In this thesis numerous nanowire samples have been analyzed using x-ray diffraction to find out more about their properties, in particular the lattice structure of hexagonal SiGe forming one shell of the nanowire structure. No previous knowledge existed about the lattice constants of hexagonal SiGe, since this material has so far not been realized in bulk form. Core/shell nanowires open the possibility to grow hexagonal SiGe as a shell around a hexagonal GaP core with an intermediate hexagonal Si shell layer serving as a "virtual" substrate for the hexagonal SiGe. GaP/Si/SiGe core-shell-shell nanowires have been fabricated at TU Eindhoven in the framework of the FET-open EC project "SiLAS". TEM has been used by our project partners to study the geometrical details of the core/shell/shell nanowires and the Ge content of the SiGe shell. We have used x-ray diffraction in combination with finite element modelling to determine the lattice parameter of hexagonal SiGe as a function of the Ge composition. This is relevant, since hexagonal SiGe is predicted to exhibit a direct band gap above a certain Ge composition, and accurate theoretical calculations on the band structure depend sensitively on the lattice parameters. The peaks and reflections of the samples were recorded at beamline P08 at Petra III in Hamburg Germany or at the rotating anode setup at JKU x-ray lab. Phyton was then utilized to plot the different reflections in reciprocal space maps and gain information of the various peaks. Lattice constants were calculated using the observed q positions in these maps. The TEM results of our samples were studied to determine content values and necessary simulation parameters, which were then fed to a FEM simulation accounting for the mutual strain within the core and the two shell layers and its effect on the wires.