Competing Phases, Complex Structure, and Complementary Diffraction Studies of R_3‑δFeAl_4‑xMg_<i>x</i>Tt₂ Intermetallics (R = Y, Dy, Er, Yb; Tt = Si or Ge; <i>x</i> < 0.5)

Four new intermetallic phases R_3‑δFeAl_4‑<i>x</i>Mg_<i>x</i>Si₂ (R = Yb, Dy) and R_3‑δFeAl_4‑<i>x</i>Mg_<i>x</i>Ge₂ (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 <i>P</i>4<i>/mbm</i> (<i>a</i> = 13.3479(9) Å, <i>c</i> = 4.0996(3) Å, <i>Z</i> = 4, and R1 = 0.0176 for Yb_2.77FeAl_3.72Mg_0.28Si₂). The structure features iron in trigonal prismatic coordination by aluminum atoms. The prisms share trigonal faces to form chains running along the <i>c</i>-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 Yb_2.77FeAl_3.72Mg_0.28Si₂ 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 determined by the rare earth ions. Y_3‑δFeAl_4‑<i>x</i>Mg_<i>x</i>Ge₂ is Pauli paramagnetic; the Yb³⁺ cations in Yb_2.77FeAl_3.72Mg_0.28Si₂ exhibit Curie–Weiss behavior with no ordering in the temperature range observed. Er_3‑δFeAl_4‑<i>x</i>Mg_<i>x</i>Ge₂ and Dy_3‑δFeAl_4‑<i>x</i>Mg_<i>x</i>Si₂ order antiferromagnetically at <i>T</i>_N = 2.8 and 4.0 K, respectively; the former undergoes a spin reorientation at ∼4400 G according to the ac field dependence of magnetization