How to build the structure file of an alloy

Many compounds are in reallity alloys; so to bulid their structure file we need to bring the crystal structure of the parent compound and derive the crystal structure of the alloy.

We will take the example of the transition metal compound cristallized in the NiAs structure with the space group of P63/mmc with #194.


_NiAs
Stoichiometry = AX

Space Group = P6₃/mmc (194)

a = ~ [2.968 - 4.340 angstroms]
c = ~ [4.058 - 6.748 angstroms]

Atomic Positions

        Ni    2a     0, 0, 0
        As   2c     1/3, 2/3, 1/4




If we want to build the crystal structure of the alloy or the derived compound NiFeAs we use the supercell tool to do that as follows:


- Supercell 1x1X1 P to make the atoms Ni inequivalent and we will get:

Ni1 (0,0,0)  and Ni2 (0,0,1/2)

- We replace the second atom Ni2 by Fe to get 

Ni (0,0,0)  and Fe (0,0,1/2)

- For the 2 atoms of As, we have two choices:

  *  make them inequivalent by adding nimbers to them to get As1 and As2 or

  * let them equivalent by adding nothing

- Run the sgroup program to get the new space group

-  For example, if you let the two atoms As equivalent and running the space group you will get the new space group P-3m1 with #164 which is a subgroup of the parent space group P6₃/mmc (#194).


Good luck