crystallizes with orthorhombic symmetry (space group: Cmcm; a =
4.159(1), b=23.510(5), c=3.775(1) Å). The basic crystal structure
was determined from single crystal X-ray data. The structure contains
trigonal Yb6 prisms that are fully occupied by Si forming
zigzag chains which run along the c-axis. Residual electron density
intensity that was detected in the other Yb6
prisms can be explained by Si split positions, leading to an occupancy
of ~80% of the corresponding prisms. Since the nature of this feature
could not be unraveled from the X-ray data alone, a comprehensive
TEM investigation of YbSi1.4 has been started.
Electron diffraction pattern (a) and HRTEM image (b) along 
electron microscopy (TEM) investigations showed the presence of
two distinct ordering variants: Additional reflections in the electron
diffraction pattern indicate that YbSi1.4-I (Imm2; aI
= 4.16, bI = 7.56, cI = 23.51 Å) has
a c axis that is twice as long as that of the basic structure. In
the structural model derived from HRTEM images, Si systematically
fills 3/4 of the partly occupied Yb6 prisms. In YbSi1.4-II,
the additional reflections are split (Fig. a). This points to an
incommensurate modulation that can be described in the 3+1 dimensional
superspace group Cmcm(10γ) (No. 63.3). The evaluation of HRTEM
images of YbSi1.4-II revealed parallel domains with the
structure of YbSi1.4-I (Fig. b). The modulation is caused
by a shift of these domains in respect of each other.
Real Structure of YbSi1.4 - Commensurately and Incommensurately
Modulated Silicon Substructures
C. Kubata, F. Krumeich, M. Wörle und N. Nesper, Z. anorg.
allg. Chem. 631 (2005) 546-555 DOI.