Bernhard Eck

Theoretical calculations on the structures, electronic and magnetic properties of binary 3d transition metal nitrides

The electronic structures of a number of binary 3d transition metal and iron nitrides, some of which still need to be synthesized, have been investigated by means of spin-polarized first principles band structure calculations (TB-LMTO-ASA). The chemical bonding in all compounds has been clarified in detail through the analysis of total and local densities-of-states (DOS) and crystal orbital Hamilton populations (COHP). The binary transition metal nitride set includes ScN, TiN, VN, CrN, MnN, FeN, CoN and NiN, both in the sodium chloride as well as in the zinc blende structure type. Antibonding metal-metal interactions for higher electron counts are significantly weaker in the zinc blende type, thus favoring this structural alternative for the later transition metal nitrides.

First‐Principles Studies of Extended Nitride Materials

Modern electronic structure theory is a valuable tool for the chemistry and physics of extended materials. This contribution illustrates some recent examples on how structures, bonding, and physical properties of solid nitrides containing transition metals, lanthanides, and main group elements may be theoretically accessed and, in selected cases, how their syntheses may be more rationally planned.

Electronic and magnetic structure of transition-metal carbodiimides by means of GGA+U theory.

The electronic structures and magnetic properties of MNCN (M = Fe, Co, and Ni) have been investigated by density-functional theory including explicit electronic correlation through an ad hoc Coulomb potential (GGA+U). The results evidence CoNCN and NiNCN as type-II anti-ferromagnetic semiconductors (that is, intralayer ferromagnetic and interlayer anti-ferromagnetic), in accordance with experimental observations. Just like the prototype MnNCN, the MNCN phases, with M = Ni and Co, thus resemble the corresponding MO monoxides with respect to their magnetic and transport properties. By contrast, FeNCN remains (semi)metallic even upon applying a strong Coulomb correlation potential. This, most probably, is in contradiction with its observed optical transparency and expected insulating behavior and points toward a serious density-functional theory problem.

Atomistic simulations of solid-state materials based on crystal-chemical potential concepts: applications for compounds, metals, alloys, and chemical reactions

Based upon a partitioning and potential concept for the chemical bonding in solids, we illustrate a number of crystal–chemical simulations for various kinds of structures and bonding types on the picosecond time scale using the aixCCAD computer program. These include ionic/covalent materials (NaCl, ZnO, AlN), ternary oxides (LiAlO2 and its crystallographic phases), main-group (Ga, Al) as well as transition (3d, 4d, 5d) metals, various intermetallics (b.c.c.- and f.c.c.-like), as well as complex Fe/AlN nano composites. The simulations give access to detailed energetics, ionic mobilities, crystallographic structures, bulk moduli, and questions of chemical reactivity.

Atomistic simulations of solid-state materials based on crystal-chemical potential concepts: basic ideas and implementation

Abstract Using crystal–chemical knowledge we show how to conceptionally partition the chemical bonding properties of any given inorganic crystal structure into a few fundamental types; their potentials are presented in analytical form. The parameterizations are based on the bond valence concept (BVC), the universal bonding energy-distance relationship (UBER), and the concept of absolute electronegativity and hardness (AEH). The approach has been implemented into the recently developed aixCCAD computer program, intended to establish solid-state atomistic simulations (molecular dynamics-type) in which the atomic charges are dynamical variables of freedom.

Chemical Reactions within Fe/AlN Layered Nanocomposites: A Simulation Study based on Crystal-Chemical Atomic Dynamics

Layered nanocomposites made of metallic iron and aluminum nitride are subject to unexpected chemical reactions, resulting in a spontaneous formation of iron nitrides and a partial reduction to metallic aluminum. Since bulk thermochemical data are unable to rationalize the above finding, atomistic computer simulations based on the crystal-chemical atomic dynamics (CCAD) approach have been performed in the search for an explanation. The computational setup mimics a total number of about 1000 atoms moving over a time frame of 74 ps. When AlN molecules are sputtered on the iron surface under the experimental radio frequency (rf) conditions, the molecules are found to be chemically unstable upon hitting the surface, immediately breaking apart into individual atoms. Atomic nitrogen enters the Fe crystal to acquire quasi-octahedral coordination, leaving Al atoms behind on the surface. The reaction results in a stronger bonding of the nitride ion in the crystal compared to the covalently bonded nitrogen atom in the molecule. As a consequence, a small amount of Fe lattice expansion (2.5%) as well as a partial buildup of an iron/aluminum alloy is observed in the reaction zone near the surface of the bulk material.