Unité Matériaux et Transformations, UMR 8207 CNRS-Université Lille 1, Univ Lille Nord de France, F-59655 Villeneuve d'Ascq, France.
Ultramicroscopy. 2010 Aug;110(9):1166-77. doi: 10.1016/j.ultramic.2010.04.010. Epub 2010 Jun 10.
Precession electron diffraction is used to distinguish between the hexagonal beta high-temperature and the trigonal alpha low-temperature phases of SiO2 quartz. The structures just differ by a kink of the SiO4 tetrahedra arranged along spiraling chains, which induces a loss of the two-fold axis and subsequent twinning in the low-temperature phase. Conventional selected-area electron diffraction (SAED) does not enable the phases distinction since only the intensity of reflections is different. It becomes possible with precession that reduces the dynamical interactions between reflections and makes their intensity very sensitive to small variations of the electron structure factors. Distinction between the twinned individuals in the low-temperature phase is then easily made and the twin law is characterized using stereographic projections. The actual symmetry of precessed zone axis patterns is also examined in detail. Using dynamical intensity simulations, it is shown that under certain thickness conditions, the diffraction class symmetry can be observed on selected area patterns that are to be used in the case of beam sensitive materials such as quartz.
进动电子衍射可用于区分 SiO2 石英的六方β高温相和三方α低温相。这两种结构仅相差沿螺旋链排列的 SiO4 四面体的扭曲，这导致低温相中失去了二重轴并随后发生孪晶。由于只有反射强度不同，传统的选区电子衍射（SAED）无法进行相区分。进动可以实现这一点，因为它降低了反射之间的动力学相互作用，使得它们的强度对电子结构因子的微小变化非常敏感。然后可以轻松区分低温相中的孪晶个体，并使用赤平投影法来描述孪晶律。还详细检查了进动轴带图案的实际对称性。通过动力学强度模拟，表明在某些厚度条件下，可以在选区图案上观察到衍射类对称性，这种选区图案可用于像石英这样对光束敏感的材料。
