The Real Structure of YbSi1.4 ‐ Commensurately and Incommensurately Modulated Silicon Substructures

Abstract

AbstractA systematic thermochemical investigation of the binary system Yb/Si revealed the phase YbSi1.4 in pure form that is accessible by reacting the elements at 890 °C in sealed niobium ampoules. According to XRD investigations, the basic structure of YbSi1.4 is orthorhombic (space group: Cmcm; a = 4.159(1), b = 23.510(5), c = 3.775(1) Å). The structure contains blocks with three sheets of trigonal Yb6 prisms that are connected to each other by face‐sharing. Two of these prism sheets are fully occupied by Si whereas the middle one is occupied by ∼ 80 % only, as obtained from the electron density distribution by XRD. Transmission electron microscopy (TEM) investigations show the presence of two distinct ordering variants: YbSi1.4‐I has a unit cell with doubled volume (space group of the structural model: Imm2; aI = 4.16, bI = 7.56, cI = 23.51Å). In the structural model derived from HRTEM images, Si fills 3/4 of the partly occupied Yb6 prisms in a regular way. YbSi1.4‐II exhibits 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, with the modulation due to a shift of the domains with respect to each other.