Structural characterization and magnetic property determination of nanocrystalline Ba3Fe2WO9 and Sr3Fe2WO9 perovskites prepared by a modified aqueous sol–gel route

Abstract

Sr₃Fe₂WO₉ and Ba₃Fe₂WO₉ triple perovskites have been prepared in nanocrystalline form using a modified sol–gel route with citric acid as a chelating agent. These materials have been studied by X-ray powder diffraction (PXRD), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and unpolarized Raman spectroscopy, while their magnetic properties have been determined using SQUID measurements. At room temperature, the crystal structure of 6H-perovskite Ba₃Fe₂WO₉ is hexagonal, space group P6₃/mmc, with a = 5.7577(2) and c = 14.1095(5) Å, whereas Sr₃Fe₂WO₉ presents a tetragonal lattice, space group I4/m, with a = 5.5803(1) and c = 7.8745(3) Å. Both compounds are nanocrystalline with average crystallite sizes of 25.6(1) nm for Ba₃Fe₂WO₉ and 35.9(1) nm for Sr₃Fe₂WO₉ with moderate microstrain. They exhibit a disordered perovskite structure, in which Fe³⁺ and W⁶⁺ cations distribute randomly at the B-sites of the perovskite lattice: Sr₂[Fe_0.72W_0.28]^2a[Fe_0.61W_0.39]^2bO₆ and Ba₆[Fe_1.71W_0.29]^2a[Fe_2.29W_1.71]^4fO₁₈. The detected hysteresis loops with non-zero remanent magnetization and a rather large coercive field reveal ferrimagnetic ordering with ordering temperatures of 373 K for Sr₃Fe₂WO₉ and 330 K for Ba₃Fe₂WO₉. Magnetic measurements show the onset of ferrimagnetic ordering at relatively high temperatures of 717 and 711 K for the Sr and Ba compounds, respectively; however the magnetization isotherms at 2 K exhibit, for H = 50 kOe, maximum magnetization values close to 0.8 μ_B (f.u.)⁻¹ and 0.35 μ_B (f.u.)⁻¹ for Sr and Ba compounds, respectively, although full saturation is not reached.