single crystal orientation

[TonyChem]

I found this forum rather helpful when seeking solutions to some problems in using ES.
I am dealing with a hexagonal single crystal (space group No 176) along c and a/b directions using EPR. To further simulate the spectra, I have to state the crystal orientation to other coordinate frames in the code.

According to the manual, for lab frame, zL is along the static magnetic field. For crystal frame zC is along the six-fold axis, that is the c axis in my case while xC and yC are arbitrary.

If the spectrum is collected along the c axis, i think the code below would work.
Exp.CrystalSymmetry = 'P63/m'; % space group symbol
Exp.CrystalSymmetry = 176; % space group number 176
Exp.CrystalOrientation = [0 0 0] *pi/180 ; %angles for lab frame to crystal frame

If the data come from the observation along the a axis, it seems not possible to have the crystal frame overlapped with the lab frame because a and b are not perpendicular to each other in the hexagonal system.

For instance, a axis can be overlapped by y is not.
Exp.CrystalOrientation = [0 90 30] *pi/180 ; %angles for lab frame to crystal frame

Can you suggest me how to deal with this problem. Thank you.
Tony

[Stefan Stoll]

EasySpin's crystal frame (xC,yC,zC) is orthogonal, whereas the (a,b,c) frame in the space group P63/m (#176) is not. EasySpin's convention in this case is that the zM axis is along the six-fold symmetry axis, c. You can then have xC along any direction in the ab plane. Since the angle between a and b is not 90 degrees, you cannot have xC align with a and yC with b at the same time.

If you choose xC to be along the a axis of your crystal, then here is how you can put the a axis (=xC) along the static field (=zL):

Code: Select all

Exp.CrystalOrientation = [0, pi/2, 0];

If you want to put the b axis along the static field (=zL), you have to add a 120 degree rotation:

Code: Select all

Exp.CrystalOrientation = [2*pi/3, pi/2, 0];