Four High-Temperature Ferromagnets in the Hf–Fe–Sn System

We report the synthesis and characterization of four new ferromagnetic compounds discovered using Sn flux: Hf_1.823(16)Fe₅Sn_3.815(14), HfFe_2–<i>x</i>Sn_<i>x</i>, and two polymorphs of Hf_1–<i>x</i>Fe₂Sn_<i>x</i>. All are closely related to HfFe₂ Laves phase parent structures. HfFe_2–<i>x</i>Sn_<i>x</i> (<i>x</i> ≈ 0.3–0.4) adopts the MgZn₂-type (C14) crystal structure, whereas Hf_1–<i>x</i>Fe₂Sn_<i>x</i> (<i>x</i> ≈ 0.1–0.4) adopts both the MgCu₂-type (C15), and MgNi₂-type (C36) structures. They crystallize in <i>P</i>6₃/<i>mmc</i>, <i>Fd</i>3̅<i>m</i>, and <i>P</i>6₃/<i>mmc</i>, respectively, with measured unit-cell parameters of <i>a</i> = 4.9238(7) Å and <i>c</i> = 7.9643(12) Å; <i>a</i> = 7.068(2) Å; and <i>a</i> = 4.9944(4) Å and <i>c</i> = 16.2604(15) Å, although phase width leads to a range of unit cell edge lengths. Hf_1.823(16)Fe₅Sn_3.815(14) adopts a more complicated, incommensurately modulated structure in the superspace group <i>Xmmm</i>(00γ)­000 with an orthorhombic subcell <i>a</i> = 9.7034(12) Å, <i>b</i> = 16.823(2) Å, and <i>c</i> = 8.4473(10) Å, three centering vectors of (1/2 0 0 1/2), (0 1/2 0 1/2), and (1/2 1/2 0 0), and a single-component modulation vector <i>q</i> = 0.2768(8)<i>c</i>*. The structure is composed of alternating slabs of the Fe-bonded Kagomé nets observed in the HfFe₂ parent structures alternated with Sn-rich Th₂Zn₁₇-type slabs, with Hf atoms primarily occurring at the interfaces between the slabs. All four compounds are ferromagnetic metals at room temperature, with Curie temperatures ranging from 467(2) to 658(2) K. Their coercive fields are remarkably low, between 2(1) and 15(2) Oe. Interestingly, in two of three cases the addition of nonmagnetic Sn atoms in place of magnetic Hf or Fe atoms in the HfFe₂ structure seems to strengthen rather than weaken magnetic coupling and increase <i>T</i>_C. Fits to electrical resistivity data for the compound suggest that electron scattering in the Laves phase polymorphs shows substantial contributions from electron–magnon and/or electron–electron scattering, while the electrical behavior of Hf_1.823(16)Fe₅Sn_3.815(14) is dominated by electron–phonon scattering, as is the case in most metals