Metallic and nonmetallic double perovskites: A case study of A(2)FeReO(6) (A = Ca, Sr, Ba)

We have investigated the structure and electronic properties of ferrimagnet ic double perovskites, A(2)FeReO(6) (A = Ca, Sr, Ba). The A = Ba phase is c ubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2(1) /n) a nd nonmetallic. Fe-57 Mossbauer spectroscopy shows that iron is present mai nly in the high spin (S = 5/2) Fe3+ state in the Ca compound, while it occu rs in an intermediate state between high spin Fe2+ and Fe3+ in the Ba compo und. It is argued that a direct Re t(2g) - Re t(2g) interaction is the main cause for the metallic character of the Ba compound; the smaller size of C a and the monoclinic distortion (which lifts the degeneracy of t(2g) states ) seem to disrupt the Re-Re interaction in the case of the Ca compound, mak ing it nonmetallic for the same electron count.