The problem: Structure of Li2CoPO4F cannot be solved or refined from bulk diffraction.The solution: Precession electron diffraction càn get the job done. This presentation describes the procedure in the shortest way possible.
For a more detailed treatment:“Solving the Structure of Li Ion Battery Materials withPrecession Electron Diffraction: Application to Li 2CoPO4F”in Chemistry of Materialsby Joke Hadermann, Artem M. Abakumov, Stuart Turner,Zainab Hafideddine, Nellie R. Khasanova, Evgeny V. Antipovand Gustaaf Van TendelooPublished online July 11, 2011http://pubs.acs.org/doi/abs/10.1021/cm201257b
First, electrondiffraction patterns aretaken, using theprecession attachment. Non main zones have more reliable peak intensities than main zones.
All patterns can be indexed using the cellparameters and space groups known from XRD:a= 10.452(2) Å, b= 6.3911(8) Å, c=10.874(2) Å Pnma
The intensities of the observed peaks are extracted (ELD software).Treated: symmetry equivalent reflections merged, geometric corrections applied Merged into one list (Triple software) We now have intensities of 237 symmetry unique reflections
a= 10.452(2) Å, Intensities of 237b= 6.3911(8) Å, & symmetry unique c=10.874(2) Å reflections Pnma INTO Direct Methods
Result: R=31%CO and P positions similar to Li2FePO4F butLi, O, F mixed over remaining positions Assign F: tetrahedra around PO: complete the octahedra around Co Remaining positions (purple) Li or ghosts? Difference Fourier maps including Co,P,O,F
The difference Fourier maps clearly show the real peaksThe Li-atoms are now also located
Straight from direct methods:too many Li(?) peaks Difference Fourier allows to eliminate the grey ones Structure is solved !
Separate list of intensities per zone wereput into Jana using separate scale factors for each list on input & Use PO4 rigid units: 18 variables reduced to 6 R=24% (reasonable for precession electron diffraction data)
Conclusion Li2CoPO4F was successfully solved and refined from precession electron diffraction PED can be successfully appliedfor the crystallographic characterization of Li-based battery materials