Ab initio structure determination using nano electron diffraction

Abstract

This thesis is the summary of the development of electron diffraction into a mature method for ab initio structure determination. The progress is illustrated with the structure determination of some complicated unknown inorganic materials. The main step forwards is that it is now possible to apply electron crystallography method on solving the structure with a relatively large unit cell dimension along all directions and with both very heavy atoms and very light atoms. One main issue about structure determination using nano-electron diffraction discussed in the thesis is how to deal with all the difficulties caused by the dynamical effect of electron scattering. We have exemplified these with the structure determination of two compounds. One is monoclinic Eu2CaCu2O6 (a=6.08Å, b=6.14Å, c=15.34Å, ?=108.3°), and the other is cubic Mg10Ir19B16 (a=10.568Å). The success of solving these two complicated structures relies on three innovations comparing to the traditional structure analysis method (Exit wave reconstruction combined with Dynamical electron diffraction refinement) in our group. They are 1) the combined use of the chemical information of bond length in the step of constructing a reasonable initial structure model; 2) phase estimation of electron diffraction by Maximum Entropy method. 3) the combined use of first-principles calculation in the step of structure refinement to solve the possible local minimum problem of dynamical electron diffraction refinement. Based on the experiences of processing these problems evolved with structure determination using electron diffraction, a correlative structure analysis approach of combination all possible methods is proposed and discussed. So far, it is also introduced in the thesis an improved TEM sample preparation method for providing thin TEM sample for the consideredKavli Institute of Applied SciencesApplied Science