504 publications from this institution
Ab initio molecular dynamics studies of the structural and electronic properties of liquid Te and tellurium-rich K–Te alloys are presented. Our work was done on pure l-Te at 748 and 1123 K, on the K12Te88-alloy at 723 K and at the equiatomic composition at 770 K. Our research has shown that the Te-chains are disturbed by threefold co-ordinated Te atoms forming bridges between the chains and onefold co-ordinated marking chain-ends. As the occurrence of these `defects' is very large (∼20% each type) liquid tellurium is better described as an atomic network. On adding a small amount of potassium (12%) the dangling bonds are saturated, stabilizing the chain-like structure. Equiatomic K–Te shows a different picture. Due to the charge transfer from K to Te the Te− atoms are isoelectronic to iodine, forming polyanionic clusters. As in the crystalline phase Te2 dumbells are built. Additionally one can find isolated Te atoms as well as short chains consisting of three or four atoms.
The structural and electronic properties of the crystalline semimetals As and Sb may be understood in terms of a Peierls distortion from the simple cubic structure bonds: as the (ppσ)-band is exactly half-filled, the cubic structure is unstable against dimerization dividing the six nearest-neighbour bonds into three strong and three weak bonds and opening a deep gap in the electronic density of states at the Fermi level. On the basis of neutron-diffraction experiments it has been suggested that some of the anomalous structural and electronic properties survive in the liquid phase. Here, an ab initio investigation of the structural, electronic, and dynamic properties of molten Sb that is based on the full set of quantum-many-body forces is presented. The results confirm the picture of a Peierls-distortion in the liquid.
An efficient and robust on-the-fly machine learning force field method is developed and integrated into an electronic-structure code. This method realizes automatic generation of machine learning force fields on the basis of Bayesian inference during molecular dynamics simulations, where the first principles calculations are only executed, when new configurations out of already sampled datasets appear. The developed method is applied to the calculation of melting points of Al, Si, Ge, Sn and MgO. The applications indicate that more than 99 \% of the first principles calculations are bypassed during the force field generation. This allows the machine to quickly construct first principles datasets over wide phase spaces. Furthermore, with the help of the generated machine learning force fields, simulations are accelerated by a factor of thousand compared with first principles calculations. Accuracies of the melting points calculated by the force fields are examined by thermodynamic perturbation theory, and the examination indicates that the machine learning force fields can quantitatively reproduce the first principles melting points.
We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.
Oxidation is often associated with corrosion, but under the right conditions it can lead to oxide layers which can be applied e.g. as protective layers against corrosion, as insulating layers in microelectronic devices and as catalytic devices[1, 2]. For late transition metals and noble metals (e.g. Pd and Ag), it is now understood that the oxidation proceeds through ultra-thin oxide layers, which are thermodynamically stable at intermediate oxygen potentials and can exhibit astonishing complexity[3–6]. It is still unclear whether the same holds for transition metals and to this end the Rh (111) surface was studied extensively experimentally [7]. To supplement these experimental studies, we performed density functional calculations with the Vienna Ab Initio Simulation Package (VASP) [8] using plane waves and the PAW method [9] as well as generalized gradient approximations [10]. The experimental studies show a Moire like pattern in STM, indicative of a (8×8) oxide layer on a (9×9) supercell of the Rh(111) substrate. This phase is formed at intermediate oxygen pressures, whereas thick corundum-like Rh2O3 is only formed at significantly higher pressures and temperatures. The theoretical calculations indicate that the Moire phase can be rationalised by an ultrathin O-Rh-O trilayer surface oxide. Contrary to the experimental observation, the bulk Rh2O3 oxide is however thermodynamically more stable than ultra-thin layers. This discrepency can be understood by the surface phase diagram shown in Fig. 1(a). It indicates that a single oxygen trilayer on Rh(111) is in fact only metastable, as the trilayer forms only under conditions, where bulk Rh2O3 is already stable (to the right of the thick grey line in Fig. 1(a)). Hence the trilayer is only kinetically stabilised, contrary to the situation on Pd(111) where a Pd5O4 ad-layer is thermodynamically stable for intermediate oxygen potentials [4]. An important hint to the reason of the kinetic stability of the O-Rh-O trilayer is given by the results for the three and four layer thick oxides. Four (three) layer oxides have a lower stability than the single trilayer for oxygen potentials μO <−0.99 eV (μO <−0.78 eV) corresponding to 10 mbar (1 bar) at 800 K. The formation of the bulk oxide must however proceed through thicker oxide layers that present a kinetic barrier for the formation of the bulk oxide at too low chemical potentials. The most favorable structures for 2, 3 and 4 oxygen layers are depicted in Fig. 1(b). In contrast to oxygen atoms on the clean metal, the oxygen atoms at the oxide/metal interface are located preferentially on top of the surface Rh atoms (shifted slightly towards the bridge site). Remarkably, the trilayer termination remains favorable even for thicker oxides. For three oxygen layers (3L) the topmost surface layer contains three Rh atoms, and a fourth single Rh atom is located in the second oxide layer. For four oxygen layers (4L), the trilayer is found at both sides of the oxide, and a single Rh atom interlinks the two O-Rh-O layers. Poster
Using correlated wave function based methods, the modeling of promising new materials is elevated to a new level. For the first time, a realistic phonon dispersion relation is predicted for the infinite linear carbon chain.
The determination of the local structure of cobalt- or nickel-promoted MoS2-based hydrodesulfurization catalysts is of interest for understanding the mechanism leading to an increased activity brought by cobalt or nickel, the so-called synergetic effect. For that reason, we carried out ab initio calculations using density functional theory under the generalized gradient approximation for periodic systems. The edge substitution model emerges as the most stable structure and provides an excellent agreement with local structures experimentally determined on real catalysts by in situ extended X-ray absorption fine structure. We studied the adsorption of sulfur on the active edge surface of the promoted MoS2 catalyst and determined the equilibrium coverage under sulfiding conditions. It is demonstrated that the incorporation of promoter atoms has a strong influence on the sulfur–metal bond energy at the surface and in particular leads to a reduction of the equilibrium S coverage of the active metal sites. A comparative study on the effects of Co, Ni, and Cu atoms as promoters was performed. Detailed results on the surface electronic structure of promoted MoS2 are presented.
A study of the electronic structure of about thirty transition-metal sulphides (TMS) of various stoichiometries and crystal structures is presented, supplementing recent studies of their structural and cohesive properties (P Raybaud, G Kresse, J Hafner and H Toulhoat, preceding paper). The electronic structure of the TMS is found to be determined by short-range interactions in the S 3p - TM d band complex, with the ligand-field splitting of the TM d states in the environment of the S atoms determining the structure of the d band. For the layered group VI disulphides, for and for the group VIII pyrites this leads to the formation of a gap at the Fermi surface. Semiconducting properties are predicted also for the monosulphides PtS and PdS and for and . We show that the semiconducting TMS have a higher catalytic activity for hydro-desulphurization than the metallic sulphides. We suggest a correlation between the catalytic activity and the characters of the highest occupied states (the frontier orbitals).
