Unique pressure versus temperature phase diagram for antiferromagnets Eu₂Ni₃Ge₅ and EuRhSi₃

We studied the magnetic properties of the antiferromagnets Eu₂Ni₃Ge₅ and EuRhSi₃ by measuring their electrical resistivity, specific heat, magnetic susceptibility, magnetization, and thermoelectric power, together with the electrical resistivities at high pressures of up to 15 GPa. These compounds have almost divalent Eu ions at ambient pressure and order antiferromagnetically with a successive change in the antiferromagnetic structure at T_N = 19 K and T_N′ = 17 K in Eu₂Ni₃Ge₅, and at T_N = 49 K and TN = 45 K in EuRhSi₃. Magnetic field versus temperature (H-T) phase diagrams were constructed for both compounds from the magnetization measurements. The Néel temperature in Eu₂Ni₃Ge₅ was found to increase up to 7 GPa but to decrease continuously with further increasing pressure, without the so-called valence transition. Under a high pressure of 15 GPa, Kondo-like behavior of the electrical resistivity was observed, suggesting the existence of the heavy-fermion state at low temperatures. A similar trend is likely to occur in EuRhSi₃. The present P-T phase diagrams for both compounds are the first cases that are reminiscent of the phase diagram of EuCu₂(Si_xGe_1-x)₂.