Hartwig, Steffen: Neutron Diffraction in Extreme Conditions. , Berlin, Technische Universität, Diss., 2016
10.14279/depositonce-5448
Open Access Version
Abstract:
In this thesis three different families of systems were examined. Firstly, the unusual magnetic properties of three intermetallic cerium compounds. Secondly, two members of the field of iron based superconductors. At last, also the results of some preparatory experiments on U2Rh2Sn single crystals will be shown. The three cerium compounds are CeRuSn, Ce2RuZn4 and CePdAl. Their common feature is a significantly reduced signal in magnetization measurements. Both Ce2RuZn4 and CeRuSn exhibit multiple sublattices, which provide different local environments for the cerium atoms with sometimes remarkably small interatomic distances. This leads to different cerium valences and subsequently different magnetic contributions. CeRuSn is especially interesting, since the two sublattices are at low temperature replaced by an incommensurate crystal and magnetic structure. CePdAl possesses a reduced macroscopic magnetic moment as well. In contrast to the former mentioned compounds, this behaviour originates from the geometrical arrangement of the magnetic ions. Neutron dffraction experiments were capable of proving, that two of the sites are permanently aligned in an antiparallel order. The third cerium is frustrated and aligned only at very low temperatures and is easily disturbed by the application of pressure. The studied materials from the family of iron based superconductors were the systems Fe(1+y)Se(x)Te(1-x) and the oxychalcogenide La2O3(Fe(1-x)Mn(x))2Se2. Fe(1+y)Se(x)Te(1-x) is a well known and intensely analyzed superconductor containing iron. However, during this thesis, it occurred that the established model of the superconductivity has to be adjusted. Proper sample preparation and a detailed characterization of the material pointed to a necessary secondary phase to obtain superconductivity. A perfect single crystal of optimal stoichiometry showed no bulk superconductivity without this phase. It was determined to be a member of the Fe3Se(x)Te(4-x) system. A perturbation of this composition was frequently mentioned in the literature, but never considered as enhancing the superconducting behaviour. The properties of Fe3Se(x)Te(4-x) were studied as well. Multiple indicators suggested the appearance of superconducting behaviour in the oxychalcogenide compound La2O3(Fe(1-x)Mn(x))2Se2. However, magnetization measurements and measurements of the electrical resistivity could not verify it. It was mentioned that for certain compositions the magnetic ordering vanishes. Via detailed neutron powder diffraction measurements, this feature was confirmed. Measurements under very large pressures were performed to gain knowledge about the compressibility of La2O3(Fe(1-x)Mn(x))2Se2 and the stability of its magnetic order. Both information are necessary to improve the theoretical models. In case of U2Rh2Sn, which was a candidate for neutron measurements in the High-Field magnet in Berlin, the sample characterization and measurements of specific heat, electrical resistivity and magnetic properties is presented.