Hole doping effects in Sr2FeMo1−xWxO6 (0 ≤ x ≤ 1) double perovskites: a neutron diffraction study

Abstract

The effect of the localization of electrons as Mo is substituted by W in theSr2FeMo1−xWxO6 (0≤x≤1)series has been studied by neutron powder diffraction (NPD) and SQUID magnetometry. The samplesfor x = 0.2,0.5,0.8 and 1 were prepared by soft-chemistry procedures and annealed under suitable reducing conditionsfor each member of the series. As the number of itinerant electrons drastically changes fromSr2FeMoO6 (one electron performula unit) to Sr2FeWO6 (no itinerant electrons), this series constitutes an ideal system to explore band filling effectsin the magnetic structure and the Curie temperature of half-metallic ferromagnetic doubleperovskites. The room-temperature crystal structure of the former members of the series(x≤0.5) istetragonal (I4/m) and it is characterized by a single antiphase tilt of theFeO6 and(Mo,W)O6 octahedraalong 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 drivingforce of the structural phase transition is the promotion of the voluminousFe2+ cations upon W substitution, as demonstrated by a bond valence study. Thephase transition is accompanied by a sudden decrease of the distortion of theFeO6 and (Mo,W)O6 octahedra. Our results show that the progressive localization of carriers upon Wsubstitution provides a good description of the magnetic and structural properties alongthe series. The study of the low-temperature (10 K) NPD pattern of the heavily W-dopedSr2FeMo0.2W0.8O6 suggests a lack of long-range magnetic ordering, which is consistent with the presence ofisolated ferromagnetic clusters in the insulating, antiferromagnetic matrix created byFe–O–W–O–Fe superexchange interactions.