Zhai Yongqing, Zhang Zhang, Feng Shihua, Huo
Guoyan, Shi Lingjuan, Hao Xiaohong Abstract The sol-gel method is used to
prepare double perovskite oxide Sr1.9K0.1FeMoO6. In this
paper, the products at different stage during the process of Sr1.9K0.1FeMoO6
synthesis were analyzed by X-ray diffraction, infra-red spectra, thermal gravimetric and
differential thermal, and X-ray energy disperse spectra. The results reveal the primary
mechanism of sol-gel method: component elements still exist as the form of amorphous
citric complexes after the pyrolysis of gel. Then the precursor after pyrolysis is
transformed into simple compounds SrMoO4 and SrFeO3-x. Finally,
reduction reaction leads to the formation of Fe-O-Mo bond, i.e., generating the final
product Sr1.9K0.1FeMoO6. 1 INTRODUCTION 2 EXPERIMENTAL Polycrystalline Sr1.9K0.1FeMoO6 samples were prepared by Sol-Gel method. Stoichiometric powders of Sr(NO3)2, Fe(NO3)3·9H2O and (NH4)6Mo7O4·4H2O were mixed with citric acid and then dissolved. The pH of the solution was adjusted by aqueous ammonia. A slight green gel was formed after several hours under the condition of water bath. Then the gel was put into oven and dried at 90°C. Subsequently, the dry gel was pyrolyzed at 180℃. The dry gel was frothed, fumed, and finally formed a dry black sponge (which was called precursor). Then the precursor was pre-sintered at 600°C. Finally, the powder was mixed with appropriate active carbon powder and then sintered in the reducing atmosphere provided by active carbon particles to obtain the final product. 2.2 Characterization Differential thermal analysis (DTA) and thermogravimetric (TG) analysis were carried with a DT-40 analyzer, at a rate of 20°C/min under static air conditions in the temperature range of 20 to 735°C, to analyze the decomposition reaction and phase transformation of the precursor. I.R. spectra were recorded on a Nicolet 380 spectrometer (400-4000 cm-1, KBr pellets). The crystal structure and the phase purity of the samples were examined by X-ray powder diffraction (XRD) using a Y2000 diffractometer with Cu Ka radiation (30kV×20mA) at room temperature. XRD was performed on powdered samples over the 2q range of 15° to 75°. A step scan mode was employed with a step width of 2q = 0.06° and a sampling time of 1s. The size and morphology of the samples were investigated with a KYKY-2800B scanning electron microscope (SEM). The content of the cations were analyzed by X-ray energy dispersion spectrometer (EDS). The field dependence of the magnetization was recorded by a vibrating-sample magnetometer (WKVSM) in a field of 500Oe. 3 RESULTS AND DISCUSSION The TG-DTA curves of the precursor are shown in Fig.1. It shows that there is a little weight loss in the stage of 50~250°C in the TG curve, which is due to the dehydration of the precursor. The weight loss mainly occurs in the stage of 250~620°C and has little change after 700°C. Two small exothermic peaks at 422°C and 497°C in DTA curve can be assigned as the thermal decomposition of NH4NO3 and free citric acid. The large composite exothermic peak in the stage of 500~620°C can be ascribed to the thermal decomposition and oxidation of the citrate complex. Thermal effect is not distinctive after 700°C, which indicates that the complex has decomposed completely. Fig.1 TG-DTA curves of precursor obtained after pyrolysis 3.2 XRD analysis The XRD pattern of the product obtained after pre-sintering under 600°C is showed in Fig.3. Through comparing Fig.2 with Fig.3, it can be seen that the precursor has decomposed after pre-sintering and formed the mixture of SrMoO4 and SrFeO3-x. A little amount of SrCO3 is found in this product, which may results from the CO2 in the air during the process of the pre-sintering. Fig.2 XRD pattern of precursor obtained after pyrolysis Fig.3 XRD pattern of product obtained after pre-sintering The XRD patterns of Sr1.9K0.1FeMoO6 and Sr2FeMoO6 are showed in Fig.4. It shows that Sr1.9K0.1FeMoO6 has the same double perovskite structure as Sr2FeMoO6. All the reflections can be indexed based on the space group I4/mmm by the Jade5 program and the appearance of the superstructure reflections (101) and (211) at 19° and 37° indicates the ordering of the Fe and Mo atoms in our samples. Fig.4 XRD patterns of Sr2FeMoO6 and Sr1.9K0.1FeMoO6 From the data in Fig.4 and Table 1, it can be seen that the partly substitution of K+ for Sr2+ in Sr2FeMoO6 results in the diffraction peaks slightly shift to large angle, while the crystal plane index has no significant change and the unit-cell parameters decrease slightly. This can be explained by two factors: one is the steric effect; the other is the electron-doping effect. On the one hand, the cell volume of the compounds will be increased because of the substitution of bigger alkali metal ions for comparing the data of ionic radius: r(Sr2+) = 112pm, r(K+) = 138pm. On the other hand, as reported in Refs. [14], in the rare earth doped double perovskite compound Sr2-xLaxFeMoO6, electron doping can increase the unit-cell volume of the compounds because the extra electron provides delocalized carriers selectively to the spin-down t2g metallic spin channel, which will increase the density of states at the Fermi level. These carriers can modify the bond lengths through screening of the ionic potentials that determine the interatomic distances. For the as-synthesized Sr1.9K0.1FeMoO6, the doped K+ is bigger than Sr2+ but has fewer electrons, which may lead to the opposite electron-doping effect, i.e. the doping of K+ will reduce to the the decrease of cell volume. Thereby, for the sample Sr1.9K0.1FeMoO6, the electron-doping effect is slightly stronger than the steric effect, which results in the decrease of the cell volume. Table 1 XRD data and unit cell parameters of Sr2FeMoO6 and Sr1.9K0.1FeMoO6
3.3 IR analysis Fig.5 IR spectra of precursors obtained after pyrolysis, pre-sintering and the final product 3.4 EDS analysis Fig.6 shows the X-ray energy dispersion spectrum (EDS) of the final products. Based on it, the molar ratio of Sr, K, Fe and Mo in this sample obtained by quantitative analysis is nearly agreement with the designed ratio (1.9 : 0.1 : 1 : 1), which indicates that the composition elements of the raw material have transformed into final product and the final product is expected Sr1.9K0.1FeMoO6. 3.5 Magnetism of sample The magnetization versus field curve (M-H) of Sr1.9K0.1FeMoO6 is showed in Fig.7. It shows that the magnetization was still not saturated as the applied field rose to 0.7 T from 0 T. The saturation magnetization of the sample is 1.62mB/f.u., which is obtained as the linear part of magnetization versus 1/H curve was extrapolated to 1/H = 0. Fig.7 M-H curve of Sr1.9K0.1FeMoO6 4 CONCLUSIONS The primary mechanism of sol-gel method for the synthesis of Sr1.9K0.1FeMoO6 were obtained by the means of X-ray diffraction, infra-red spectrum, thermal gravimetric and differential thermal, and X-ray energy disperse spectrum. It is as follow: component elements still exist as the form of amorphous citric complexes after the pyrolysis of gel at 180°C and no corresponding oxides are formed; then the precursor obtained after pyrolysis is decomposed and transformed into simple compounds SrMoO4 and SrFeO3-x by pre-sintering at 600°C; finally, thermal reduction reaction at1200°C leads to the formation of Fe-O-Mo bond, i.e., generating the final product Sr1.9K0.1FeMoO6. REFERNECES 翟永清,张张,冯仕华,霍国燕,石领娟,郝晓红 (河北大学化学与环境科学学院,河北 保定,071002) 摘要 采用溶胶-凝胶法制备了双钙钛矿Sr1.9K0.1FeMoO6,通过对溶胶-凝胶法合成Sr1.9K0.1FeMoO6的各阶段产物的X射线衍射、红外光谱、差热-热重分析及X射线能量色散谱的分析研究,获得了其初步的合成机理:凝胶经热解后,组分元素仍以非晶态的柠檬酸络合物形式存在,经过预烧反应,转化为相应的简单化合物SrMoO4和SrFeO3-x,最后通过还原反应,促成了Fe-O-Mo键的形成,生成最终产物Sr1.9K0.1FeMoO6。 关键词 溶胶-凝胶法,双钙钛矿,Sr1.9K0.1FeMoO6,合成机理 |
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