Homoatomic Clustering in the intermediate Valence Compounds Yb_1-yAl_3-xSi_x and Yb_1-yAl_3-xGe_x

Two new phases, Yb1-xAl3-xSix and Yb1-yAl3-xGex, were found by systematic investigations of the according ternary systems. The crystal structures of Yb1-yAl2.8Si0.2 and Yb1-yAl2.8Ge0.2 (defect HT-PuAl3 type) were studied by X-ray powder methods (CuKalpha(1) radiation, lambda = 1.54056 Angstrom, hexagonal system, space group P6(3)/mmc (No. 194), a = 6.009(1) and 6.015(1) Angstrom, c = 14.199(2) and 14.241(5) Angstrom, V= 444.0(2) and 446.2(3) Angstrom(3), 93 and 92 reflections, and 8200 and 8000 profile points for silicide and germanide, respectively). Full profile refinements with 11 and 13 structural parameters resulted in R-I = 0.049 and 0.054, and R- p = 0.088 and 0.104, respectively. The ternary structures are distorted closest packings in comparison with the binary YbAl3 compound with AuCu3-tYPe structure. They are characterized by the formation of Al-3- Si-3-, and Ge-3-homoatomic clusters and aluminum networks. Magnetization measurements show that both the silicide and germanide are valence fluctuation compounds with enhanced electronic density of states at the Fermi level similar to the binary YbAl3. The characteristic maximum of the magnetic susceptibility increases from approximate to 120 K for YbAl3 to approximate to 140 K for Yb(1-y)Al(2.8)Si(0.2)or Yb1- yAl2.8Ge0.2 and further to approximate to 150 K for Yb1- yAl2.75Si0.25. The S-shape of the electrical resistivity curves is also characteristic of valence fluctuations. (C) 2002 Elsevier Science