Hole doping effects in Sr₂FeMo_1−<i>x</i>W_<i>x</i>O₆(0 ≤<i>x</i>≤ 1) double perovskites: a neutron diffraction study

Abstract

The effect of the localization of electrons as Mo is substituted by W in the Sr2FeMo1−xWxO6 (0≤x≤1) series has been studied by neutron powder diffraction (NPD) and SQUID magnetometry. The samples for x = 0.2,0.5,0.8 and 1 were prepared by soft-chemistry procedures and annealed under suitable reducing conditions for each member of the series. As the number of itinerant electrons drastically changes from Sr2FeMoO6 (one electron per formula unit) to Sr2FeWO6 (no itinerant electrons), this series constitutes an ideal system to explore band filling effects in the magnetic structure and the Curie temperature of half-metallic ferromagnetic double perovskites. The room-temperature crystal structure of the former members of the series (x≤0.5) is tetragonal (I4/m) and it is characterized by a single antiphase tilt of the FeO6 and (Mo,W)O6 octahedra along the c axis; this structure evolves to the more distorted monoclinic (P 21/n) for x = 0.8 and 1, containing three kinds of non-equivalent oxygen atoms. The driving force of the structural phase transition is the promotion of the voluminous Fe2+ cations upon W substitution, as demonstrated by a bond valence study. The phase transition is accompanied by a sudden decrease of the distortion of the FeO6 and (Mo,W)O6 octahedra. Our results show that the progressive localization of carriers upon W substitution provides a good description of the magnetic and structural properties along the series. The study of the low-temperature (10 K) NPD pattern of the heavily W-doped Sr2FeMo0.2W0.8O6 suggests a lack of long-range magnetic ordering, which is consistent with the presence of isolated ferromagnetic clusters in the insulating, antiferromagnetic matrix created by Fe–O–W–O–Fe superexchange interactions.