Sergei I. Simak

Van der Waals density functional for layered structures.

To understand sparse systems, we must account for both strong local atom bonds and weak nonlocal van der Waals forces between atoms separated by empty space. A fully nonlocal functional form [Phys. Rev. B 62, 6997 (2000)]] of density-functional theory (DFT) is applied here to the layered systems graphite, boron nitride, and molybdenum sulfide to compute bond lengths, binding energies, and compressibilities. These key examples show that the DFT with the generalized-gradient approximation does not apply for calculating properties of sparse matter, while use of the fully nonlocal version appears to be one way to proceed.

Temperature dependent effective potential method for accurate free energy calculations of solids

We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic ...

Screened Coulomb interactions in metallic alloys. II. Screening beyond the single-site and atomic-sphere approximations

A quantitative description of the configurational part of the total energy of metallic alloys with substantial atomic size difference cannot be achieved in the atomic-sphere approximation: It needs to be corrected at least for the multipole-moment interactions in the Madelung part of the one-electron potential and energy. In the case of a random alloy such interactions can be accounted for only by lifting the atomic-sphere and single-site approximations, in order to include the polarization due to local environment effects. Nevertheless, a simple parametrization of the screened Coulomb interactions for the ordinary single-site methods, including the generalized perturbation method, is still possible. We obtained such a parametrization for bulk and surface NiPt alloys, which allows one to obtain quantitatively accurate effective interactions in this system.

Hard numbers on soft matter

Graphitic systems are ideal examples of the soft-matter challenge for quantum mechanics that must account for both strong local atom bonds and weak nonlocal van der Waals interactions between atoms separated by empty space. Here a new density functional (DF), which encompasses nonlocal correlations among the electrons, is applied successfully to obtain structure, bonding, and compressibility, documenting the decisive role of the interlayer vdW interactions. The understanding emerging from this new extended-DF formulation has bearing on broad classes of problems, including surfaces, in physics, chemistry, biology, medicine, and technology, where we can now provide an efficient and accurate DF-theory account.

Stability of gold nanowires at large Au-Au separations

The unusual structural stability of gold nanowires at large separations of gold atoms is explained from first-principles quantum mechanical calculations. We show that undetected light atoms, in particular hydrogen, stabilize the experimentally observed structures, which would be unstable in pure gold wires. The enhanced cohesion is due to the partial charge transfer from gold to the light atoms. This finding should resolve a long-standing controversy between theoretical predictions and experimental observations.

Hemispherical anisotropic patterns of the Earth’s inner core

It has been shown that the Earth’s inner core has an axisymmetric anisotropic structure with seismic waves traveling ∼3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth’s core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres. The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth’s rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth’s inner core.

Synthesis of Ti3AuC2, Ti3Au2C2 and Ti3IrC2 by noble metal substitution reaction in Ti3SiC2 for high-temperature-stable Ohmic contacts to SiC

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti3AuC2 and Ti3Au2C2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Ti3IrC2 is subsequently produced by a substitution reaction of Ir for Au in Ti3Au2C2. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 °C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.

Density-functional bridge between surfaces and interfaces

Interfaces are brought into focus by many materials phenomena, e.g., contacting, materials strength, and wetting. The class of interfaces includes ultra-high-vacuum surfaces, which provide a meeting place for numerous accurate experimental techniques and advanced theory. Such meetings stimulate detailed comparisons on the quantum level between experiment and theory, which develop our theoretical tools and understanding. This creates good positions for broadened applications, e.g., other interfaces, which typically lack adequate experimental tools. Density-functional theory is one key bridge between surfaces and other interfaces. The paper presents some recent typical applications from our group, including brief reports on interface structures (VN/Fe, TiC/Co, TiC/Al 2 O 3 ), dynamic processes at surfaces and interfaces (O 2 /Al(111), scanning-tunneling microscopic spectroscopy and manipulation), adsorption and desorption (CO, N 2 , NO, and O 2 on Al(111)), electronic and magnetic properties at surfaces and interfaces (magnetic effects on TiC/Co, surface state on κ-Al 2 O 3 (001)), and epitaxial growth on surfaces (Al(111) and alike). Similar progress in many worldwide materials groups and networks gives a basis for the ongoing paradigm shift in materials science.

Bonding and elastic properties of superconducting MgB2

Abstract Bonding and elastic properties of the recently discovered superconductor MgB2, have been studied from first-principles. Calculated lattice parameters, bulk modulus and its pressure derivative are in excellent agreement with the recently reported experimental data. Values of all independent elastic constants (c11, c12, c13, c33, and c55) as well as bulk moduli in the a and c directions (Ba and Bc, respectively) are predicted. Based on an analysis of the electron localization function we show that the bonding in the system is diverse and anisotropic. This explains the high anisotropy of the calculated elastic moduli, indicating that MgB2 should be rather brittle.

From V8Ga36.9Zn4.1 and Cr8Ga29.8Zn11.2 to Mn8Ga27.4Zn13.6: A Remarkable Onset of Zn‐Cluster Formation in an Intermetallic Framework

The series of isotypic compounds V8Ga41 --> V8Ga36.9Zn4.1 --> Cr8Ga29.5Zn11.2 --> Mn8Ga27.4Zn13.6 with the V8Ga41 structure type (space group R3, Z = 3) was prepared and structurally characterised by X-ray diffraction experiments (V8Ga41: a 13.9351(5), 14.8828(12); V8Ga36.9Zn4.1: a = 13.9244(7), c = 14.8660(9): Cr8Ga29.8Zn11.2: 13.7153(5), c = 14.6872(9); Mn8Ga27.4Zn13.6: a = 13.6033(6), c = 14.6058(16)). The site occupancies of the ternary compounds were refined from neutron powder-diffraction data and exposed a startling segregation of Zn and Ga, which finally resulted in the formation of separated Zn13 cluster entities-corresponding to almost ideal centred cuboctahedra or small pieces of fcc metal-in the Mn compound, which has the highest Zn content in the series. The homogeneity ranges of the underlying phases T8Ga41 xZnx were determined to be 0 < x < 4.1(3), 8.7(3) < x < 11.2(3) and 13.6(4) < x < 16.5(3) for T = V, Cr and Mn, respectively. The different ranges of composition of the phases reflect the requirement of an optimum electron concentration for a stable V8Ga41-type structure, which is in the narrow range between 159 and 165 electrons per formula unit. First-principles electronic-structure calculations could explain this fact by the occurrence of a pseudo gap in the density of states at which the Fermi level is put for this particular electron concentration. Furthermore the nature of the Zn/Ga segregation was revealed: T-Zn interactions were found to be considerably weaker than those for T-Ga. This places the Zn atoms as far as possible from the T atoms, thus leading to the formation of cuboctahedral Zn13 entities.