65 research outputs found
Ab initio vibrational free energies including anharmonicity for multicomponent alloys
A density-functional-theory based approach to efficiently compute numerically
exact vibrational free energies - including anharmonicity - for chemically
complex multicomponent alloys is developed. It is based on a combination of
thermodynamic integration and a machine-learning potential. We demonstrate the
performance of the approach by computing the anharmonic free energy of the
prototypical five-component VNbMoTaW refractory high entropy alloy
Low temperature features in the heat capacity of unary metals and intermetallics for the example of bulk aluminum and AlSc
We explore the competition and coupling of vibrational and electronic
contributions to the heat capacity of Al and AlSc at temperatures below 50
K combining experimental calorimetry with highly converged finite temperature
density functional theory calculations. We find that semilocal exchange
correlation functionals accurately describe the rich feature set observed for
these temperatures, including electron-phonon coupling. Using different
representations of the heat capacity, we are therefore able to identify and
explain deviations from the Debye behaviour in the low-temperature limit and in
the temperature regime 30 - 50 K as well as the reduction of these features due
to the addition of Sc.Comment: 10 pages, 6 figures in total, paper submitted to Physical Review
Structural stability and thermodynamics of CrN magnetic phases from ab initio and experiment
The dynamical and thermodynamic phase stabilities of the stoichiometric
compound CrN including different structural and magnetic configurations are
comprehensively investigated using a first-principles density-functional-theory
(DFT) plus U approach in conjunction with experimental measurements of the
thermal expansion. Comparing DFT and DFT+U results with experimental data
reveals that the treatment of electron correlations using methods beyond
standard DFT is crucial. The non-magnetic face-centered cubic B1-CrN phase is
both, elastically and dynamically unstable, even under high pressure, while CrN
phases with non-zero local magnetic moments are predicted to be dynamically
stable within the framework of the DFT+U scheme. Furthermore, the impact of
different treatments for the exchange-correlation (xc)-functional is
investigated by carrying out all computations employing the local density
approximation and generalized gradient approximation. To address
finite-temperature properties, both, magnetic and vibrational contributions to
the free energy have been computed employing our recently developed spin-space
averaging method. The calculated phase transition temperature between
low-temperature antiferromagnetic and high-temperature paramagnetic (PM) CrN
variants is in excellent agreement with experimental values and reveals the
strong impact of the choice of the xc-functional. The temperature-dependent
linear thermal expansion coefficient of CrN is experimentally determined by the
wafer curvature method from a reactive magnetron sputter deposited single-phase
B1-CrN thin film with dense film morphology. A good agreement is found between
experimental and ab initio calculated linear thermal expansion coefficients of
PM B1-CrN. Other thermodynamic properties, such as the specific heat capacity,
have been computed as well and compared to previous experimental data.Comment: 10 figure
Dislocations in Laves phases – Phantastical beasts and how to understand them
How macroscopically hard and brittle materials deform is not well understood in many cases with not even the dominant slip systems known and no critical stresses or dislocation mechanisms available. This is true even for the most abundant type of intermetallic phase, the Laves phases. However, this knowledge is essential to improve many metallic-intermetallic composite alloys, such as Mg with an interconnected Laves network in Mg-Al-Ca alloys preventing creep.
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Structural anomaly in the high-entropy alloy ZrNbTiTaHf
We present a high-resolution scanning transmission electron microscopy study on the microstructure of a non-equiatomic high-entropy alloy with the composition of Zr12.7 Nb30.8 Ti17.7 Ta30.8 Hf8.0 (at.%). We identify a novel inter-grain phase (IGP) that compositionally and structurally differs from the surrounding body-centred cubic host. In particular, we find that the IGP has a composition of Zr52.8 Nb6.9 Ti4.6 Ta20.6 Hf15.1 (at.%) and that it solidifies in a face-centred cubic crystal lattice structure. The occurrence of the latter is unexpected and remarkable since all possible binary phase diagrams of the involved elements only show body-centred cubic and hexagonal close-packed crystal structures. Therefore, to validate our experimental findings we have conducted parameter-free ab-initio calculations based on density-functional theory and the coherent-potential approximation. The simulations support our experimental findings showing that for the composition of the IGP, the face-centred cubic crystal structure is indeed the most stable one
Dynamic stabilization of perovskites at elevated temperatures: A comparison between cubic BaFeO and vacancy-ordered monoclinic BaFeO
The impact of ordered vacancies on the dynamic stability of perovskites is
investigated under the framework with a focus on cubic
BaFeO () and vacancy-ordered monoclinic BaFeO
(). The harmonic approximation shows that both structures are
dynamically unstable at 0 K. For the monoclinic structure, the instability is
related to rotational distortions of the Fe coordination tetrahedra near the
ordered vacancies. molecular dynamics simulations in
combination with the introduced structural descriptor demonstrate that both
structures are stabilized above 130 K. Our results suggest that the ordered
vacancies do not significantly alter the critical temperature at which
BaFeO perovskites are dynamically stabilized. Further, strong
anharmonicity for the vacancy-ordered structure above its critical temperature
is revealed by a significant asymmetry of the trajectories of O anions near the
ordered vacancies.Comment: 13 pages, 7 figure
Electronic Moment Tensor Potentials include both electronic and vibrational degrees of freedom
Abstract We present the electronic moment tensor potentials (eMTPs), a class of machine-learning interatomic models and a generalization of the classical MTPs, reproducing both the electronic and vibrational degrees of freedom, up to the accuracy of ab initio calculations. Following the original polynomial interpolation idea of the MTPs, the eMTPs are defined as polynomials of vibrational and electronic degrees of freedom, corrected to have a finite interatomic cutoff. Practically, an eMTP is constructed from the classical MTPs fitted to a training set, whose energies and forces are calculated with electronic temperatures corresponding to the Chebyshev nodes on a given temperature interval. The eMTP energy is hence a Chebyshev interpolation of the classical MTPs. Using the eMTP, one can obtain the temperature-dependent vibrational free energy including anharmonicity coming from phonon interactions, the electronic free energy coming from electron interactions, and the coupling of atomic vibrations and electronic excitations. Each of the contributions can be accessed individually using the proposed formalism. The performance of eMTPs is demonstrated for two refractory systems which have a significant electronic, vibrational and coupling contribution up to the melting point—unary Nb, and a disordered TaVCrW high-entropy alloy. Highly accurate thermodynamic and kinetic quantities can now be obtained just by using eMTPs, without any further ab initio calculations. The proposed construction to include the electronic degree of freedom can also be applied to other machine-learning models
High-accuracy thermodynamic properties to the melting point from ab initio calculations aided by machine-learning potentials
Abstract Accurate prediction of thermodynamic properties requires an extremely accurate representation of the free-energy surface. Requirements are twofold—first, the inclusion of the relevant finite-temperature mechanisms, and second, a dense volume–temperature grid on which the calculations are performed. A systematic workflow for such calculations requires computational efficiency and reliability, and has not been available within an ab initio framework so far. Here, we elucidate such a framework involving direct upsampling, thermodynamic integration and machine-learning potentials, allowing us to incorporate, in particular, the full effect of anharmonic vibrations. The improved methodology has a five-times speed-up compared to state-of-the-art methods. We calculate equilibrium thermodynamic properties up to the melting point for bcc Nb, magnetic fcc Ni, fcc Al, and hcp Mg, and find remarkable agreement with experimental data. A strong impact of anharmonicity is observed specifically for Nb. The introduced procedure paves the way for the development of ab initio thermodynamic databases
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