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 ferrimagnetic double perovskites, A2FeReO6 (A= Ca, Sr, Ba). The A=Ba phase is cubic (Fm3m) and metallic, while the A=Ca phase is monoclinic (P21/n) and nonmetallic.   57Fe 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 t2g - Re t2g interaction is the main cause for the metallic character of the Ba compound; the high covalency of Ca-O bonds and the monoclinic distortion (which lifts the degeneracy of t2g states) seem to disrupt the Re-Re interaction in the case of the Ca compound, making it non-metallic for the same electron count. 
J. Gopalakrishnan, A. Chattopadhyay, S. B. Ogale, T. Venkatesan, R. L.   Greene, A. J. Millis, K. Ramesha, B. Hannoyer and G...