Magnetic and 4f quadrupolar behavior of Yb2Co3Al9 and the Kondo lattice compound Yb2Co3Ga9 -: art. no. 094423

Yb2Co3Al9 and Yb2Co3Ga9 are isostructural compounds with an orthorhombic crystal structure having a unique Yb site. The Yb ions are in a trivalent state in Yb2Co3Al9 and order magnetically near 1.2 K inferred both from low-temperature heat capacity and Yb-170 Mossbauer spectroscopy measurements. A well-resolved hyperfine field split spectrum at 0.06 K yields a spontaneous moment of 0.93 mu (B)/Yb. From the thermal variation of the 4f specific heat and of the 4f quadrupolar moment in the paramagnetic phase. we could determine the energies of the four crystal-field states of Yb3+ and we find that the overall splitting is about 180 K. By contrast, the 4f electronic state of Yb ions in Yb2Co3Ga9 is strongly hybridized with the conduction-electron band states. with valency close to 2.9 as determined by L-111-edge x-ray-absorption measurements. The Pauli paramagnetic ground state. with chi similar or equal to 12.5x10(-3) emu (Yb mol)(-1) and the thermal variation of the susceptibility on the one hand, the Sommerfeld coefficient gamma similar or equal to 120 mJ (Yb mol)(-1) K-2 and the thermal variation of the 4f heat capacity on the other hand, are quantitatively well explained in the Coqblin-Schrieffer limit of the J = 7/2 Kondo impurity, with a characteristic hybridization (or Kondo) scale T-0=235 K. From the Yb-170 Mossbauer spectra recorded up to 77 K and Yb-172 perturbed angular correlation data recorded up to 1000 K, we find that the thermal variation of the 4f quadrupolar moment Q(4f)(T) in Yb2Co3Ga9 scales with the magnetic susceptibility chi (T), as predicted by theory to occur in the case the Kondo scale is larger than the crystal-field splittings.