1: a) Relax unit cell structure, b) build a supercell e.g 2x2x2, from step a).

2: Do count total number of La-atoms in your atomic structure (in supercell, let it be y).

3: For x-percent substitutional doping of Na, remove 'z' number of La-atoms from the supercell structure, and place 'z' number of Na atoms there. Here, atomic doping percentage can be calculated as x-percent=(z/y)*100. [e.g If y=16, then 25% doping=4/16*100,, z=4]

Dear Muhammad Mushtaq ,

Thanks a lot for your valuable suggestions. But what will be the status of symmetry, If I substitute the La atom by Na. As I know replacement of these atom will change the symmetry of the crystal system. Will it belong to same space group as it was before doping?

Some of the researchers have suggested about the use of atomic composition in researchgate. But why these methods are not effective for doping ( a video has been attached).

If we can replace atoms in supercell then why not in unit cell? Please clarify the point I am missing. Thanks in advance again.

Dear Rinku Majumder,

Muhammad Mushtaq is correct, that is a good way to model doping. The only thing to add to his method is that this will lead to a periodic doping, i.e. the same La in each unit cell will be replaced by Na. It is more realistic to take a larger supercell and replace more than one La by Na, though of course it's a more computationally expensive simulation and now you have to think about which La atoms to substitute (they aren't equivalent after the first atom is substituted, and some will be more energetically favourable than others). In general, you need to converge your results with respect to the supercell size.

You're absolutely right that the symmetry will (in general) change upon substitution. If you're writing your own input files for CASTEP then you just have to ensure the CELL file has the keyword

symmetry_generate

rather than explicit symmetry operations. If you're using Materials Studio then its symmetry finder should update the symmetry for you, and put the updated symmetry operations into the CELL file for you. CASTEP does check that the input geometry has the symmetry operations its given, so there shouldn't be any danger of running a calculation with the wrong symmetries, it should just abort with an error message in the ERR file.

I strongly recommend *not* following the doping methods you link to. These will use the Virtual Crystal Approximation (VCA) to create a "mixture atom". Essentially this mixes the pseudopotentials of the 2 atoms linearly to create a pseudopotential for a "mixture atom". This is only valid when the properties of interest vary linearly with the fraction that is substituted, and not always very accurate even then -- in particular, different properties show very different behaviour. It can be useful as a quick exploratory simulation if the substitution is isoelectronic and you expect a linear behaviour, but I recommend never using it for any real research.

The difference between doing the supercell method Muhammad Mushtaq sets out, and doing the VCA, is that in the former method you replace an *entire* atom of La with one of Na, which is absolutely fine -- the La pseudopotential for some of the atomic sites is replaced by a Na pseudopotential.

Hope that helps,

Phil Hasnip

(CASTEP developer)

Dear Philip James Hasnip,

Thank you very much for your response. I think your suggestions will be helpful for further proceedings of my project.

Yes you can, With the following steps with VCA

I don't recommend you to use VCA is indeed not a very safe method specially in your case (La and Na are not the same. they have different ionic radii and electron number .... ).

It is expected to work reasonably well in the isovalent case

you should use supercell approach and increase the number of unit cell if you can,

test all substitution possibility and choose the most energetically favorable case

I recommended Muhammad Mushtaq answer, and you are must check the symmetry

Philip James Hasnip

"You're absolutely right that the symmetry will (in general) change upon substitution. ... If you're using Materials Studio then its symmetry finder should update the symmetry for you, and put the updated symmetry operations into the CELL file for you."

I have one question is: In CASTEP, is there anyway to calculate the doped supercell with the symmetry before doping?

Nguyen Trung Hieu No, I don't think so. You can tell CASTEP which symmetries you want to impose, but I think it checks whether they are valid and that check will fail if the dopant breaks them. Is there a particular reason why you'd like to be able to do this?

thanks Philip James Hasnip . The reason is I want to compare the undoped and doped materials. I encountered a problem that the symmetries are different between them, both symmetries were checked automatically by CASTEP for supercells. As a consequence, CASTEP built different unit cells when it ran geometry optimization, thus I can't compare lattice parameters, i.e can't derive the dependence of lattice cell on doping.

Nguyen Trung Hieu , if you want to compare the undoped and doped materials by using supercell in CASTEP code, don't accepte the symetry proposed by the code, and in this case you can compare the lattice parameters by deviding the lattice parameters of supercells by 2, 6 or 8, it dependes on your supercells.

Thanks Ahmed Gueddouh !

I recommended Muhammad Mushtaq answer.

1: a) Relax unit cell structure, b) build a supercell e.g 2x2x2, from step a). 2: Do count total number of La-atoms in your atomic structure (in supercell, let it be y). 3: For x-percent substitutional doping of Na, remove 'z' number of La-atoms from the supercell structure, and place 'z' number of Na atoms there. Here, atomic doping percentage can be calculated as x-percent=(z/y)*100. [e.g If y=16, then 25% doping=4/16*100,, z=4]

I want to dope a bulk poshpor material like this system Na[Li3SiO4]:Eu2+

but i need to calculate all the possible structures with all doping possibilities. if anyone can help me with the script or give me details i will be Thankful.