Large Magnetoresistance And Volume Expansion Associated With Valence Transition In Eu(Rh1-Xirx)(2)Si-2

We report lattice parameters as a function of temperature, magnetization curves, and magnetoresistance of poly-crystalline samples of Eu(Rh1-xIrx)(2)Si-2. The lattice parameters a and c for x = 0.3, 0.4, and 0.5 jump at around the valence transition temperature T-v. For these samples, the high-temperature (larger a, c) and low-temperature (smaller a, c) phases, the volume fraction of which changes with temperature, coexist at around T-v, which means that the valence transition is of the first order. The magnetization curves of x = 0.3, 0.4, and 0.5 at T = 4.2K exhibit a metamagnetic transition with a large hysteresis, implying a first-order field-induced valence transition. The transition field B-v is proportional to T-v, B-v = alpha T-v with alpha = 0.36 T/K. For x = 0.3, a large negative magnetoresistance associated with the field-induced valence transition is observed. The origin of the negative magnetoresistance is discussed.