| Zdroj: |
Aachen : Shaker, Berichte aus der Chemie IV, 119, 90 S. : Ill., graph. Darst. (2009). = Zugl.: Aachen, Techn. Hochsch., Diss., 2008 |
| Popis: |
In the present thesis the synthesis of RhFe3N was optimized by studying the influence of the temperature profile during synthesis on the phase purity. For this purpose it was necessary to set up a new oven system. Thus, the phase purity of RhFe3N was raised up to 94%. With the improved product quality the magnetic and optical characterization was possible. Like the archetype Fe4N, RhFe3N can be classified as a semi-hard ferromagnet. The atomic saturation magnetization is slightly lower, while the coercive field is only a fifth of the Fe4N value. With regard to a possible application as material for magnetic recording heads RhFe3N possesses even more promising properties than Fe4N. The Rietveld analysis based on XRD data shows that position 1a is the only position for rhodium, thus it is not occupying position 3c (space group Pm-3m). This finding was already predicted by the results of DFT calculation and was furthermore confirmed by Mössbauer experiments. The established synthesis method for RhFe3N was adopted for the first-time synthesis of new quaternary nitrides within the iron and platinum group. Structurally, these nitrides also derive from Fe4N. The first attempt was the partial substitution of rhodium in the ternary nitride RhFe3N by nickel to yield a nitride in the quaternary system Ni_xRh_(1-x)Fe3N. It was shown that a synthesis with x > 0.50 yields phase pure products. The phase purity in comparison to the synthesis of RhFe3N was also improved within the system Co_xRh_(1-x)Fe3N. DFT calculations revealed that for both systems the nitrides with x = 0.50 are thermodynamically more stable than the corresponding ternary nitrides (x = 0 and x = 1). Furthermore, the occupation for the substitutional metals is again predicted to be exclusively on position 1a for all investigated quaternary nitrides (space group Pm-3m). Next to the systems Ni/Rh and Co/Rh many more quaternary systems were predicted to be thermodynamically stable and were, in accordance to that, synthesized for the first time. Systems containing nickel as one substitutional metal showed up excellent phase purities (Co_0.50Ni_0.50Fe3N, Ni_0.75Ru_0.25Fe3N, Ni_0.50Rh_0.50Fe3N and Ni_0.50Pd_0.50Fe3N). Among these systems especially those containing cobalt or ruthenium next to nickel are worth mentioning because their corresponding ternary nitrides were synthesized with a much lower occupation of cobalt or ruthenium. In systems of the higher homologues of nickel, i.e. Pd_xM'_(1-x)Fe3N and Pt_xM'_(1-x)Fe3N, the phase purity decreases in correspondence to the higher thermodynamic stability of the ternary nitrides PdFe3N and PtFe3N. The stability of quaternary with the other platinum elements can be estimated as Rh > Co > Ru. This order is based on the large number of newly synthesized quaternary nitrides during this thesis and is also corresponding to the predicted stability of ternary nitrides by DFT calculations. Thus, except a few examples, the theoretical and experimental results match each other perfectly (e.g. lattice parameters, magnetism). All systems predicted to be thermodynamically stable could be synthesized. In order to optimize the phase purity of those systems to excellent values the composition (x <> 0.5) range was varied throughout the experimental investigation. Beyond this, the ternary nitride GaFe3N was synthesized as phase pure product and also magnetically characterized for the first time. The investigation evidences a dependency between the magnetic ordering and the gallium amount in the system Ga_xFe_(4-x)N. With values of x = 0.5 and x = 1.5 a ferromagnetic ordering was observed. In contrast to that, with x = 1.0 an antiferromagnetic ordering was found. These findings are in good agreement with DFT calculations. As conclusion the systematic optimization of the synthesis of those nitrides is the basis for future investigations regarding a multitude of questions that were partly already addressed in the results and discussion part of this thesis. As outlook the experimental results should be supported by additional DFT calculations with varying compositions (x <> 0.5). The synthesis should be further optimized with regard to other parameters, e.g. the partial pressure of nitrogen. The occupation of the substitutional metals should be analyzed in more details by Mössbauer and neutron scattering experiments. This might help to get a better understanding of their influence on the structural as well as the magnetic properties of such nitrides. Therefore, the complex magnetic characterization should be completed in order to predict and adjust the magnetic properties as needed. The exciting magnetic properties of GaFe3N should be analyzed in more detail using other experimental methods (e.g. Mössbauer and neutron scattering experiments) and also by further theoretical calculations. The first step should be the investigation of the changing spin ordering from antiferromagnetism to ferromagnetism as a function of the composition. |
