Ga substitution effect on the valence transition of Eu₂Pt₆Al₁₅

We have synthesized polycrystalline samples of Eu₂Pt₆(Al₁−_xGa_x)₁₅ (x = 0, 0.05, 0.1, 0.2, 0.3, and 1) and examined electrical resistivity, magnetic susceptibility, high-field magnetization, and X-ray absorption spectra at the Eu L₃-edge. With increasing x up to 0.1, the first-order valence transition is shifted to low temperatures. For x ≥ 0.1, a nearly divalent state is stabilized down to the lowest temperature, and an antiferromagnetic order emerges below T_N = 13–16 K. For x = 0 and 0.05, magnetization curves in pulsed magnetic fields at T = 4.2 K clarify a field-induced valence transition at B_v = 29 and 19 T, respectively. The ratio of B_v to the valence transition temperature T_v is close to a universal value of other Eu-based systems with a different crystal structure (ThCr₂Si₂ type). The phase boundary curve in the T–B phase diagram is discussed thermodynamically.