Estimation of atomic displacement in a unit cell by means of convergent-beam elctron diffraction

用大角度会聚束电子衍射(LACBED)测定了α-Al_2O_3.1的结构因子。在g=006l时，L*5的消光距离比样品厚度(150-200 nm)大得多，这表明电子衍射的运动学理论是可以应用的。当入射光束的会聚角很大，使得L从5到9的多次反射可以同时满足布拉格条件时，光束照射到一小块试件上，可以用运动学近似来分析布拉格反射的强度。Al离子在氧化铝中的位置沿C轴偏离理想位置0.02435 nm。计算得到的静态位移与用电子衍射动力学理论得到的位移和用X射线衍射法得到的位移符合得很好，证明了本文提出的运动学处理的正确性。如果在分析中忽略温度因素，则Al离子的位移被高估了约10%。当样品被冷却时，测量精度提高，因为在较低的温度下，0 0 6L反射强度随Al离子的位移而变化更敏感。在本实验中，比较了相邻反射，以减少由背景强度和胶片暗度的非线性度引起的误差。如果使用成像板或慢扫描CCD相机来代替胶片，精度将会提高，实验也会很容易。

Large angle convergent-beam electron diffraction (LACBED) has been applied to determine structure factors, and following results have been obtained for alpha Al_2O_3.1. The extinction distance xi_<kappa> for g=0 0 6l for l*5 was found to be much larger than specimen thickness of 150-200 nm suggesting that the kinematical theory of electron diffraction can be applied.2. When convergent angle of the incident beam is so large that several reflections with l's from 5 to 9 may satisfy Bragg conditions simultaneously and the beam is illuminated onto a small area of specimen, intensities of the Braggreflections can be analyzed with the kinematical approximation.3. The position of Al ion in alumina was found to be displaced by 0.02435 nm along a C-axis from the ideal position. The static displacement thus evaluated is in good agreement with the one derived by using the dynamical theory of electron diffraction and those obtained by X-ray diffraction, demonstrating the adequateness of the present kinematical treatment.4. If the temperature factor is ignored in the analysis, the displacements of Al ion is over estimated by about 10%. An accuracy in measurements is improved when a specimen is cooled, since intensities of 0 0 6l reflections change more sensitively with the displacement of Al ion at lower temperature.5. In the present experiment neighboring reflections were compared to reduce errors originated from background intensities and from non-lineality of darkness of films. The accuracy will be improved and experiments will be easy provided imaging plates or a slow scan CCD camera will be used in place of films.