Structural characterization and magnetic property determination of nanocrystalline Ba 3 Fe 2 WO 9 and Sr 3 Fe 2 WO 9 perovskites prepared by a modified aqueous sol–gel route

Sr 3 Fe 2 WO 9 and Ba 3 Fe 2 WO 9 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 Ba 3 Fe 2 WO 9 is hexagonal, space group P 6 3 / mmc , with a = 5.7577(2) and c = 14.1095(5) Å, whereas Sr 3 Fe 2 WO 9 presents a tetragonal lattice, space group I 4/ 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 Ba 3 Fe 2 WO 9 and 35.9(1) nm for Sr 3 Fe 2 WO 9 with moderate microstrain. They exhibit a disordered perovskite structure, in which Fe 3+ and W 6+ cations distribute randomly at the B-sites of the perovskite lattice: Sr 2 [Fe 0.72 W 0.28 ] 2a [Fe 0.61 W 0.39 ] 2b O 6 and Ba 6 [Fe 1.71 W 0.29 ] 2a [Fe 2.29 W 1.71 ] 4f O 18 . 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 Sr 3 Fe 2 WO 9 and 330 K for Ba 3 Fe 2 WO 9 . 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.