Application of Doniach diagram on valence transition in EuCu ₂(Si_xGe_1-x)₂

To investigate the valence transition of Eu in the pseudo-binary compounds series, EuCu₂(Si_xGe_1-x)₂, measurements of the lattice constants, the X-ray absorption spectroscopy (XAS), the magnetic susceptibility, χ, the electric resistivity, ρ, the thermoelectric power, S, and the specific heat, C, have been carried out. The antiferromagnetic order for the samples with 0 ≤ x ≤ 0.50, the temperature-induced valence transition for the samples with 0.68 ≤ x ≤ 1.0 and the state with a high electronic specific heat coefficient for the samples having the intermediate range of x of the above two were observed. The plotting of the Néel temperature, T_N, and of the valence transition temperature, T_V, as the function of x is found to resemble very well to the Doniach diagram, giving an evidence of the competition of the two interactions, the one for the valence transition, the other for the magnetic order. The observation of the sign change of S near to the temperature T _V is found to be a new feature of Eu compounds.