽  
Vibrational Entropy ◽  
Quantum Mechanical Calculations ◽  
Spatial Redistribution

AbstractThe spatial redistribution of niobium atoms near the surface of pure nickel has been considered in the low niobium concentration limit. The calculation of free energy includes lattice relaxation around niobium atoms by using molecular dynamics (MD) incorporating atomistic potentials based on ab-initio quantum mechanical calculations and includes vibrational entropy phenomenologically within the local harmonic approximation.

Lattice dynamics and Debye–Waller factors of some compounds with the sodium chloride structure

1999 ◽  
Vol 55 (6) ◽  
pp. 1014-1025 ◽  
Author(s):  
H. X. Gao ◽  
L.-M. Peng ◽  
J. M. Zuo
Keyword(s):  
Sodium Chloride ◽  
Lattice Dynamics ◽  
Polynomial Regression ◽  
Phonon Dispersion ◽  
Harmonic Approximation ◽  
Dispersion Curves ◽  
Phonon Dispersion Curves ◽  
Shell Models ◽  
Downhill Simplex ◽  
Physical Quantities

The lattice dynamics of 19 compounds with the sodium chloride structure have been investigated using shell models. The models are compared with existing experimentally measured phonon-dispersion curves and refined using a multidimensional downhill simplex method. Debye–Waller factors for these compounds are calculated over the temperature range from 1 to 1000 K where appropriate and the results are fitted analytically using polynomial regression. The results are compared with experimentally measured room-temperature Debye–Waller factors and for most of the compounds the agreement is found to be better than 10%. At lower temperatures, it is expected that these results would be more accurate, since the harmonic approximation, which is crucial to the calculation of the Debye–Waller factors, works better. In choosing the models for particular applications, it is recommended that the model with the smallest standard error \sigma is used for fitting the experimentally measured phonon-dispersion curves, or the model that shows best agreement with reliable experimental measurements of more relevant physical quantities, such as Debye–Waller factors in crystallography.

Theoretical Prediction on Mechanical, Dynamic, and Thermodynamic Properties of C11b-Structured WSi2 under Pressure

2018 ◽  
Vol 74 (1) ◽  
pp. 83-89 ◽  
Author(s):  
ShunRu Zhang ◽  
DuoPeng Zeng ◽  
HaiJun Hou ◽  
You Yu
Keyword(s):  
Thermodynamic Properties ◽  
Elastic Constants ◽  
Phonon Dispersion ◽  
Structural Parameters ◽  
Harmonic Approximation ◽  
Three Dimensional ◽  
Mechanical Anisotropy ◽  
Phonon Dispersion Curves ◽  
Lattice Constants ◽  
Experimental Findings

AbstractThe structural parameters, mechanical, dynamic, and thermodynamic properties of WSi2 with the C11b structure under pressure were systematically explored by using first-principles calculations. The results shown that calculated lattice constants and elastic constants of WSi2 were consistent with previous experimental findings and theoretical values. Our obtained elastic constants revealed that WSi2 was mechanically stable from 0 to 100 GPa. The values of anisotropic indexes, three-dimensional surface constructions, and two-dimensional projections under pressure indicated that WSi2 showed mechanical anisotropy. Additionally, the phonon density of state and phonon dispersion curves under pressure were obtained, and all vibration modes were analyzed. Finally, thermodynamic properties were also predicted based on quasi-harmonic approximation.

Ab initio study of the structural, vibrational and thermal properties of Ge2Sb2Te5

2015 ◽  
Vol 04 (02) ◽  
pp. 1550009
Author(s):  
Henry Odhiambo ◽  
Herick Othieno
Keyword(s):  
Thermal Properties ◽  
Phonon Dispersion ◽  
Hexagonal Phase ◽  
Harmonic Approximation ◽  
Experimental Studies ◽  
Energy Functional ◽  
Residual Entropy ◽  
Cubic Phase ◽  
Phonon Dispersion Curves ◽  
Valence States

The structural, vibrational and thermal properties of hexagonal as well as cubic Ge 2 Sb 2 Te 5 (GST) have been calculated from first principles. The relative stability of the possible stacking sequences of hexagonal GST has been confirmed to depend on the choice for the exchange-correlation (XC) energy functional. It is apparent that without the inclusion of the Te 4d orbitals in the valence states, the lattice parameters can be underestimated by as much as 3.9% compared to experiment and all-electron calculations. From phonon dispersion curves, it has been confirmed that the hexagonal phase is, indeed, stable whereas the cubic phase is metastable. In particular, calculations based on the quasi-harmonic approximation (QHA) reveal an extra heat capacity beyond the Dulong–Petit limit at high temperatures for both hexagonal and cubic GST. Moreover, cubic GST exhibits a residual entropy at 0 K, in agreement with experimental studies which attribute this phenomenon to substitutional disorder on the Sb / Ge / v sublattice.

scholarly journals Neural Network Sampling of the Free Energy Landscape for Nitrogen Dissociation on Ruthenium

2021 ◽  
Author(s):  
Elizabeth Lee ◽  
Thomas Ludwig ◽  
Boyuan Yu ◽  
Aayush Singh ◽  
François Gygi ◽  
...  
Keyword(s):  
Free Energy ◽  
Heterogeneous Catalysis ◽  
Density Functional ◽  
Energy Surface ◽  
Molecular Nitrogen ◽  
Harmonic Approximation ◽  
Molecular State ◽  
Transition Metal Catalysts ◽  
Vibrational Entropy ◽  
Free Energy Surface

<p>In heterogeneous catalysis, free energy profiles of reactions govern the mechanisms, rates, and equilibria. Energetics are conventionally computed using the harmonic approximation (HA), which requires determination of critical states <i>a priori</i>. Here, we use neural networks to efficiently sample and directly calculate the free energy surface (FES) of a prototypical heterogeneous catalysis reaction—the dissociation of molecular nitrogen on ruthenium—at density functional theory-level accuracy. We find vibrational entropy of surface atoms, often neglected in HA for transition metal catalysts, contributes significantly to the reaction barrier. The minimum free energy path for dissociation reveals an “on-top” adsorbed molecular state prior to the transition state. While a previously reported flat-lying molecular metastable state can be identified in the potential energy surface, it is absent in the FES at relevant reaction temperatures. These findings demonstrate the importance of identifying critical points self-consistently on the FES for reactions that involve considerable entropic effects.</p>

scholarly journals First-Principles Study of Mechanical and Thermodynamic Properties of Binary and Ternary CoX (X = W and Mo) Intermetallic Compounds

Materials ◽  
2021 ◽  
Vol 14 (6) ◽  
pp. 1404
Author(s):  
Yunfei Yang ◽  
Changhao Wang ◽  
Junhao Sun ◽  
Shilei Li ◽  
Wei Liu ◽  
...  
Keyword(s):  
Free Energy ◽  
Thermodynamic Properties ◽  
Gibbs Free Energy ◽  
Density Functional ◽  
Vibrational Entropy ◽  
Functional Theory ◽  
Lattice Constants ◽  
The Density Functional Theory ◽  
Ductile Behavior ◽  
Pseudo Potential

In this study, the structural, elastic, and thermodynamic properties of DO19 and L12 structured Co3X (X = W, Mo or both W and Mo) and μ structured Co7X6 were investigated using the density functional theory implemented in the pseudo-potential plane wave. The obtained lattice constants were observed to be in good agreement with the available experimental data. With respect to the calculated mechanical properties and Poisson’s ratio, the DO19-Co3X, L12-Co3X, and μ-Co7X6 compounds were noted to be mechanically stable and possessed an optimal ductile behavior; however, L12-Co3X exhibited higher strength and brittleness than DO19-Co3X. Moreover, the quasi-harmonic Debye–Grüneisen approach was confirmed to be valid in describing the temperature-dependent thermodynamic properties of the Co3X and Co7X6 compounds, including heat capacity, vibrational entropy, and Gibbs free energy. Based on the calculated Gibbs free energy of DO19-Co3X and L12-Co7X6, the phase transformation temperatures for DO19-Co3X to L12-Co7X6 were determined and obtained values were noted to match well with the experiment results.

The crystal dynamics of cuprous halides

1982 ◽  
Vol 60 (11) ◽  
pp. 1589-1594 ◽  
Author(s):  
Manvir S. Kushwaha
Keyword(s):  
Phonon Dispersion ◽  
Characteristic Temperature ◽  
Force Model ◽  
Phonon Dispersion Curves ◽  
Debye Characteristic Temperature ◽  
Zinc Blende ◽  
Crystal Dynamics ◽  
Bond Bending ◽  
Dynamical Description ◽  
Good Agreement

The lattice dynamics of cuprous halides have been thoroughly investigated by means of an 8-parameter bond-bending force model (BBFM), recently developed and applied successfully to study phonons in various II–VI and III–V compound semiconductors having zinc-blende (ZB) structure. The application of BBFM is made to calculate the phonon dispersion relations, phonon density of states, and temperature variation of the Debye characteristic temperature [Formula: see text] of CuCl, CuBr, and CuI. The room-temperature neutron scattering measurements for phonon dispersion curves along three principal symmetry directions and calorimetric experimental data for the Debye characteristic temperature have been used to check the validity of BBFM for the three crystals. The overall good agreement between theoretical and experimental results supports its use as an appropriate model for the dynamical description of ZB crystals.

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