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Structural characterization and magnetic properties determination of nanocrystalline Ba3Fe2WO9 and Sr3Fe2WO9 perovskites prepared by the modified aqueous sol-gel route

Perovskite materials exhibit intriguing physical properties that have been extensively studied for various practical applications - from the discovery of ceramic high-temperature superconductors to the hybrid organic–inorganic semiconductors for high-efficiency solar cells - the materials science and applications of perovskites have been a very broad research area open to many revolutionary discoveries for new device concepts. The ideal stoichiometry of a perovskite (ABX3) can be modified by changing one, two or three different types of atoms in the A or B cation positions, hence because of this flexibility, perovskites compounds can adopt more complex structures, such as: A2BB’O6, AAˈB2O6, AA’BB’O6, A3B2B’O9, A2A’B2B’O9. In particular, triple perovskites A3B2B’O9 are exhibiting a range of interesting physical properties such as relaxor ferromagnetic, multiferroicity and spin-glass, unfortunately often they are reported without any detailed structural characterization. Sr3Fe2WO9 and Ba3Fe2WO9 triple perovskites have been prepared in nanocrystalline form using a modified sol-gel route with citric acid as chelating agent. These materials have been studied by X-ray powder diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), 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 = 5.5724(1) and c=7.9022(2) Å. Both compounds exhibit a disordered perovskite structure, in which Fe3+ and W6+ cations distribute randomly at the B-sites of the perovskite lattice: Sr2[Fe0.78W0.22]2a[Fe0.59W0.41]2bO6 and Ba6[Fe1.69W0.31]2a[Fe2.31W1.69]4fO18. The detected hysteresis loops with non-zero remanent magnetization and rather large coercive field reveal ferrimagnetic ordering with ordering temperatures of 373 K for Sr3Fe2WO9 and 330 K for Ba3Fe3WO9. 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 mu(B) f.u.(-1) and 0.35 mu(B) f.u.(-1) for Sr and Ba compounds respectively, although full saturation is not reached.

citrate route ; multiferroics ; perovskites ; sol-gel methods ; tungsten

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