Intermetallics as Zintl phases: Yb₂Ga₄Ge₆ and RE₃Ga₄Ge₆ (RE = Yb, Eu): Structural response of a [Ga₄Ge₆]^4- framework to reduction by two electrons

Two new intermetallic compounds, Yb₂Ga₄Ge₆ and Yb₃Ga₄Ge₆, were obtained from reactions in molten Ga. A third compound, Eu₃Ga₄Ge₆, was produced by direct combination of the elements. The crystal structures of these compounds were studied by single-crystal X-ray diffraction. Yb ₂Ga₄Ge₆ crystallizes in an orthorhombic cell with a=4.1698(7), b= 23.254(4), c=10.7299(18) A in the polar space group Cmc2₁. The structure of RE₃Ga₄Ge₆ is monoclinic, space group C2/m, with cell parameters a= 23.941(6), b=4.1928(11), C= 10.918(3) Å, β=91.426(4)° for RE=Yb, and a=24.136(2), b=4.3118(4), c=11.017(1) Å, β=91.683(2)° for RE=Eu. The refinement [I>2σ(I)] converged to the final residuals R ₁/wR₂=0.0229/0.0589, 0.0411/0.1114, and 0.0342/0.0786 for Yb₂Ga₄Ge₆, Yb₃Ga₄Ge ₆, and Eu₃Ga₄Ge₆, respectively. The structures of these two families of compounds can be described by a Zintl concept of bonding, in which the three-dimensional [Ga₄Ge ₆]^n- framework serves as a host and electron sink for the electropositive RE atoms. The structural relation of RE₃Ga ₄Ge₆ to of Yb₂Ga₄Ge₆ lies in a monoclinic distortion of the orthorhombic cell of Yb₂Ga ₄Ge₆ and reduction of the [Ga₄Ge₆] network by two electrons per formula unit. The results of theoretical calculations of the electronic structure, electrical transport data, and thermochemical and magnetic measurements are also reported.