PRECESSION ELECTRON DIFFRACTION

 


PRECESSION ELECTRON DIFFRACTION:

SOLUTION FOR STRUCTURE DETERMINATION


WHY X-RAY IS LIMITED FOR NANOCRYSTALS STRUCTURE ANALYSIS?

    X-Ray crystallography is well adapted to structure analysis of perfect single crystals larger than a few micrometers. X-Ray crystal interactions are kinematical and thus the structure factors can be directly derived from the diffracted intensity data. For nanocrystal structural studies use of powder X-Ray diffraction present severe limitations as the grain size is very small and there is a severe peak broadening. ; as a consequence it becomes very difficult to solve ab-initio unknown  nanostructures from  X-Ray diffraction ( see  figure below ).




Transmission Electron Microscopy (TEM) is very well adapted to the imaging and the analysis of
nanocrystals.


By means of the electron micrograph of the studied specimen, it is possible to select (possibly in a defect free area) and probe a tiny  area, smaller than the nanocrystal size, in order to obtain an electron diffraction pattern.





 

ELECTRON DIFFRACTION AND NANOCRYSTALS STRUCTURE ANALYSIS


Despite these interesting features, conventional  electron diffraction was rarely used in the past as a standard tool for crystal identification mainly because the electron interactions with matter are about 10,000 times stronger than that of X‑rays. As a result, the scattering is not kinematic but dynamical, so that the diffracted intensities are so much altered that they cannot be trusted and used for crystal structure determination, unless the crystal thickness is very thin or very demanding dynamical calculations are undertaken.


PRECESSION  ELECTRON DIFFRACTION


Electron diffraction precession technique proposed by Vincent & Midgley [1] offers a solution to this problem by decreasing the dynamical behaviour of electron diffraction. This technique is equivalent to the Buerger precession technique used in X-ray diffraction, where the specimen is precessed with respect to the incident X-ray beam. In the electron precession technique, the electron beam is tilted and precessed along a conical surface, having a common axis with the TEM optical axis.



As a result of  precession  diffraction:


    -many more reflections in the reciprocal space are visible than conventional selected  area diffraction patterns (SAED) take  a look at the video


    - reflection diffracted intensity  is much closer to the integrated intensity values


    - resulting  precession diffraction pattern can be considered  much less dynamical and much closer to  kinematical  ( like X-Ray case )


With precession electron diffraction kinematically forbidden reflections and multiple scattering are greatly reduced, making space group identification easier. Precession electron diffraction also reduces sensitivity of ED intensities to crystal thickness, misorientation effects and Ewald sphere curvature.


Using precession in a ED pattern  results to huge extension of  visible reflections at very high angles (diffraction  order up to 0.05 nm) and an important redistribution of electron  diffraction intensities due to the practical  elimination of dynamical diffraction/multiple diffraction contributions.


Several minerals, catalysts, and complex oxides have  been solved  ab initio from quasi‑kinematical  precession diffraction  intensities [2].


HOW CAN WE REALIZE PRECESSION DIFFRACTION IN TEM ?


NanoMEGAS  unique  digital  precession  inteface  DigiSTAR  can be user-retrofitted  to your commercial TEM (LaB6, FEG 100 - 400 kv ). By precessing  incident beam at a  constant angle around a zone axis in combination with  a similar  precession (descan) of the ED pattern below the specimen, the equivalent mechanism of the precession  of the specimen is obtained. DigiSTAR  takes control  of  TEM coils in order that scanning and de-scanning  of the beam are exactly compensated for any spot size and  any TEM ( even without STEM presence ) and in order that  ED  pattern remain stationary during precession.


Resulting precession electron diffraction (PED) patterns change from conventional  dynamical diffraction (0 mrad precession angle) to quasi-kinematical (50 mrad or more, TEM dependant)



[1]  R.Vincent, P,Midgley, Ultramicroscopy, 1996

[2]  Ultramicroscopy   Special issue  , proceedings Elcryst2005 vol 107, June –July 2007, 2007

     STEP  1    with  DigiSTAR obtain precession patterns  from one or several ZA of  same nanocrystal

     STEP  2    measure precession intensities from one or several   ZA patterns using film, CCD or

                      image plates ; for high accuracy use  electron diffractometer Pleiades

     STEP  3    merge several ED zone axis intensities 

     STEP  4    solve structure using direct method software (SIR2008, MICE , SHELX , FULPROF etc..)

                       most  probable nanostructure solution will show up with list of all atomic positions

     

    Take a look  at  BIBLIOGRAPHY for  methods  solving crystal  structures   

X-Ray diffraction techniques (synchrotron or conventional sources) are currently used  for standard ab-initio structure determination; however in many cases structure solution is not possible for several  reasons : reflection overlap inherent to powder data , poor crystallization , peak broadening related to nm crystal size or existence of  unknown polymorphs. Taking into account that X-Ray and electron scattering factors show similar trends with sinθ/λ, strong and weak reflections in X-Rays are also observed as strong and weak quasi-kimematical PED reflections. Information coming from PED data (using TEM equipped with DigiSTAR) is very useful for identifying weak X-Ray reflections, estimating accurate individual  hkl  intensity contributions in case of overlapping  X-Ray intensities, and obtaining crystallographic phases of hkl  reflections in ZA projections.


Information from PED can be combined with hkl reflections from X-Ray powder diffraction to accurately solve and refine ab-initio structures using  direct methods

Combining precession electron diffraction - powder X-Ray diffraction to solve complex structure

  1. useful for poorly crystallized / nm size pollycrystalline materials

  2. useful for solving structures of complex organic and inorganic materials

  3. useful in presence of unknown phases in X-Ray powder pattern

SOLUTION FOR STRUCTURE  DETERMINATION

(Variation of electron diffraction patern of mayenite mineral (111 Zone axis) with precession angle (continious variation from 0 to 50 mrad)

Precession Diffraction. READ MOREhttp://www.nanomegas.com/Documents/Precession%20Applications.pdfhttp://www.nanomegas.com/Documents/Precession%20Applications.pdfhttp://www.nanomegas.2syp.com/Documents/Precession%20Applications.pdfshapeimage_3_link_0shapeimage_3_link_1

DigiSTAR unit and dedicated PC

Statistics of crystal structures solved using different techniques

Electron Crystallography

Electron Microscopy and Electron Diffraction


Authors:

Xiaodong Zou, Sven Hovmöller and Peter Oleynikov

First  textbook in Electron Crystallography for inorganic chemistry/materials science students

Electron crystallography is beginning an exciting expansion with new instrumentation and software

The combination of theory and practice offers a unique way of mastering the topic of the book

Caters for readers at different levels: from practical guidance to full mathematical description

Richly illustrated with clear and exciting figures, including images with atomic detail

Grown out of lecture notes, class-tested and improved at university level

Read more about the book or order:

From Oxford University Press.

From Amazon: Amazon UK or Amazon USA.

ATLAS  OF  ELECTRON  DIFFRACTION  ZONE-AXIS PATTERNS                                      

Prof. J.P Morniroli   Univ of Lille  (France)

FREE  DOWNLOAD    (Atlas + several  lectures  on  Precession Diffraction)

www.Electron-Diffraction.fr