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 EuCo2 Si2, the heavy fermion state in EuNi2 P2, the temperature-induced valence transition in EuPd2 Si2, the pressure-induced valence transition in EuRh2 Si2 and EuRu2 P2, and the heavy fermion state approaching to the quantum critical point with increasing pressure in Eu2 Ni3 Ge5.
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