高压下β-FeOOH纳米固体结构转变的研究

 以化学水解法合成的β-FeOOH纳米微粉（平均粒径在12 nm左右）为原料，分别在0.0~4.5 GPa和200~350 ℃的压力和温度范围进行冷压和热压处理。实验结果表明，冷压对β-FeOOH纳米固体的结构没有明显影响，但却使它的热致相变（从β-FeOOH相到α-Fe₂O₃相）温度从常压下的203.8 ℃提高到4.5 GPa压力下的274 ℃，接近常规体相材料的相变温度。而在一定的热压条件处理下，首次发现了从β-FeOOH相到α-FeOOH相的结构转变，并在4.5 GPa、200 ℃的热压条件下得到了转变过程中的一个新的亚稳相。从压力和温度对纳米微粒的作用角度，对上述实验结果进行了讨论。

The Structure Transformation in the β-FeOOH Nanometer Solid under High Pressure

It is well known that the research on nanometer solid has brought to people's great interest in recent years, but only a few works has been done so far about the properties of nanocrystalline under high pressure. In the present paper, we have synthesized a series of β-FeOOH nanometer solid by treating the ultrafine powder under the pressure range of 0.0～4.5 GPa and the temperature range of 200～350 ℃. The ultrafine powder was synthesized by using chemical hydrolysis method and its average diameter is 12 nm. Using X-ray diffraction (XRD) and differential thermal analysis (TG-DTA), we investigated the structural transformation of these nanometer solids. The results of XRD and TG-DTA experiments show that under cold pressure of 0.0～4.5 GPa, the structure of the nanometer solids do not change considerably, but on the other hand, the temperature of the heat-induced phase transformation (from β-FeOOH phase to α-Fe₂O₃ phase) in these β-FeOOH nanometer solids is raised obviously with increasing pressure, from 203.8 ℃ at normal pressure to 274 ℃ at 4.5 GPa. This variation can be interpreted by the effect of pressure on the coordinate state of the OH^- group in the interface of nanometer solid. After treating under hot pressure, both the formation and temperature of the structural transformation of β-FeOOH nanometer solid are very different from the results under cold pressure. Under the hot pressure of 4.5 GPa, the temperature of the structural transformation from β-FeOOH phase to α-Fe₂O₃ phase is far above 350 ℃. However under the condition of 3.0 GPa, 200 ℃ and 4.5 GPa, 350 ℃, we firstly observe the structural transformation, from β-FeOOH phase to α-FeOOH phase. It is more interesting that we obtain a new metastable phase of FeOOH in the above structural transformation at the condition of 4.5 GPa and 200 ℃. These special variation can be interpreted by considering the effect of pressure combining temperature on the OH^- group in the nanometer solid at the same time.