Pressure Evolution of Characteristic Electronic States in EuRh₂Si₂ and EuNi₂Ge₂

EuRh₂Si₂ and EuNi₂Ge₂ are known to reveal the valence transition at pressure of about 1 and 2 GPa, respectively, from a divalent state (Eu²⁺) to a nearly Eu-trivalent state (Eu^3-δ′). We have succeeded in growing single crystals of EuRh₂Si₂ by the Bridgman method and EuNi₂Ge₂ by the In-flux method. In order to clarify the transition of electronic properties from Eu²⁺ to Eu^3-δ′ in detail, we carried out electrical resistivity measurements under pressure. EuRh₂Si₂ indicates a remarkable first-order valence transition in the pressure range from 1 to 2 GPa with a sharp hysteresis in the electrical resistivity. At 2.12 GPa, the temperature dependence of the electrical resistivity exhibit a broad peak, implying a moderate heavy-fermion state such as EuPd₂Si₂ and EuIr₂Si₂. While, EuNi₂Ge₂ indicates a first-order valence transition in the pressure range from 2 to 3.22 GPa with a hysteresis in the electrical resistivity. At 3.60 GPa, the electrical resistivity of EuNi₂Ge₂ also shows a characteristic behavior for a moderate heavy-fermion compound. Pressure - temperature phase diagrams for both compounds are constructed.