Methods for determining crystal structure and apparatus for carrying out the methods

A method for determining the absolute structure of a non-centrosymmetric crystal, wherein the crystal is capable of electron diffraction comprising steps:

• obtaining electron diffraction data from the crystal by three-dimensional electron diffraction, wherein the data comprising information on the crystal diffraction patterns, wherein each diffraction pattern comprising information on the scattered electron intensities in each direction and the crystal orientation to the incident electron beam during data acquisition, wherein the crystal is rotating or tilting during data acquisition;
• and
• processing the data determining crystal lattice parameters, crystal orientation at the time of the diffraction pattern acquisition, indices of each reflection and the scattered electron intensity of each reflection on each diffraction pattern, thereby producing the processed data;
• generating virtual diffraction frames by a computer program, wherein each virtual diffraction frame comprising a list of integrated scattered electron intensities for each direction with summed contributions from all experimental patterns forming one virtual frame, the virtual diffraction frames being formed by calculation from the provided three-dimensional electron diffraction data;
• providing an approximate crystal structure model;
• and
• calculating quality indicators of the approximate crystal structure model using the dynamical diffraction theory to calculate the model intensities;
• creating a model of opposite absolute structure to the current approximate crystal structure model, thereby creating an inverted model;
• calculating quality indicators of the inverted crystal structure model using the dynamical diffraction theory to calculate the model intensities;
• and
• comparing quality indicators of the inverted and non-inverted models, wherein the correct structure is the one that leads to the smallest deviation between the experimental data and the data calculated on the basis of the refined structure model.

An apparatus for determining the crystal structure model comprising:

• an electron source capable of emitting an electron beam;
• a crystal holder adapted to perform a rotational or tiling movement with the crystal;
• a detector detecting scattered electrons by the crystal, the detector is connected to a data storage and processing unit, wherein
• the processing unit is adapted to determine crystal lattice parameters, crystal orientation at the time of the diffraction pattern acquisition, indices of each reflection and the scattered electron intensity of each reflection on each diffraction pattern, thereby producing the processed data;
• and
• generating virtual diffraction frames by a computer program, wherein each virtual diffraction frame comprising a list of integrated scattered electron intensities for each direction with summed contributions from all experimental patterns forming one virtual frame, the virtual diffraction frames being formed by calculation from the provided three-dimensional electron diffraction data;
• and wherein
• the data storage is configured to provide an approximate crystal structure model or the processing unit is adapted to perform analysis of the processed data leading to obtaining an approximate structure model;
• and wherein
• the processing unit is adapted to refine the structure model.