Phase transformation driven by valence electron concentration: Tuning interslab bond distances in Gd5GaxGe4-x

Abstract

X-ray single crystal and powder diffraction studies on the Gd5GaxGe4-x system with 0 less than or equal to x less than or equal to 2.2 reveal dependence of interslab T-T dimer distances and crystal structures themselves on valence electron concentration (T is a mixture of Ga and Ge atoms). While the Gd5GaxGe4-x phases with 0 less than or equal to x less than or equal to 0.6 and valence electron concentration of 30.4-31 e(-)/formula crystallize with the Sm5Ge4-type structure, in which all interslab T-T dimers are broken (distances exceeding 3.4 Angstrom), the phases with 1 less than or equal to x less than or equal to 2.2 and valence electron concentration of 28.8-30 e-/formula adopt the Pu5Rh4- or Gd5Si4-type structures with T-T dimers between the slabs. An orthorhombic Pu5Rh4-type structure, which is intermediate between the Gd5Si4- and Sm5Ge4-type structures, has been identified for the Gd5GaGe3 composition. Tight-binding linear-muffin-tin-orbital calculations show that substitution of three-valent Ga by four-valent Ge leads to larger population of the antibonding states within the dimers and, thus, to dimer stretching and eventually to dimer cleavage.