Entwicklung eines hochauflösenden Röntgen-Einkristalldiffraktometers und Strukturuntersuchungen an La 2 - x Sr x CuO 4 - Einkristallen
The first part of this thesis describes the development of a new high resolution x-ray single crystal diffractometer (RED) and its multiple applications. It is shown that the application of the presently available technologies allows very flexible and even completely new possibilities of structural investigations of single crystals with x-rays in laboratories. The quick exchange between different beam optics allows flexible and efficient scientific work. Available are standard optics for Bragg data collection (Cu-K_alpha- and Mo-K_alpha radiation), an additional secondary monochromator unit for pure elastic scattering experiments and a Ge-fourfold monochromator for high resolution measurements using pure Cu-K_alpha_1 radiation. The standard optics allow in combination with the large dynamical ranges (max. 500.000 cps linear) of the available detectors detailed investigations of structures with extreme differences between integrated intensities. Furthermore, detailed temperature dependent measurements can be performed down to 20 K by using an additional closed-cycle cryostat, e.g. for structural investigations at phase transitions. The Ge-fourfold monochromator yields high resolutions comparable to those of similar instruments at synchrotron sources (e.g. Si-(111) reflection with a halfwidth of < 0.0030°, Rocking scan). This allows a very precise analysis of reflection profiles for characterization of sample quality and the observation of smallest structural distortions. On principle the available radiation intensity of the RED is significantly smaller than that of a synchrotron. Nevertheless measurements of several samples show that the flexibility of the RED yields scientific possibilities for structural investigations at the home laboratory which are normaly available only at synchrotron facilities. The second part of this work describes structural investigations with neutron diffraction of strontium doped lanthanium cuprate La(2-x)Sr(x)CuO4 (LSCO), a high-temperature-superconductor (HTSC). LSCO is a layered perovskite which undergoes a structural transition from a high-temperature tetragonal phase (HTT) to a low temperature orthorhombic phase (LTO) up to x=0.21. In the LTO-phase the CuO6 octahedrons are tilted in an ordered and well defined manner against the CuO4-planes. Up to now, the question of the possible coupling between superconductivity and lattice structure of the HTSC could not be finally answered. As a result of this experts are highly interested in the recent conclusions of different working groups using local methods on LSCO with x=0.15 (max. Tc=38 K). They deviate from their results partially strong deviations of the local structure from the mean structure. Diffraction with fast neutrons enables the accurate distinction of the mean square displacements (msd) of the atoms. This allows clear statements about the upper limits of the local deviations from the mean structure, which gives the opportunity to check the different postulated models.Therefore, a set of neutron diffraction experiments was performed during this thesis on LSCO samples (x=0.15) at different temperatures above and below their structural phase transitions. The results show an enlargement of the msd of the oxygens into the tilt directions of the CuO6-octahedrons which is independent from temperature. This yields small (<2°) variations around the temperature dependent average values and is compatible – also quantitatively – to harmonic lattice dynamical calculations which take a statical disorder caused by Sr-doping into account. Because of this, different postulated local deviations like an order-disorder transition or a large tilt (about 16°) of a larger number of CuO6-octahendrons can be excluded. The results of this work limit the possible interpretations of the results from local structure investigations significally. Superconductivity itself has – on a macroscopic scale – no significant influence on the structure except of the freeze-in of the tilt angles. The investigation of the oxygen content shows a significant influence on the phase transition of the LSCO structure even for small variations. This emphasizes the importance of an accurate characterization of LSCO samples – especially of their stochiometry – to assure the comparability of results from different investigations and sources.
info:eu-repo/classification/ddc/550, Cuprate, Hochtemperatursupraleiter, Lanthanverbindungen, Neutronendiffraktometrie, Röntgendiffraktometer, Strontiumverbindungen, Geowissenschaften
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