Metallic and nonmetallic double perovskites:: A case study of A2FeReO6 (A = Ca, Sr, Ba)

We have investigated the structure and electronic properties of ferrimagnetic double perovskites, A(2)FeReO(6) (A = Ca, Sr, Ba). The A = Ba phase is cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2(1) /n) and nonmetallic. Fe-57 Mossbauer spectroscopy shows that iron is present mainly in the high spin (S = 5/2) Fe3+ state in the Ca compound, while it occurs in an intermediate state between high spin Fe2+ and Fe3+ in the Ba compound. 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 Ca 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, making it nonmetallic for the same electron count.