Divalent, trivalent, and heavy fermion states in Eu compounds

Most of the Eu compounds are in the divalent (Eu2+) electronic state and order magnetically. On the other hand, the Eu-trivalent (Eu3+) compounds exist but are small in number. An energy difference between the Eu2+ and Eu3+ states is, however, not extremely large. Therefore, the valence transition occurs in some Eu compounds. We present the characteristic properties of the Eu compounds from several viewpoints: a canting magnetisation in the Eu2+-antiferromagnets, the Fermi surface property in the Eu3+ compounds of EuPd3 and EuCo2Si2, the heavy fermion state in EuNi2P2, the temperature-induced valence transition in EuPd2Si2, the pressure-induced valence transition in EuRh2Si2 and EuRu2P2, and the heavy fermion state approaching to the quantum critical point with increasing pressure in Eu2Ni3Ge5.