Browsing by Author "Glensk, Albert"

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Ab initio based method to study structural phase transitions in dynamically unstable crystals, with new insights on the β to ω transformation in titanium
(2019) Korbmacher, Dominique; Glensk, Albert; Duff, Andrew Ian; Finnis, Michael W.; Grabowski, Blazej; Neugebauer, Jörg
We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable β (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal ω phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of [111]β linear-chain defects that exhibit randomly disordered arrangements in the (111)β plane.
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, Björn
We 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.
Open Access
An insight into using DFT data for Calphad modeling of solid phases in the third generation of Calphad databases, a case study for Al
(2019) Bigdeli, Sedigheh; Zhu, Li-Fang; Glensk, Albert; Grabowski, Blazej; Lindahl, Bonnie; Hickel, Tilmann; Selleby, Malin
In developing the next generation of Calphad databases, new models are used in which each term contributing to the Gibbs energy has a physical meaning. To continue the development, finite temperature density-functional- theory (DFT) results are used in the present work to discuss and suggest the most applicable and physically based model for Calphad assessments of solid phases above the melting point (the breakpoint for modeling the solid phase in previous assessments). These results are applied to investigate the properties of a solid in the super- heated temperature region and to replace the melting temperature as the breakpoint with a more physically based temperature, i.e., where the superheated solid collapses into the liquid. The advantages and limitations of such an approach are presented in terms of a new assessment for unary aluminum.
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, Michael
We 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.
Open Access
Strong impact of lattice vibrations on electronic and magnetic properties of paramagnetic Fe revealed by disordered local moments molecular dynamics
(2016) Alling, Björn; Körmann, Fritz; Grabowski, Blazej; Glensk, Albert; Abrikosov, Igor A.; Neugebauer, Jörg
We study the impact of lattice vibrations on magnetic and electronic properties of paramagnetic bcc and fcc iron at finite temperature, employing the disordered local moments molecular dynamics (DLM-MD) method. Vibrations strongly affect the distribution of local magnetic moments at finite temperature, which in turn correlates with the local atomic volumes. Without the explicit consideration of atomic vibrations, the mean local magnetic moment and mean field derived magnetic entropy of paramagnetic bcc Fe are larger compared to paramagnetic fcc Fe, which would indicate that the magnetic contribution stabilizes the bcc phase at high temperatures. In the present study we show that this assumption is not valid when the coupling between vibrations and magnetism is taken into account. At the γ-δ transition temperature (1662 K), the lattice distortions cause very similar magnetic moments of both bcc and fcc structures and hence magnetic entropy contributions. This finding can be traced back to the electronic densities of states, which also become increasingly similar between bcc and fcc Fe with increasing temperature. Given the sensitive interplay of the different physical excitation mechanisms, our results illustrate the need for an explicit consideration of vibrational disorder and its impact on electronic and magnetic properties to understand paramagnetic Fe. Furthermore, they suggest that at the γ-δ transition temperature electronic and magnetic contributions to the Gibbs free energy are extremely similar in bcc and fcc Fe.
Open Access
Temperature dependence of the Gibbs energy of vacancy formation of fcc Ni
(2018) Gong, Yilun; Grabowski, Blazej; Glensk, Albert; Körmann, Fritz; Neugebauer, Jörg; Reed, Roger C.
Quantum-mechanical calculations are used to determine the temperature dependence of the Gibbs energy of vacancy formation in nickel. Existing data reveal a discrepancy between the high-temperature estimates from experiments and low-temperature approximations from density functional theory. Our finite-temperature calculations - which include the effects of magnetism and fully interacting phonon vibrations - demonstrate that this discrepancy is mostly caused by the previously neglected explicit anharmonic contribution.