Competing Phases, Complex Structure, and Complementary Diffraction Studies of R3-δFeAl4-xMgxTt2 Intermetallics (R = Y, Dy, Er, Yb; Tt = Si or Ge; x < 0.5)

Four new intermetallic phases R3-δFeAl4-xMgxSi2 (R = Yb, Dy) and R3-δFeAl4-xMgxGe2 (R = Er, Y) were synthesized in Mg/Al (1:1 mol ratio) molten flux. These phases have a new structure type in tetragonal space group P4/mbm (a = 13.3479(9) A, c = 4.0996(3) A, Z = 4, and R1 = 0.0176 for Yb2.77FeAl3.72Mg0.28Si2). The structure features iron in trigonal prismatic coordination by aluminum atoms. The prisms share trigonal faces to form chains running along the c-axis, similar to the chains seen in several related structures, including that of the previously reported competing phases R5Mg5Fe4Al12Si6 (R = Gd, Dy, and Y). Occupancies of Mg, Al, and Si sites in Yb2.77FeAl3.72Mg0.28Si2 were determined by single crystal X-ray and neutron diffraction, bond length analysis, and comparison to atom positions and bond lengths in the isostructural germanides. Electronic structure calculations indicate these phases are polar intermetallics with pseudogaps near the Fermi level. The magnetic properties of these phases are dete...