Simulated Annealing

sirous

Dear All,

I am working on structure solution of my synthesized powder using simulated annealing method and I use the instructions mentioned in the following website:

https://community.dur.ac.uk/john.evans/ta_tutorials/html/p…

I have to solve a structure in which I have some dopants and at the last stage which is structure solution, I learnt that all the coordinates for each atom must be set to zero, all the occupancy must be set to 1 and only the fractional atomic coordinates and the scale parameter will be refined. The symmetry will operate on them and locate them inside the unit cell which its dimension is already obtained from indexing Right?

For example in the case of PbSO4 it is :

site Pb x @ 0 y @ 0 z @ 0 occ Pb+2 1 beq 1
site S x @ 0 y @ 0 z @ 0 occ Sr 1 beq 1
site O1 x @ 0 y @ 0 z @ 0 occ Al-2 1 beq 1
site O2 x @ 0 y @ 0 z @ 0 occ Al-2 1 beq 1
site O3 x @ 0 y @ 0 z @ 0 occ Al-2 1 beq 1
site O4 x @ 0 y @ 0 z @ 0 occ Al-2 1 beq 1

In my case, according to CIF file, my unit cell has 17 atomic positions. Now, if I want to include my dopants (i.e Eu and Boron) among my initial atoms , how many atomic position I should consider for each of the dopants in my structure? How can I realize it?

Best,
Sirous

johnsoevans

I think you're getting confused between Rietveld refinement and structure solution. If you have a .cif file your basic structure is known. You then just need to undertake a refinement process to determine which site your dopant sits on. I suggest you look at a text such as the IUCr book edited by Young on "The Rietveld Method".

sirous

Thanks John.

I will definitely study the book. Therefore, how do people come up with the number of atomic sites when they proceed on the last step of the structure solution where all the coordinates must be set to zero and refine? How do they realize that how many atomic sites must be defined for the solution? ( I mean the last step of PbSO4 example in your page?)

thanks,
Sirous