Universität Stuttgart
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Item Open Access Phonon lifetimes throughout the Brillouin zone at elevated temperatures from experiment and ab Initio(2019) Glensk, Albert; Grabowski, Blazej; Hickel, Tilmann; Neugebauer, Jörg; Neuhaus, Jürgen; Hradil, Klaudia; Petry, Winfried; Leitner, MichaelWe obtain phonon lifetimes in aluminium by inelastic neutron scattering experiments, by ab initio molecular dynamics, and by perturbation theory. At elevated temperatures significant discrepancies are found between experiment and perturbation theory, which disappear when using molecular dynamics due to the inclusion of full anharmonicity and the correct treatment of the multiphonon background. We show that multiple-site interactions are small and that local pairwise anharmonicity dominates phonon-phonon interactions, which permits an efficient computation of phonon lifetimes.Item Open Access Anomalous phonon lifetime shortening in paramagnetic CrN caused by spin-lattice coupling: a combined spin and ab Initio molecular dynamics study(2018) Stockem, Irina; Bergman, Anders; Glensk, Albert; Hickel, Tilmann; Körmann, Fritz; Grabowski, Blazej; Neugebauer, Jörg; Alling, BjörnWe study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temperature compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature.Item Open Access Calculating free energies of point defects from ab initio(2018) Zhang, Xi; Grabowski, Blazej; Hickel, Tilmann; Neugebauer, JörgThe formation and lifetime of point defects is governed by an interplay of kinetics and thermodynamic stability. To evaluate the stability under process conditions, empirical potentials and ab initio calculations at T 1⁄4 0 K are often not sufficient. Therefore, various concepts to determine the full temperature dependence of the free energy of point defects with ab initio accuracy are reviewed. Examples for the importance of accurately describing defect properties include the stabilization of vacancies by impurities and the non-Arrhenius behaviour of vacancy formation energies due to anharmonic lattice vibrations.Item Open Access A combined experimental and first-principles based assessment of finite-temperature thermodynamic properties of intermetallic Al3Sc(2021) Gupta, Ankit; Tas, Bengü; Korbmacher, Dominique; Dutta, Biswanath; Neitzel, Yulia; Grabowski, Blazej; Hickel, Tilmann; Esin, Vladimir; Divinski, Sergiy V.; Wilde, Gerhard; Neugebauer, JörgWe present a first-principles assessment of the finite-temperature thermodynamic properties of the intermetallic Al3Sc phase including the complete spectrum of excitations and compare the theoretical findings with our dilatometric and calorimetric measurements. While significant electronic contributions to the heat capacity and thermal expansion are observed near the melting temperature, anharmonic contributions, and electron–phonon coupling effects are found to be relatively small. On the one hand, these accurate methods are used to demonstrate shortcomings of empirical predictions of phase stabilities such as the Neumann–Kopp rule. On the other hand, their combination with elasticity theory was found to provide an upper limit for the size of Al3Sc nanoprecipitates needed to maintain coherency with the host matrix. The chemo-mechanical coupling being responsible for the coherency loss of strengthening precipitates is revealed by a combination of state-of-the-art simulations and dedicated experiments. These findings can be exploited to fine-tune the microstructure of Al-Sc-based alloys to approach optimum mechanical propertiesItem Open Access Temperature dependence of the stacking-fault Gibbs energy for Al, Cu, and Ni(2018) Zhang, Xi; Grabowski, Blazej; Körmann, Fritz; Ruban, Andrei V.; Gong, Yilun; Reed, Roger C.; Hickel, Tilmann; Neugebauer, JörgThe temperature-dependent intrinsic stacking fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals Al, Cu, and Ni. All relevant temperature-dependent contributions are considered including electronic, vibrational, magnetic, and explicit anharmonic Gibbs energy contributions as well as coupling terms employing state-of-the-art statistical sampling techniques. Particular emphasis is put on a careful comparison of different theoretical concepts to derive the stacking fault energy such as the axial-next-nearest-neighbor-Ising (ANNNI) model or the vacuum-slab approach. Our theoretical results are compared with an extensive set of previous theoretical and experimental data. Large uncertainties in the experimental data highlight the necessity of complementary parameter-free calculations. Specifically, the temperature dependence is experimentally unknown and poorly described by thermodynamic databases. Whereas calphad derived data shows an increase of the stacking fault energy with temperature for two of the systems (Cu and Ni), our results predict a decrease for all studied systems. For Ni, the temperature induced change is in fact so strong that in the temperature interval relevant for super-alloy applications the stacking fault energy falls below one third of the low temperature value. Such large changes clearly call for a revision of the stacking fault energy when modeling or designing alloys based on such elements.