Chemical Pressure and Rare-Earth Orbital Contributions in Mixed Rare-Earth Silicides La5–xYxSi4 (0 ≤ x ≤ 5)
A crystallographic study and theoretical analysis of the structural and La/Y site preferences in the La5–xYxSi4 (0 ≤ x ≤ 5) series prepared by high-temperature methods is presented. At room temperature, La-rich La5–xYxSi4 phases with x ≤ 3.0 exhibit the tetragonal Zr5Si4-type structure (space group P41212, Z = 4, Pearson symbol tP36), which contains only Si–Si dimers. On the other hand, Y-rich phases with x = 4.0 and 4.5 adopt the orthorhombic Gd5Si4-type structure (space group Pnma, Z = 4, Pearson symbol oP36), also with Si–Si dimers, whereas Y5Si4 forms the monoclinic Gd5Si2Ge2 structure (space group P21/c, Z = 4, Pearson symbol mP36), which exhibits 50% “broken” Si–Si dimers. Local and long-range structural relationships among the tetragonal, orthorhombic, and monoclinic structures are discussed. Refinements from single crystal X-ray diffraction studies of the three independent sites for La or Y atoms in the asymmetric unit reveal partial mixing of these elements, with clearly different preferences for these two elements. First-principles electronic structure calculations, used to investigate the La/Y site preferences and structural trends in the La5–xYxSi4 series, indicate that long- and short-range structural features are controlled largely by atomic sizes. La 5d and Y 4d orbitals, however, generate distinct, yet subtle effects on the electronic density of states curves, and influence characteristics of Si–Si bonding in these phases.
Reprinted (adapted) with permission from Inorg. Chem., 2011, 50 (24), pp 12714–12723. Copyright 2011 American Chemical Society.