A transmission electron microscopy study on the real structure of synthetic hematite
A transmission electron microscopy study on the real structure of synthetic hematite
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DOI：
10.1007/s10853-007-1718-3
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发表时间：
2009-03
影响因子：
4.5
通讯作者：
R. Theissmann;H. Fuess,;K. Tsuda;M. Terauchi
中科院分区：
文献类型：
--
作者：
R. Theissmann;H. Fuess,;K. Tsuda;M. Terauchi
赤铁矿（a-Fe_2 O_3）是最重要的铁矿石，其磁性已被广泛研究。它表现出一种Morin转变--从反铁磁态到弱铁磁态的磁相变。低温相的自旋沿[111]方向反铁磁沿着排列。这些高温相序在（111）面内具有反铁磁性，并有轻微的面外倾斜，这导致了铁磁成分。已知纯赤铁矿中的桑色素转变强烈依赖于所研究样品的真实的结构：晶粒尺寸、由其他金属取代Fe、由于各种合成路线而引入的水或OH-基团影响赤铁矿的自旋有序。Dang等人[1]回顾了已发表的工作，并成功地建立了晶格参数和Morin转变之间的相关性。他们将不同的晶格参数与不同的缺陷结构相关联，根据Wolska和Schwertmann [2]，这些缺陷结构被称为原赤铁矿和水赤铁矿。磁性和晶格缺陷之间的这种相关性使得仔细研究赤铁矿的真实的结构变得非常重要。Matijevic和Schreiner [3]是第一个引入强制水解的方法，在高温下从金属盐水溶液中生产合成赤铁矿。通过控制温度、pH值和不同阴离子的浓度，得到了粒径分布较窄的金属（水合）氧化物颗粒。但是，对用这种方法制备的赤铁矿颗粒结构的研究表明，在单个颗粒中可能存在小的、不完全有序的亚基。因此，Matijevic et al. [4]，拉特等. [5]和Sahu et al. [6]从X射线粉末衍射图的线宽得出结论，所研究的颗粒可能不是单晶，而是由更小的亚基组成的。目前的TEM研究集中在这个问题上，合成赤铁矿是否由建议的亚基组成。比较了一个商业赤铁矿样品和一个通过强制水解获得的自己生产的样品。使用不同的TEM方法来识别颗粒的内部结构，包括高分辨率（HR）TEM以及会聚束电子衍射（CBED）和大角度（LA）CBED。后者对晶格缺陷非常敏感[7-9]。即使在常规TEM和HRTEM方法失败的情况下，它们仍然提供关于样品的真实的结构的信息。它将被证明，我们的研究，无论是购买和自己生产的合成赤铁矿颗粒，显示出“假单堆叠”，而一个自然生长的赤铁矿的模拟研究表明一个完美的结晶度。
The magnetic properties of hematite (a-Fe2O3), the most important iron ore, have been extensively studied. It shows a Morin transition—a magnetic phase transition from an antiferromagnetic state to a weakly ferromagnetic one. The spins of the low-temperature phase order antiferromagnetically along the rhombohedral [111] direction. Those of the high-temperature phase order antiferromagnetically in the (111) plane, with a slight tilt out of plane, which leads to the ferromagnetic component. The Morin transition in pure hematite is known to be strongly dependent on the real structure of the investigated sample: Grain size, substitution for Fe by other metals, incorporation of water or OH–groups due to various synthesis routes influence the spin ordering of the hematite. Dang et al.[1] reviewed published work and successfully established a correlation between the lattice parameters and the Morin transition. They correlated the varying lattice parameters to different defect structures, which have been called protohematite and hydrohematite with reference to Wolska and Schwertmann [2]. This correlation between magnetic properties and lattice imperfections makes it important to carefully study the real structure of hematite. Matijevic and Schreiner [3] were the first to introduce the method of forced hydrolysis to produce synthetic hematite from an aqueous metal salt solution at elevated temperatures. By controlling the temperature, pH-value and concentration of different anions, particles of metal (hydrous) oxides with narrow size distribution were obtained. But investigations of the structure of hematite particles prepared by this method have brought up the possible existence of small, not perfectly ordered subunits within single particles. Thus, Matijevic et al.[4], Rath et al.[5] and Sahu et al.[6] concluded from the linewidth of x-ray powder diffraction patterns that the investigated particles could not be single crystalline, but were composed of smaller subunits.The present TEM-study is focused on the question, whether synthetic hematite is composed of the suggested subunits. A commercial hematite sample as well as a sample of own production obtained by forced hydrolysis were compared. Different TEM methods were used to identify the internal structure of the particles, including high resolution (HR) TEM as well as convergent-beam electron diffraction (CBED) and large-angle (LA) CBED. The latter ones are very sensitive to lattice defects [7–9]. They still provide information about the real structure of a sample even in cases if conventional TEM and HRTEM methods fail. It will be demonstrated that the synthetic hematite particles of our study, both bought and from own production, show a ‘pseudo single crystallinity’, whereas an analogue study on a naturally grown hematite indicated a perfect crystallinity.