Metallic and nonmetallic double perovskites: a case study of A<SUB>2</SUB> FeReO<SUB>6</SUB>(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 Ret2g-Ret2g 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 t2g states) seem to disrupt the Re-Re interaction in the case of the Ca compound, making it nonmetallic for the same electron count