The physical properties of the single-crystalline U2Ir3Si5, a new ternary uranium compound with U2Co3Si5-type orthorhombic structure, are investigated by means of magnetic susceptibility χ(T), specific heat C(T), electrical resistivity ρ(T), and high-field magnetization M(H,T) measurements. U2Ir3Si5 undergoes an antiferromagnetic transition at TN=36 K followed by a first-order phase transition at T0=25.5 K. The sharp peak in C(T) at T0 and the obvious hysteresis behavior in χ(T),ρ(T), and M(H,T) around T0 provide strong evidence for the first-order phase transition. The ρ(T) measurements along the a and b axes reveal the negative temperature coefficients of resistance over a wide temperature range, which can be understood based on the semiconductorlike narrow band gap model or the Kondo effect. The M(H) curve measured at 4.2 K along the b axis shows three-step metamagnetic transitions within a narrow field region around 200 kOe and a large hysteresis near the first transition field, while the MH?a(H) and MH?c(H) curves show no transitions up to 560 kOe suggesting the strong magnetic anisotropy. A possible mechanism of the first-order phase transition at T0 is the occurrence of a magnetic quadrupolar order, resulting from the quasi-one-dimensional uranium zigzag chain.
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