Structural, magnetic, transport and thermal properties of YbCu5-xAux alloys with Au concentration between the limit of structural stability of AuBe5 type at x = 0.4 up to x = 0.7 are reported. The outstanding features of this system are: i) the const
ant and record high values of Cm /T 7J/molK^2 below a characteristic temperature T*, ranging between 150 mK and 350 mK. ii) A power law thermal dependence dependence Cm/T(T>T*)=A/T^q, with q = 1.3 +/- 0.1, and iii) an arising incoherent electronic scattering observed in the resistivity at T < 1K for x < 0.6 despite the fact that Yb magnetic atoms are placed in a lattice. Magnetic frustration, originated in the tetrahedral distribution of Yb atoms, appears as the responsible of the exotic behavior of this system.
