Ordering Phenomena in YbSi_1.4

YbSi_1.4 crystallizes with orthorhombic symmetry (space group: Cmcm; a = 4.16, b = 23.51, c = 3.78 Å). The basic crystal structure was determined from single crystal X-ray data. It 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 described 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 YbSi_1.4 was conducted.

Transmissions electron microscopy (TEM) investigations revealed the presence of two distinct ordering variants: Additional reflections in the electron diffraction pattern indicate that YbSi_1.4-I (Imm2; a_I = 4.16, b_I = 7.56, c_I = 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 YbSi_1.4-II, the additional reflections are split (Fig. a). This observation 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 YbSi_1.4-II revealed parallel domains with the structure of YbSi_1.4-I (Fig. b). The modulation is caused by a shift of these domains in respect of each other.

The Real Structure of YbSi_1.4 - Commensurately and Incommensurately Modulated Silicon Substructures
C. Kubata, F. Krumeich, M. Wörle und N. Nesper, Z. anorg. allg. Chem. 631 (2005) 546-555 DOI.