oxygen vacancy: double donor (missing O fails to bind two electrons)
Gd³⁺ substituted for Eu²⁺: single donor

The systems are not diluted: every cation is magnetic

Electrons increase T_c to ~150 K (Shafer and McGuire, 1968)
Mechanism: carrier-mediated, see Lecture 3

Electrons lead to metal-insulator transition close toT_c: