Synthesis and Properties of New Multinary Silicides R5Mg5Fe4AlxSi18–x (R = Gd, Dy, Y, x ≈ 12) Grown in Mg/Al Flux

Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R5Mg5Fe4AlxSi18–x (x ≈ 12). These phases have a new structure type in tetragonal space group P4/mmm (a = 11.655(2) Å, c = 4.0668(8) Å, Z = 1 and R1 = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and 27Al and 29Si MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at TN = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field.