Evidence of Homogeneous Intermediate Valence Coexisting with the Long-Range Magnetic Order in EuCu2(Si, Ge)2

The phase diagram of the intermediate valence system EuCu2Ge2 is investigated by measuring the temperature dependencies of the heat capacity and the electrical resistance as a function of pressure up to 15 GPa. Transition to the magnetic ordered state is observed at pressures up to 9 GPa. It is established that for EuCu2Ge2 system the effects of the hydrostatic and chemical (substitution of Ge for Si) pressure on Eu‑valence are equivalent from the point of view of the transition to the antiferromagnetic ordered ground state. The coexistence of the homogeneous intermediate valence state and long-range magnetic order in EuCu2(Si, Ge)2 has been confidently established. The magnetic phase diagram is presented as a function of Eu average valence. The probable background for the formation of the magnetically ordered intermediate valence state of Eu is considered. There is an indication that it develops on the basis of competition of the two magnetic configurations of Eu, in some analogy with the case of TmSe intermediate valence system.