Maßgeschneidertes und Analytik-Ersatz : über die quantenchemischen Untersuchungen einiger ternärer intermetallischer Verbindungen

摘要

This work manages to apply modern density-functional methods (DFT) to analyse as well as to predict new tailormade intermetallic materials. It has been shown that DFT in combination with increasing computation power, nowadays, could be ideal for combinatorial work. Magnetic measurements of single crystalline CaMn2Sb2 pointed towards its antiferromagnetic nature. The calculation was accomplished before an intricate neutron diffraction approved the structure's exact spin orientation. The orientation of the spins ferromagnetically within each layer and antiparallel between two layers of the structure's Mn–Mn double layer was predicted via crystal-orbital-Hamilton-population analysis (COHP). The LMTO calculations could also been utilized to describe the electric conductivity along the Mn–Mn double layer. Modern phase change materials are of high technical interest due to their usage in optical storage media. They are able to be quickly switched between an amorphous and a crystalline state and are utilized for re-writable DVDs and CDs. Only experimentalists have been improving these materials (by now). The quantum-physical prediction of stability differences within new "GeTeSb" compositions adapting a rocksalt-like structure could be explained via bonding analysis. With its help, reasons for the increase of vacancy concentration and local structure distortion have been found in the avoidance of antibonding Ge-Te interactions. The predicted compound Ge1.5Sb2Te4 could be subsequently prepared by experimentalists via modern epitaxy methods. The material's phase change property was ascertained. Thus, it has been managed to apply DFT to the tailormade prediction of a new material. In the field of data storage magnetic materials also play an important role. Within this work a theoretical research on the thermochemical stabilities and the magnetism of full Heusler phases (X2YZ) has been accomplished. The calculation of 810 possible phases found a plethora (48%) of yet unknown, but stable Heusler structured compounds. An ad hoc constructed periodic system of Heusler phases made it possible to visualise periodicities of magnetic moments of all these compounds. The most promising magnetic properties were looked at in detail for the compounds Rh2YZ (Y~=~Fe, Mn, Cr; Z~=~Al, In, Bi). With the help of electronic structure analysis it was demonstrated that it is possible to finetune the properties by applying the rigid-band model. The predicted stable Heusler phases could be successfully inscribed into a Slater-Pauling curve, in which the dependency of the magnetic moment from the valence electron concentration is shown. Finally, a look at the material's prices lead to the suggestion to produce the low-cost Fe2CuAl, Ni2CrGa and Fe2NiGe. A magnetic moment between 3 and 4 µB could be predicted for these phases. The suggested new compound Pt2ScSn is already been prepared by experimentalists. The comparison of the measured and predicted lattice parameter, considering the systematic error of the GGA method, shows the high accuracy of the calculations. In this part of the thesis the use of DFT methods for combinatorial purposes, giving reliable results, could be proven. Following this idea, 810 inversely arranged Heusler phases {(XY)XZ} were object to research. X atoms formerly in tetrahedral voids have been partially exchanged with Y atoms, formerly being in an octahedral coordination sphere. Only 27 phases were more stable in the inverse structure. Three of these atom combinations were stable only due to this inversion. Bonding inspections of Fe2CuGa via COHP analyses could explain the reasons for that. The COHP diagrams also revealed structural instabilities pointing towards a distortion. According to further calculations 14 of 27 phases would tetragonally elongate. On of theses phases is Fe2CuGa, which is stable with a lattice parameter ratio of 1.505. The tetragonal regularily arranged Fe2CuGa will have a higher magnetic saturation moment of 4.69 µB than its cubic pendant. Hopefully these computer experiments could also lead to a preparation of these phases. The usability of DFT for prediction is not only a dream, anyway.