Michael Gilleßen

A combinatorial study of inverse Heusler alloys by first‐principles computational methods

In continuation of our recent combinatorial work on 810 X2YZ full Heusler alloys, a computational study of the same class of materials but with the inverse (XY)XZ crystal structure has been performed on the basis of first‐principles (GGA) total‐energy calculations using pseudopotentials and plane waves. The predicted enthalpies of formation evidence 27 phases to be thermochemically stable against the elements and the regular X2YZ type. A chemical‐bonding study yields an inherent tendency for structural distortion in a majority of these alloys, and we predict the existence of the new tetragonal phase Fe2CuGa (P42/ncm; a = 5.072 Å, c = 7.634 Å; c/a ≈ 1.51) with a saturation moment of μ = 4.69 μB per formula unit. Thirteen more likewise new, isotypical phases are predicted to show essentially the same behavior. Six phases turn out to be the most stable in the inverse tetragonal arrangement. The course of the magnetic properties as a function of the valence‐electron concentration is analyzed using a Slater‐Pauling approach.

Electronic structure, chemical bonding, and finite-temperature magnetic properties of full Heusler alloys

The electronic structure, chemical bonding, and magnetic properties of 15 full Heusler alloys X2MnZ have been studied on the basis of density‐functional theory using the TB‐LMTO‐ASA approach and the local‐density (LDA), as well as the generalized‐gradient approximation (GGA). Correlations between the chemical bondings derived from crystal orbital Hamilton population (COHP) analysis and magnetic phenomena are obvious, and different mechanisms leading to spin polarization and ferromagnetism are derived. As long as a magnetically active metal atom X is present, antibonding XX and XMn interactions at the Fermi level drive the systems into the ferromagnetic ground state; only if X is nonmagnetic (such as in Cu2MnZ), antibonding MnMn interactions arise, which again lead to ferromagnetism. Finite‐temperature effects (Curie temperatures) are analyzed using a mean‐field description, and a surprisingly simple (or, trivial) relationship between structural properties (MnMn interatomic distances) and TC is found, being of semiquantitative use for the prediction of the latter.

New quaternary complex borides, Ti9M2Ru18B8 (Cr, Mn, Co, Ni, Cu, Zn): synthesis, crystal structure and bonding analysis

Abstract Powder samples and single crystals of the Ti9M2Ru18B8 (Cr, Mn, Co, Ni, Cu, Zn) phases were synthesized from the elements and characterized by powder and single-crystal X-ray diffraction as well as energy-dispersive X-ray analysis. The new phases are all isotypic and crystallize in the tetragonal system as substitutional variants of the Zn11Rh18B8-type structure (space group P4/mbm, no. 127). M2 dumbbells are observed and interconnect to each other along the [001] direction to build “ladders”. The M–M dumbbell distances vary from 2.48 to 2.50 Å and the distances between two dumbbells (M2 · · · M2) are all close to 2.97 Å, whereas the chains are well separated from each other by distances of at least 11.20 Å. A strong variation of the unit cell volume with increasing valence electron count is observed in the series. According to the results of tight-binding electronic structure calculations, Ru–B and Ti–Ru contacts are responsible for the structural stability of these phases. The strength of the M–M and M–Ru interactions decreases with increasing valence electron count in the series. Non vanishing density of states at the Fermi level indicates metallic character for all phases.