Phase Transformation Driven by Valence Electron Concentration: Tuning Interslab Bond Distances in Gd<sub>5</sub>Ga<i><sub>x</sub></i>Ge<sub>4</sub><sub>-</sub><i><sub>x</sub></i>
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X-ray single crystal and powder diffraction studies on the Gd5GaxGe4-x system with 0 ≤ x ≤ 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 ≤ x ≤ 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 Å), the phases with 1 ≤ x ≤ 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.
