Properties of the ferrimagnetic double perovskites A2FeReO6 (A = Ba and Ca)

Ceramics of A(2)FeReO(6) double perovskites have been prepared and studied for A = Ba and Ca. Ba2FeReO6 has a cubic structure (Fm3m) with a approximate to 8.0854(1) Angstrom whereas Ca2FeReO6 has a monoclinic symmetry with a approximate to 5.396(1) Angstrom, b approximate to 5.522(1) Angstrom, c approximate to 7.688(2) Angstrom and beta = 90.4 degrees (P21/n). The barium compound is metallic from 5 K to 385 K, i.e. no metal-insulator transition has been seen up to 385 K, and the calcium compound is semiconducting from 5 K to 385 K. At 5 K, we observed a negative magnetoresistance of 10% in a magnetic field of 50 kOe for Ba2FeReO6. Magnetization measurements show a ferrimagnetic behaviour for both materials, with T-c approximate to 315 K for Ba2FeReO6 and above 385 K for Ca2FeReO6. A specific heat measurement on the barium compound gave an electron density of states at the Fermi level, N(E-F), equal to 5.9 x 10(24) eV(-1) mol(-1). Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2Fe(Mo, Re)O-6.