现代结构分析方法20098(免费阅读).ppt

The Powder Method The Powder Method In order to record a proper powder pattern, the sample should be rotated or more grains should be added. The middle image shows a 2D diffraction pattern obtained from 200 grains. The corresponding 1D trace along [h00] (red curve) shows a much better representation of the intensities. Still some of the reflections do not show the correct intensity. ?Use powder (or polycrystalline) sample to guarantee some particles will be oriented properly such that every possible set of crystallographic planes will be available for diffraction. Each material has a unique set of planar distances and extinctions, making X-ray diffraction useful in analysis of an unknown material. Diffractometer Technique X-Ray power diffraction pattern How to index a known powder pattern Calculate the d list Find out the indices for each peak How to index an unknown powder pattern Survey the literature Calculate the d list Compare with XRD cards * * The next set of images shows a powder composed from 4 single crystals in random orientation (left) and the corresponding diffraction pattern (middle). The individual diffraction patterns plotted in the same color as the corresponding crystal start to add up to rings of reflections. With just four reflection its difficult though to recognise the rings. The right image shows a diffraction pattern of 40 single crystal grains (black). The colored spots are the peaks from the 4 grain powder shown in the middle image. Now the powder rings are clearly visible. * 现代结构分析方法(09－10年度第一学期) 第8讲 倒易点阵、正点阵与衍射 S0/l= k0 S/l= k ghkl (hkl) 000 相机长度 Rhkl b a 正交点阵沿c轴投影 a b c a b c 入射束 d100 d110 7 衍射图标定?相；取向7.1 单晶衍射图的产生与标定 When is q small (electron diffraction), Rd=lL=diffraction constant, the reflection sphere cuts a 2D reciprocal plane. Indexing: the plane normal [uvw] and at least one low index spot (hkl) (normally two spots), with hu+kv+lw=0. S0/l= k0 S/l= k ghkl (hkl) 000 相机长度 Rhkl [-101] (101) (010) Plot the fcc [110] diffractio