HETEROGENEOUS YB3-YB2+ MIXED-VALENCY AND UNUSUAL KONDO GROUND-STATE IN YB4AS3()

We present Yb-170 Mossbauer spectroscopy data in the temperature range 0.045-50 K and magnetic susceptibility and isothermal magnetisation m easurements in the temperature range 0.6-50 K in the heavy fermion com pound Yb4As3. The presence of approximately 20% of Yb+3 ions is detect ed in the Mossbauer spectra with an applied magnetic field of 7 T, yie lding a microscopic proof of the existence of a heterogeneous mixed va lence state containing Yb3+ and Yb2+ ions. No magnetic ordering of the Yb3+ sublattice (with moments bigger than 0.15 mu(B)) is detected dow n to 0.045 K. The low temperature upturn of the magnetic susceptibilit y is shown to be an intrinsic feature of Yb4As3, suggesting that the g round state is of an unconventional Kondo lattice type.