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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.

In der vorliegenden Arbeit wurden sowohl die martensitische Umwandlung als auch die magnetischen Eigenschaften Mangan-reicher Ni50Mn50-xSnx- und Ni50Mn50-yIny-Legierungen mit 5 at.-%

In this work the electronic structures, densities of states, chemical bonding, magnetic exchange Parameters and Curie temperatures of binary and ternary ferromagnetic alloys are analyzed. The electronic structure of ferromagnetic MnAl has been calculated using density-functional techniques (TB-LMTO-ASA, FPLAPW) and quantum chemically analyzed by means of the crystal orbital Hamilton population analysis. The crystal structure of the ferromagnetic tetragonal MnAl may be understood to originate from the structure of nonmagnetic cubic MnAl with a CsCl motif through a two-step process. While the nonmagnetic cubic structure is stable against a structural deformation, antibonding Mn--Mn interactions at the Fermi level lead to spin polarization and the onset of magnetism, i.e., a symmetry reduction taking place solely in the electronic degrees of freedom, by that emptying antibonding Mn-Mn states. Residual antibonding Al--Al states can only be removed by a subsequent, energetically smaller structural deformation towards the tetragonal system. As a final result, homonuclear bonding is strengthened and heteronuclear bonding is weakened. Corresponding DFT calculations of the electronic structure as well as the calculation of the chemical bonding and the magnetic exchange interactions have been performed on the basis of LDA and GGA for a series of ferromagnetic full Heusler alloys of general formula Co2MnZ (Z=Ga, Si, Ge, Sn), Rh2MnZ (Z=Ge, Sn, Pb), Ni2MnZ (Z=Ga, In, Sn),Pd2MnZ (Z=Sn, Sb) and Cu2MnZ (Z=Al, In, Sn). There is general agreement between experimental and theoretical moments by either the LDA or the GGA, and the amount of local exchange is characteristic for the atomic nature of the transition metal and Mn atoms. The connection between the electronic spectra and the magnetic interactions have been studied. Correlations between the chemical bondings in Heusler alloys derived from COHP analysis and magnetic phenomena are obvious, and different mechanisms leading to spin polarization and ferromagnetism are derived. Different mechanisms contributing to the exchange coupling are revealed. The band dependence of the exchange parameters, their dependence on volume and valence electron concentration have been thoroughly analyzed within the Green function technique. Finite temperature effects (Curie temperatures) are analysed using the mean-field description, and a surprisingly simple relationship between structural properties and the Curie temperature is found. The alloy variations of the Curie temperatures calculated are in good agreement with experimental data. The analysis demonstrates that the exchange parameter Jij dependence on the Z atom may be described within a rigid band approximation, having straightforward implications for the influence of the atomic volume of Z, thereby allowing semi-quantitative predictions. The magnetic exchange parameters and also Curie temperatures decrease along the row Cu, Ni, Rh, Pd, in agreement with the degree of d localisation for the transition metal. The X-Mn interactions are very important for systems with sizable magnetic moments on the transition metal (Co, Rh and Ni), making the magnetic short-range order effects stronger in these materials. The X-Mn interactions are limited by first neighbors while Mn-Mn interactions are quite long ranged. Non-spin-polarized COHP bonding analyses evidence antibonding metal-metal interactions as a prerequisite for becoming ferromagnetic, and these show up for the 3d-metal-Mn and also 4d-metal-Mn combinations but not for the wider Mn-Mn interactions. As an exception to this rule, the Cu2MnZ class of compounds evidences, on the opposite, large antibonding Mn-Mn interactions at the Fermi level; different mechanisms for spin polarization are obvious.

This is a theoretical investigation of the magnetic, structural and dynamical properties of the ferromagnetic Heusler compound Ni-Mn-Ga. Its physical properties have been studied by means of the Density Functional Theory. The challenge of this work is to explain in terms of the electronic structure and phonon spectra the entire variety of different crystal structures of Ni-Mn-Ga and their stability. It was shown that this crystal exibits anomalies both in the electronic structure and the phonons. Important for Ni-Mn-Ga features include modulation shuffling of the structure, phonon softening, phonon acoustic-optical coupling, electron-phonon coupling and the charge transfer. A combination of these properties allowed to draw a solid physical view on the nature of the Heusler alloys and the alloy Ni-Mn-Ga.