Alloying and pressure effect on the mixed-valence state of Yb in YbInCu4

The susceptibility and thermal expansion of Yb1-xRxInCu4 (x less than or equal to 0.25; R = Y, La and Ce) have been measured at ambient and high pressures. These compounds undergo a first-order valence transition from an intermediate-valence state to a nearly trivalent state with increasing temperature. The valence change at the transition is deltanu similar to 0.1 for YbInCu4. It decreases on both alloying and applying pressure for all the compositions studied. From the pressure effect on the susceptibility of YbInCu4, the Gruneisen parameter for the Kondo energy Omega(K) is determined to be -34.5 and -20 for the low-temperature and the high-temperature state, respectively. The F coefficient characterizing the hybridization strength between the 4f and conduction electrons for the intermediate-valence state is nearly five times larger than that for the high-temperature local-moment state. The obtained results suggest that the change of the 4f-conduction band electron hybridization rather than the volume dependence of the Kondo interaction is responsible for a large modification of the physical properties of the YbInCu4-based compounds at the valence transition.