Cation Vacancies in NiFe2O4 During Heat Treatments at High Temperatures: Structural, Morphological and Magnetic Characterization

Nickel ferrite (NiFe2O4) was synthesized by mixing stoichiometric amounts of α-Fe2O3 and NiO using mechanical milling and heat treatments at high temperatures. The physical characterization of the samples was carried out using X-ray diffraction, infrared and Raman spectroscopies, Mössbauer spectrometry, magnetization measurements, scanning electron microscopy and energy dispersive X-ray spectroscopy. We found that NiFe2O4 production increases from 81 to 100 wt. % with increasing temperature. Additionally, the lattice parameter and the saturation magnetization increase with increasing temperature. On the other hand, Mössbauer spectrometry showed that there is a decrease in the subspectral areas ratio for Fe3+ cations at tetrahedral (A) and octahedral [B] sites, AA/AB, with the increase of the temperature. In the SEM micrographs it was observed that the samples consisted of particles with irregular shapes and micrometric sizes. From IR spectra, the intensity of the 411 cm-1 band (vibrations at octahedral sites) increases relative to the intensity of the 599 cm-1 band (vibrations at tetrahedral sites) with increasing temperature. From the results obtained in the magnetization curves, it was possible to confirm the synthesis of NiFe2O4. As the heat treatment temperature increases, hysteresis loops with S-type geometric forms were obtained. All the results suggest that a defective spinel NiFe2O4 is formed at 1000 °C, and that as the temperature increases, the defects gradually disappear. Neither cation reordering phenomena nor possible evaporation of chemical elements were the dominant effects to account for the results. The results can be explained if it is assumed that [B]-sites cation vacancies are gradually filled with cations as the temperature of the heat treatment increases.

铁酸镍（Nickel ferrite, NiFe₂O₄）通过按化学计量比混合α-Fe₂O₃与NiO，经机械球磨与高温热处理合成。本研究采用X射线衍射（X-ray diffraction）、红外光谱（infrared spectroscopy）、拉曼光谱（Raman spectroscopy）、穆斯堡尔谱（Mössbauer spectrometry）、磁化强度测量、扫描电子显微镜（scanning electron microscopy）以及能量色散X射线光谱（energy dispersive X-ray spectroscopy）对样品进行物理表征。研究发现，随着热处理温度升高，铁酸镍的产率从81 wt.%提升至100 wt.%。此外，晶格参数与饱和磁化强度亦随温度升高而增大。另一方面，穆斯堡尔谱分析结果表明，随着温度升高，四面体（A）位与八面体[B]位三价铁离子（Fe³⁺）的子谱面积比AA/AB逐渐降低。扫描电子显微镜显微图像显示，样品颗粒形貌不规则，尺寸为微米级。红外光谱分析结果表明，随着温度升高，411 cm⁻¹波段（对应八面体位振动）的强度相较于599 cm⁻¹波段（对应四面体位振动）的强度逐渐增强。通过磁化曲线的分析结果，可证实铁酸镍的成功合成。随着热处理温度升高，所得磁滞回线均呈现S型几何形貌。所有实验结果均表明，在1000 ℃时合成得到存在缺陷的尖晶石（spinel）型铁酸镍，且随着温度升高，晶体缺陷逐渐消失。阳离子重排现象与化学元素的潜在蒸发均非导致本实验结果的主导因素。若假设随着热处理温度升高，八面体[B]位的阳离子空位逐渐被阳离子填充，则可合理解释本研究的全部实验结果。