Papers by Keyword: Double-Perovskite

Abstract: The double-perovskite La_0.4Sr_1.6CoNbO_6-δ (LSCN) powders were synthesized by the solid-state reaction method. The electrical conductivities of LSCN samples were tested in air and 5 vol%H₂/Ar. The results show that the conductivity of LSCN in 5 vol%H₂/Ar (8.12 Scm^-1) at 850 °C was higher than that in air (7.03 Scm^-1). The activation energy obtained from the Arrhenius function was 0.821 eV in air and 0.707 eV in 5 vol%H₂/Ar. The analysis of XPS shows that there exit three valence states of Co (Co⁺², Co⁺³, Co⁺⁴) and two of Nb (Nb⁺⁴, Nb⁺⁵). The loss of lattice oxygen in LSCN not only produces oxygen vacancies, but also generates excess electrons, which contributes to the electrical conductivity of the LSCN samples.

Abstract: The double perovskite Sr2FeMnO6 has been synthesized by the solid-state reaction method. X-ray diffraction and subsequent Rietveld refinement confirm that Sr2FeMnO6 has the double-perovskite structure with a cubic cell (a=7.726 Å) and space group Fm3m (No. 225) . The appearance of the superlattice reflection (111) indicates that Fe and Mn atoms alternatively occupy the center of oxygen octahedron in an ordered way. Based on magnetization hysteresis curve and ESR measurements, a ferrimagnetic to paramagnetic phase transition for the double perovskite Sr2FeMnO6 was observed, and the Neel temperature was determined to be TN ~475K. In paramagnetic phase above TN, the value of gyromagnetic factor was determined to be very close to 2.

Abstract: The compounds of Fe substitution of Cr in nanotructured Sr₂Fe_1-xCr_xMoO₆ (0≤x≤0.3) double perovskite have been prepared by sol-gel method. The x-ray diffraction patterns of the samples show that the samples are in nanometer range. And the average grain size D for these samples is 39.7, 37.1, 33.4, 31.9, and 31.2nm, respectively, decreases with the increasing of Cr ion. The saturation magnetization and the magnetoresistance of the samples decrease with the increasing of Cr for the disorder defects which increase for the close ionic radii of Mo and Cr.

Abstract: We observe a large magneto-resistance in very thin Ba₂FeMoO₆ films of ~-12% at 10K, which is not significantly changed when thin NiFe/Ba₂FeMoO₆ bi-layers are made. This suggests that the magneto-resistance in both cases is dominated by inter-grain tunnelling in the Ba₂FeMoO₆ thin film. There is an anomalous peak in the resistance at ~300K in the bi-layer film that can not be easily explained. However, we know that the temperature where this peak occurs is close to the Ba₂FeMoO₆ Curie temperature.

Abstract: We report the results from magnetotransport measurements on polycrystalline Sr_2-xLa_xFeMoO₆ samples at magnetic fields of up to 8 T. We observe a normal as well as an anomalous Hall effect. We find that there is a small decrease in the normal Hall coefficient for La doped samples.

Abstract: Crystal structure and magnetic properties of the double perovskite compounds Sr₂(Fe_1-xV_x)MoO₆ (0 ≤ x ≤ 0.1) have been investigated in this work. Rietveld refinement of the crystal structure shows that the cation ordering decrease monotonously with the V content. Magnetic measurement shows that saturation magnetization of the compounds decrease with increasing x due to the reduced degree of ordering. Analysis of saturation magnetization based on the ferrimagnetic model (FIM) indicates that the V atoms in Sr₂(Fe_1-xV_x)MoO₆ selectively occupy the Mo sites instead of Fe sites.

Abstract: Hole doped double perovskite compounds (Sr, Na)₂FeMoO₆ with the Na content of x = 0, 0.02, 0.03, 0.07 and 0.17 have been synthesized by sol-gel method. Effects of hole doping on the crystal structure and magnetic properties of Sr₂FeMoO₆ have been investigated. XRD pattern indicates that all the samples are of single phase and belong to the space group I4/m. The degree of cation ordering in the Na-doped Sr₂FeMoO₆ compounds shows a non-monotonic variation with the doping level. The saturation magnetization of the compound increases with x for x<0.17. Similar to the electron doped Sr₂FeMoO₆, provided that the doped hole enters selectively the spin-down band, the variation of the saturation magnetization can be explained in light of the ferrimagnetic model (FIM).

Abstract: Rare-earth-doped compounds (Sr_1.85Ln_0.15)FeMoO₆ (Ln=Sr, La, Ce, Pr, Nd, Sm and Eu) have been prepared by solid-state reaction. Crystal structure and magnetic properties were investigated by means of X-ray diffraction and magnetic measurements. All the samples are single phase and belong to the I4/m space group. Due to the competing contributions of electron doping and steric effects, the unit-cell volume of the doped compounds changes slightly and does not vary systematically with the ionic radius of the rare-earth ions. The temperature dependence of the magnetization of (Sr_1.85Ln_0.15)FeMoO₆ indicates that the Curie temperature of the doped compounds has increased upon doping, except for the Eu-doped compound.

Abstract: New electron doped double perovskite compound (Sr2-xSmx) FeMoO6 (0≤x≤0.25) has been synthesized by solid-state reaction. Crystal structure and magnetic properties of the compounds have been investigated by X-ray powder diffraction (XRD) and magnetic measurements. XRD revealed that all the compounds were of single phase and belonged to a I 4/m lattice. The degree of cationic ordering on the B site was decreased pronouncedly by the electron doping. Different from the results of La- and Nd-doped Sr2FeMoO6, Curie temperature (TC) of (Sr2-xSmx) FeMoO6 decreased first with the doping and then increased beyond x = 0.15, indicating that steric effect was enhanced as the radius of rare-earth ions decreased.

Abstract: Double-perovskite Sr2Fe1-xScxMoO6- (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) powders have been synthesized by sol-gel citrate method. Initial powders were calcined in different temperature and atmosphere (air, H2(4vol%)/Ar), then analyzed by using the methods of X-ray, scanning electron microscopy (SEM), as well as thermal analysis. It is found that the formation of perovskite structure is related to the content of Sc, pH value, calcination temperature and sintering atmosphere. Especially, a pure perovskite structure almost completely formed after three hours sintering in atmosphere of H2(4vol%)/Ar. Although the formation of perovskite structure also happened in the air atmosphere, at the same time the SrMoO4 structure formed undesirably as a result of oxidization of Mo.