Influence of chemical composition on the X-ray photoemission, thermopower, specific heat, and magnetic properties of CeNi₂(Si_1-yGe_y)₂

We report our studies of the intermediate compositions between CeNi₂Si₂ and CeNi₂Ge₂, i.e., the alloys CeNi₂(Si_1-yGe_y)₂ by means of the thermopower, electrical resistivity, specific heat, magnetic susceptibility, and X-ray photoemission measurements. CeNi₂Si₂ is a fluctuating valence system and CeNi₂Ge₂ is known to show the heavy fermion behaviour. The change of the temperature dependence of the resistivity towards the typical metallic behaviour occurs below y ∝ 0.25. The transition between CeNi₂Si₂ and CeNi₂Ge₂ is discussed in the frames of competition between the crystal electric field and Kondo interactions. It is found that valence stabilisation occurs for Ge content y > 0.25. The hybridization energy Δ determined from the XPS Ce 3d spectrum reflects well the behaviour of the parameter E_ex obtained from the analysis of the magnetic susceptibility by the interconfiguration fluctuation model. It has been also shown that thermopower data can be successfully described employing the single ion model for 0.6 < y < 1.0 and two-band model including the crystal electric field splitting for y ≤ 0.25. (orig.)