Dmitri Souptel

Incommensurately modulated CeSi1.82

Abstract An incommensurately modulated structure of CeSi1.82 as determined by single-crystal X-ray diffraction at room temperature is reported. The observed satellite reflections reduce the orthorhombic symmetry of the X-ray pattern to a monoclinic one. This suggests the description of the modulated structure in the monoclinic superspace group I2/b(αβ0)00. The modulation wave vector was determined to q = 0.410(1) · a* + 0.610(1) · b*. The refinement was done using harmonic modulation functions for the displacement and the occupation probability of the atoms, the quality parameters converged to Robs = 3.33% (Robs = 2.56% for the main reflections, Robs = 8.88% for the 1st order satellite reflections and Robs = 33.06% for the 2nd order satellite reflections). The origin of the observed satellite reflections is attributed to a modulation of the partly occupied Si site caused by composition changes within the crystal.

Intergrowth of several solid phases from the Y-Ni-B-C system in a large YNi2B2C crystal

A YNi2B2C single crystal containing traces of foreign phases was inspected by means of neutron and X-ray diffraction as well as scanning electron microscopy and X-ray spectroscopy methods. The diffraction patterns obtained from the experiments look similar to those expected for a superstructure. Nevertheless, they can be interpreted as crystallographically oriented precipitations of YB2C2 and Ni2B within the YNi2B2C crystal, formed during the cooling process. The orientation relation between the lattices was obtained from experimental neutron and X-ray data. Structure refinements of the collected X-ray data were performed by separation of the intensity data of the individual phases. Scanning electron microscopy images of the inclusions found on a polished cross section of the crystal are presented; their chemical composition was determined using wavelength-dispersive X-ray analysis.