Pressure effect on the valence transition of EuNi2(Ge1-xSix)(2)

The temperature dependence of electrical resistivity rho has been measured for EuNi2(Ge1-xSix), with 0.0 less than or equal to x less than or equal to 0.15 at pressures to 15 kbar. At ambient pressure, all the compounds are a ntiferromagnetic with stable Eu2+ moments, but compounds with 0.10 less tha n or equal to x less than or equal to 0.15 show a pressure-induced valence transition. Just above a critical pressure, the compound undergoes a first- order valence transition. With further increase in pressure, the valence tr ansition becomes continuous. The temperature dependence of rho is discussed by regarding the system as a virtual alloy of Eup22+Eup33+, where p(2) and p(3) are the occupation probabilities of Eu2+ and Eu3+ states, respectivel y. We argue that impurity scattering due to randomness in a virtual alloy i s responsible for the temperature dependence of rho in this system. By comb ining the present results with previous determinations of the pressure depe ndence of the tetragonal unit-cell volume, a generalized pressure-temperatu re phase diagram of EuNi2Ge2 is established which indicates that applying p ressure is equivalent to Si doping in EuNi2Ge2. [S0163-1829(99)07201-X].