Guidelines to find the atomic positions
Here we will present some guidelines to find the atomic positions of a unknown compound and we will take the example of the compound CaCl2.
1- Look for the space group of the compound (there may be more than one space group for the compound) .
For the compound CaCl2 the space group is Pnnm with # 58. Now we will find the different Wyckoff Positions of the space group Pnnm from the site http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list
Wyckoff Positions of Group 58 (Pnnm)
| Multiplicity | Wyckoff letter | Site symmetry | Coordinates | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 8 | h | 1 | |||||||||
| 4 | g | ..m | |||||||||
| 4 | f | ..2 | |||||||||
| 4 | e | ..2 | |||||||||
| 2 | d | ..2/m | |||||||||
| 2 | c | ..2/m | |||||||||
| 2 | b | ..2/m | |||||||||
| 2 | a | ..2/m | |||||||||
2- We look for a similar compound with the same space group Pnnm
We have found the compound FeSe2 with the crystal structure in format cif from here Crystal structure of FeSe2
x y z
Fe 0 0 0
Se 0.212700 0.370100 0
We conclude from this that the x,y and z of Fe are always zero which are equivalent to the whyckoff position 2a and the x and y of Se are free parameters while z is always zero with are equivalent to the whyckoff position 4g. We can verify that the molecule FeSe2 is formed with one atom of Fe and two atoms of Se.
3- Now we need to define the x and y values for Cl
We can use the atomic positions of a compound of similar compound with close atomic number and then optimize them to get the real values for this compound
Result:
Ca (2a) 0 0 0
Cl (4g) x y 0 with x and y having free values similar to 0.21 and 0.37
How do you determine GaAs and Si coordinates following the same process? Would you please explain a little bit?
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