Stability of the atomic arrangement in Laves phases Be2Fe1-xOsx (0 ≤ x ≤ 0.75) and Be2Fe1-xRux (x ≈ 0.5)

The binary Be2Fe, which crystallizes in the C14-structure type hexagonal Laves phase, can dissolve a large amount of Os as a third element, without changing the crystal structure (about 3/4 of Fe can be replaced by Os). A study of the homogeneity range for the Be2Fe1-xOsx Laves phase, shows an increase of the lattice parameters with an increase of the Os content. When Ru substitutes Fe in Be2Fe, a temperature- and composition-dependent transformation from the C14 to the cubic C15 Laves phase is observed, with a narrow homogeneity range. Formation energy calculations revealed that the C14-structure type is energetically favored when Os substitutes Fe in Be2Fe, while a structural phase transition is observed between Be2Fe (C14) and Be2Fe0.5Ru0.5 (C15), all in good agreement with the experimental results. A strong charge transfer from Be to Os or Ru is observed from the analysis of electron density within the Quantum Theory of Atoms in Molecules (QTAIM) framework. A formation of multiatomic bonds is found from the electron localizability approach. Be2Fe1-xOsx undergoes a transition to a disordered ferromagnetic state at T-C = 350 K and behaves like a ferromagnet with a non-collinear magnetic structure (not all domains polarized along field). Be2Fe1-xRux orders ferromagnetically below T-C = 340 K and the very narrow hysteresis curves show that Be2Fe1-xRux is a soft ferromagnet.