Superstructure in RE2-xFe4Si14-y (RE = Y, Gd−Lu) Characterized by Diffraction, Electron Microscopy, and Mössbauer Spectroscopy
Ternary rare-earth iron silicides RE2-xFe4Si14-y (RE = Y, Gd−Lu; x ≈ 0.8; y ≈ 4.1) crystallize in the hexagonal system with a ≈ 3.9 Å, c ≈ 15.3 Å, Pearson symbol hP20−4.9. Their structures involve rare-earth silicide planes with approximate compositions of “RE1.2Si1.9” alternating with β-FeSi2-derived slabs and are part of a growing class of rare-earth/transition-metal/main-group compounds based on rare-earth/main-group element planes interspersed with (distorted) fluorite-type transition-metal/main-group element layers. The rare-earth silicide planes in the crystallographic unit cells show partial occupancies of both the RE and Si sites because of interatomic distance constraints. Transmission electron microscopy reveals a 4a × 4b × c superstructure for these compounds, whereas further X-ray diffraction experiments suggest ordering within the ab planes but disordered stacking along the c direction. A 4a × 4b structural model for the rare-earth silicide plane is proposed, which provides good agreement with the electron microscopy results and creates two distinct Fe environments in a 15:1 ratio. Fe-57 Mössbauer spectra confirm these two different iron environments in the powder samples. Magnetic susceptibilities suggest weak (essentially no) magnetic coupling between rare-earth elements, and resistivity measurements indicate poor metallic behavior with a large residual resistivity at low temperatures, which is consistent with disorder. First-principles electronic-structure calculations on model structures identify a pseudogap in the densities of states for specific valence-electron counts that provides a basis for a useful electron-counting scheme for this class of rare-earth/transition-metal/main-group compounds.