Pressure effect on the valence transition of (formula presented)

The temperature dependence of electrical resistivity (Formula presented) has been measured for (Formula presented) with (Formula presented) at pressures to 15 kbar. At ambient pressure, all the compounds are antiferromagnetic with stable (Formula presented) moments, but compounds with (Formula presented) 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 transition becomes continuous. The temperature dependence of (Formula presented) is discussed by regarding the system as a virtual alloy of (Formula presented) where (Formula presented) and (Formula presented) are the occupation probabilities of (Formula presented) and (Formula presented) states, respectively. We argue that impurity scattering due to randomness in a virtual alloy is responsible for the temperature dependence of (Formula presented) in this system. By combining the present results with previous determinations of the pressure dependence of the tetragonal unit-cell volume, a generalized pressure-temperature phase diagram of (Formula presented) is established which indicates that applying pressure is equivalent to Si doping in (Formula presented)