arXiv subjects

Alex Antonelli

Publications and source records attributed to Alex Antonelli.

17 recordsLinked to original sources

arXiv2023

Half-Heusler TiXSn (X=Pd, Pt and Ni): electronic, vibrational, and defect properties from first-principles calculations

The knowledge of Half-Heusler compounds have attracted much attention as materials for thermoelectric applications. In this work, we investigate, using first-principles calculations, the electronic, vibrational, and defect properties of TiXSn (X=Ni, Pd, Pt) half-Heusler compounds. The knowledge of such properties is vital for the understanding and improvement of thermoelectric transport properties of these materials. The band gap of the three compounds increase with the atomic number of the group 10 elements, in agreement with previous findings. The electronic effective masses of the three compounds are similar, while the heavy hole effective mass of TiPtSn is larger than those of the other two materials. Our calculations of the phonon dispersion included the calculation of the LO-TO splitting indicating that TiNiSn has a stronger ionic character and polar scattering of charge carriers by optical phonons in the case of low doping. Calculation of the formation energy of Ni interstitial defect in TiNiSn is very low, in agreement with previous results. Surprisingly, for the Pd interstitial in TiPdSn the formation energy is negative, suggesting that the full-Heusler structure can be more stable than the half-Heusler one. On the other hand, the formation energy of an interstitial in TiPtSn is significantly higher, suggesting smaller effects on both electronic structure and transport properties. The formation energy of all substitutional defects investigated are substantially higher than that of interstitial ones, suggesting that they should occur in very low concentrations.

cond-mat.mtrl-sci↗

arXiv2023

Computational prediction of high thermoelectric performance in As$_{2}$Se$_{3}$ by engineering out-of-equilibrium defects

We employed first-principles calculations to investigate the thermoelectric transport properties of the compound As$_2$Se$_3$. Early experiments and calculations have indicated that these properties are controlled by a kind of native defect called antisites. Our calculations using the linearized Boltzmann transport equation within the relaxation time approximation show good agreement with the experiments for defect concentrations of the order of 10$^{19}$ cm$^{-3}$. Based on our total energy calculations, we estimated the equilibrium concentration of antisite defects to be about 10$^{14}$ cm$^{-3}$. These results suggest that the large concentration of defects in the experiments is due to kinetic and/or off-stoichiometry effects and in principle it could be lowered, yielding relaxation times similar to those found in other chalcogenide compounds. In this case, for relaxation time higher than 10 fs, we obtained high thermoelectric figures of merit of 3 for the p-type material and 2 for the n-type one.

cond-mat.mtrl-sci↗

arXiv2023

Semiclassical Electron and Phonon Transport from First Principles: Application to Layered Thermoelectrics

Thermoelectrics are a promising class of materials for renewable energy owing to their capability to generate electricity from waste heat, with their performance being governed by a competition between charge and thermal transport. A detailed understanding of energy transport at the nanoscale is thus of paramount importance for developing efficient thermoelectrics. Here, we provide a comprehensive overview of the methodologies adopted for the computational design and optimization of thermoelectric materials from first-principles calculations. First, we introduce density-functional theory, the fundamental tool to describe the electronic and vibrational properties of solids. Next, we review charge and thermal transport in the semiclassical framework of the Boltzmann transport equation, with a particular emphasis on the various scattering mechanisms between phonons, electrons, and impurities. Finally, we illustrate how these approaches can be deployed in determining the figure of merit of tin and germanium selenides, an emerging family of layered thermoelectrics that exhibits a promising figure of merit. Overall, this review article offers practical guidelines to achieve an accurate assessment of the thermoelectric properties of materials by means of computer simulations.

cond-mat.mtrl-sci↗

arXiv2022

Outstanding thermoelectric performance predicted for out-of-plane p-doped GeSe

The record-breaking thermoelectric performance of tin selenide (SnSe) has motivated the investigation of analogue compounds with the same structure. A promising candidate that emerged recently is germanium selenide (GeSe). Here, using extensive first-principles calculations of the hole-phonon and hole-impurity scattering, we investigate the thermoelectric transport properties of the orthorhombic phase of p-doped GeSe. We predict outstanding thermoelectric performance for GeSe over a broad range of temperatures due to its high Seebeck coefficients, extremely low Lorenz numbers, ultralow total thermal conductivity, and relatively large band gap. In particular, the out-of-plane direction in GeSe presents equivalent or even higher performance than SnSe for temperatures above 500 K. By extending the analysis to 900 K, we obtained an ultrahigh value for the thermoelectric figure of merit (zT = 3.2) at the optimal hole density of 4x10^19 cm^-3. Our work provides strong motivation for continued experimental work focusing on improving the GeSe doping efficiency in order to achieve this optimal hole density.

cond-mat.mtrl-sci↗

arXiv2021

Microscopic origin of the excellent thermoelectric performance in n-doped SnSe

Excellent thermoelectric performance in the out-of-layer n-doped SnSe has been observed experimentally (Chang et al., Science 360, 778-783 (2018)). However, a first-principles investigation of the dominant scattering mechanisms governing all thermoelectric transport properties is lacking. In the present work, by applying extensive first-principles calculations of electron-phonon coupling associated with the calculation of the scattering by ionized impurities, we investigate the reasons behind the superior figure of merit as well as the enhancement of zT above 600 K in n-doped out-of-layer SnSe, as compared to p-doped SnSe with similar carrier densities. For the n-doped case, the relaxation time is dominated by ionized impurity scattering and increases with temperature, a feature that maintains the power factor at high values at higher temperatures and simultaneously causes the carrier thermal conductivity at zero electric current (k_el) to decrease faster for higher temperatures, leading to an ultrahigh-zT = 3.1 at 807 K. We rationalize the roles played by k_el and k^0 (the thermal conductivity due to carrier transport under isoelectrochemical conditions) in the determination of zT. Our results show the ratio between k^0 and the lattice thermal conductivity indeed corresponds to the upper limit for zT, whereas the difference between calculated zT and the upper limit is proportional to k_el.

cond-mat.mtrl-sci↗

arXiv2021

Nonequilibrium free-energy calculation of phase-boundaries using LAMMPS

We present a guide to compute the phase-boundaries of classical systems using a dynamic Clausius-Clapeyron integration (dCCI) method within the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code. The advantage of the dCCI method is because it provides coexistence curves spanning a wide range of thermodynamic states using relatively short single non-equilibrium simulations. We describe the state-of-the-art of non-equilibrium free-energy methods that allow us to compute the Gibbs free-energy in a wide interval of pressure and/or temperature. We present the dCCI method in details, discuss its implementation in the LAMMPS package and make available source code, scripts, as well as auxiliary files. As an illustrative example, we determine the phase diagram of silicon in a range of pressures covering from 0 to 15 GPa and temperatures as low as 400 K up to the liquid phase, in order to obtain the phase boundaries and triple point between diamond, liquid and beta-Sn phases.

cond-mat.mtrl-sci↗

arXiv2020

Boosting the efficiency of ab initio electron-phonon coupling calculations through dual interpolation

The coupling between electrons and phonons in solids plays a central role in describing many phenomena, including superconductivity and thermoelecric transport. Calculations of this coupling are exceedingly demanding as they necessitate integrations over both the electron and phonon momenta, both of which span the Brillouin zone of the crystal, independently. We present here an ab initio method for efficiently calculating electron-phonon mediated transport properties by dramatically accelerating the computation of the double integrals with a dual interpolation technique that combines maximally localized Wannier functions with symmetry-adapted plane waves. The performance gain in relation to the current state-of-the-art Wannier-Fourier interpolation is approximately 2n_s \times M, where n_s is the number of crystal symmetry operations and M, a number in the range 5 - 60, governs the expansion in star functions. We demonstrate with several examples how our method performs some ab initio calculations involving electron-phonon interactions.

cond-mat.mtrl-sci↗

arXiv2020

Investigating charge carrier scattering processes in anisotropic semiconductors through first-principles calculations: The case of p-type SnSe

Efficient ab initio computational methods for the calculation of thermoelectric transport properties of materials are of great avail for energy harvesting technologies. The BoltzTraP code has been largely used to efficiently calculate thermoelectric coefficients. However, its current version that is publicly available is based only on the constant relaxation time (RT) approximation, which usually does not hold for real materials. Here, we extended the implementation of the BoltzTraP code by incorporating realistic k-dependent RT models of the temperature dependence of the main scattering processes, namely, screened polar and nonpolar scattering by optical phonons, scattering by acoustic phonons, and scattering by ionized impurities with screening. Our RT models are based on a smooth Fourier interpolation of Kohn-Sham eigenvalues and its derivatives, taking into account non-parabolicity (beyond the parabolic or Kane models), degeneracy and multiplicity of the energy bands on the same footing, within very low computational cost. In order to test our methodology, we calculated the anisotropic thermoelectric transport properties of low temperature phase (Pnma) of intrinsic p-type and hole-doped tin selenide (SnSe). Our results are in quantitative agreement with experimental data, regarding the evolution of the anisotropic thermoelectric coefficients with both temperature and chemical potential. Hence, from this picture, we also obtained the evolution and understanding of the main scattering processes of the overall thermoelectric transport in p-type SnSe.

cond-mat.mtrl-sci↗

arXiv2019

Splitting up entropy into vibrational and configurational contributions in bulk metallic glasses: A thermodynamic approach

We applied an efficient methodology to separate vibrational and configurational entropies in bulk metallic glasses by means of molecular dynamics simulation based on a combination of non-equilibrium adiabatic switching and reversible scaling methods. This approach involves calculating the vibrational free energy using the Einstein crystal as a reference for the solid phase and the recently proposed Uhlenbeck-Ford model for the fluid phase. This methodology has the advantage that it does not require a crystalline solid phase for separating the entropies. Therefore, in principle, it is applicable to any material, regardless of whether or not it has a crystalline phase. Using this methodology, we separate the vibrational and configurational entropies of two metallic glasses with different fragilities at zero external pressure, namely, Cu$_{50}$Zr$_{50}$ and Cu$_{46}$Zr$_{46}$Al$_{8}$. We find that the results for the former alloy are in quite reasonable agreement with recent experimental work by Smith \textit{et al.}[H. L. Smith \textit{et al.}, Nat. Phys. \textbf{13}, 900 (2017)]. We also find the configurational entropy of the glass containing Al to be 70\% larger than that of the other glass. Our results suggest that, although other factors may be at play, the configurational entropy can be used to investigate the effect of the addition of a minor-alloying element on the glass-forming ability of bulk metallic glasses.

cond-mat.soft↗

arXiv2019

Revisiting the fragile-to-strong crossover in metallic glass-forming liquids: application to Cu$_x$Zr$_x$Al$_{100-2x}$

The fragile-to-strong crossover seems to be a general feature of metallic glass-forming liquids. Here, we study the behavior of shear viscosity, diffusion coefficient and vibrational density of states for Cu$_\text{x}$Zr$_\text{x}$Al$_{\text{100-2x}}$ alloy through molecular dynamics simulations. The results reveal that the fragile-to-strong temperature (T$_\text{fs}$) and the glass transition temperature (T$_\text{g}$) increase as the aluminum content becomes larger. The inverse of the diffusion coefficient as a function of temperature exhibits a dynamical crossover in the vicinity of T$_\text{g}$, at a much lower temperature than that predicted by nearly all previous studies. At the temperature in which the dynamical crossover occurs determined by the inverse of the diffusion coefficient, we found an excess of vibrational states at low frequencies, resembling a pronounced peak in the reduced vibrational density of states characteristic of a strong liquid. Finally, the behavior of the shear viscosity as a function of reduced temperature (T$_\text{g}$/T) also shows that, besides the fragile-to-strong crossover nearby T$_\text{g}$, another dynamical crossover is present near the onset of the supercooled regime.

cond-mat.soft↗

arXiv2018

Nonequilibrium free energy methods applied to magnetic systems: the degenerate Ising model

In this paper, we review the physical concepts of the nonequilibrium techniques for the calculation of free energies applied to magnetic systems using Monte Carlo simulations of different nonequilibrium processes. The methodology allows the calculation of the free energy difference between two different system Hamiltonians, as well as the free energy dependence on temperature and magnetic field for a given Hamiltonian. As an illustration of the effectiveness of this approach, we apply the methodologies to determine the phase diagram of a simple microscopic model, the degenerate Ising model. Our results show very good agreement with those obtained from analytical (theoretical) methods.

cond-mat.stat-mech↗

arXiv2018

Stochastic sampling of the isothermal-isobaric ensemble: phase diagram of crystalline solids from molecular dynamics simulation

A methodology to sample the isothermal-isobaric ensemble using Langevin dynamics is proposed, which combines novel features of geometric integrators for the equations of motion. By employing the Trotter expansion, the methodology generates a robust, symmetric and accurate numerical algorithm. In order to show that the proposed method correctly samples the phase-space, simulations in the isotropic NPT ensemble were carried out for two analytical examples. Also this method permits to study a solid-solid phase transition, by conducting a fully flexible-cell molecular dynamics simulation. Additionally, we present an efficient method to determine the Gibbs free energy in a wide interval of pressure along an isothermal path, which allows us to determine the transition pressure in a driven by pressure solid-solid phase transition. Our calculations show that the methodology is highly suitable for the study of phase diagram of crystalline solids.

cond-mat.stat-mech↗

arXiv2017

Ultralow and Anisotropic Thermal Conductivity in Semiconductor As2Se3

An ultralow lattice thermal conductivity of 0.14 W$\cdot$ m$^{-1} \cdot$ K$^{-1}$ along the $\vec b$ axis of As$_2$Se$_3$ single crystals was obtained at 300 K by first-principles calculations involving the density functional theory and the resolution of the Boltzmann transport equation. This ultralow lattice thermal conductivity arises from the combination of two mechanisms: 1) a cascade-like fall of the low-lying optical modes, which results in avoided crossings of these with the acoustic modes, low sound velocities and increased scattering rates of the acoustic phonons; and 2) the repulsion between the lone-pair electrons of the As cations and the valence $p$ orbitals of the Se anions, which leads to an increase in the anharmonicity of the bonds. The physical origins of these mechanisms lie on the nature of the chemical bonding in the material and its strong anisotropy. These results, whose validity has been addressed by comparison with SnSe, for which excellent agreement between the theoretical predictions and the experiments is achieved, point out that As$_2$Se$_3$ could exhibit improved thermoelectric properties.

cond-mat.mtrl-sci↗

arXiv2016

$zT$-factor enhancement in SnSe: predictions from first principles calculations

The electronic structure and thermoelectric properties of SnSe are studied by first-principles methods. The inclusion of van der Waals dispersive corrections improves the agreement of structural parameters with experiments. The bands structure and projected density of states justify the macroscopic anisotropy exhibited by this system. An original methodology is used to estimate the chemical potential and the relaxation time for the electrical and thermal conductivities. Following this methodology, the Seebeck coefficient and thermal conductivity for single crystals and polycrystals are described in good agreement with experimental data. As for the electrical conductivity, values calculated with a temperature-dependent relaxation time compare well with available measurements, especially for single crystals, polycrystals are better described by a constant relaxation time. Finally, the figure of merit of SnSe single crystals and polycrystals is calculated. It is found to exhibit a maximum for some "ideal" carrier concentration, and might be noticeably enhanced by using carrier concentrations higher than the experimental ones. From these findings, possible strategies to increase the figure of merit in practise are suggested.

cond-mat.mtrl-sci↗

arXiv2013

Revisiting Dynamics Near a Liquid-Liquid Phase Transition in Si and Ga: The Fragile-to-Strong Transition

Using molecular dynamics simulations we analyze the dynamics of two atomic liquids that display a liquid-liquid phase transition (LLPT): Si described by the Stillinger-Weber potential and Ga as modeled by the modified embedded-atom model (MEAM). In particular, our objective is to investigate the extent to which the presence of a dip in the self-intermediate scattering function is a manifestation of an excess of vibrational states at low frequencies and may be associated with a fragile-to-strong transition (FTST) across the LLPT, as suggested recently. Our results do not lend support to these suggestions. Specifically, in the case of Ga we observe the appearance of an excess of vibrational states at low frequencies, even in the absence of the appearance of a dip in the self-intermediate scattering function across the LLPT. Furthermore, studying the behavior of the shear viscosities traversing the LLPTs we find that, despite the development of a dip in the self-intermediate scattering function for the case of Si and its absence in Ga, both substances are fragile in character above and below their respective LLPT temperatures.

cond-mat.stat-mech↗

arXiv2005

Orientational Defects in Ice Ih: An Interpretation of Electrical Conductivity Measurements

We present a first-principles study of the structure and energetics of Bjerrum defects in ice Ih and compare the results to experimental electrical conductivity data. While the DFT result for the activation energy is in good agreement with experiment, we find that its two components have quite different values. Aside from providing new insight into the fundamental parameters of the microscopic electrical theory of ice, our results suggest the activity of traps in doped ice in the temperature regime typically assumed to be controlled by the free migration of L defects.

cond-mat.mtrl-sci↗

arXiv2004

Bundling up carbon nanotubes through Wigner defects

We show, using ab initio total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right besides a vacancy, which are present in irradiated graphite can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizeable amount, clearly indicating its importance for the mechanical properties of nanotube bundles.

cond-mat.mtrl-sci↗