[Wien] change of lattice parameters while alloying
Peter Blaha
pblaha at theochem.tuwien.ac.at
Tue May 9 13:52:40 CEST 2017
You are mixing up the concepts of space groups and
bravais lattice or unit cell.
For a certain structural input, there is only ONE space group.
However, even for one and the same SG, there are often several
"settings", which can lead to VERY different unit cells (bravais
lattices, lattice vectors) or positions of the atoms within the cell.
And besides the "standard" settings (which is taken by sgroup), there
are often very many different possible settings ("unit cells"), which
are equivalent to each other.
I've mentioned this several times on the list: if you "know" what you
are doing, there is in most cases no need to follow the setting as
suggested by sgroup (except when the size of the unit cell is reduced).
Instead, you can very often use the multiplicities as given by nn and
use the symmetry operations as given by "symmetry".
However, you should check if sgroup gives you
a) identical multiplicities
b) the same number of symmetry operations.
c) if symmetry complains, that inversion is present, but you should
shift the origin, then you must either take sgroups struct file or shift
all positions yourselv (x supercell can do that for you too).
If this is fulfilled, you do not need to take the struct file from
sgroup, which may have changed the unit cell ("lattice parameters and
angles") to cope with some standard settings.
In other words: then you can probably stay with your "cubic" perovskite
unit cell (even when you get P1 symmetry, i.e. have only ONE symmetry
operation).
On 05/09/2017 01:38 PM, Dr. K. C. Bhamu wrote:
> Dear Sir
> For your first two points, I convinced but below suggestions triggered
> me to ask/clarify my doubts more:
>
>
> Note that depending on what you want to compare in your series of
> calculations it probably is a good idea to do all calculations in
> the same space group - even if you are left with very low symmetry.
> Do this by giving all Cr in the pristine cell you want to substitute
> a different index (Cr1, Cr2 ...).
>
>
> I tried to label all Cl with a number and then replaced all Cl one by
> one with Br. But when I initialised the structure it gives me 1-P1 SG
> with reduced basis vectors and all angles =60deg.
> I wounder how to keep all six structure in the same space group?
>
> I just had an discussion with experimentlist and he suggested me that
> changing of basis vector and SG is normal but the key point I have to
> keep in mind that the SG should not leave the the SG family belonging to
> that class of (here perovskite) crystal structure. For example: we have
> various SGs for perovskite family (225, 42, 44, 107..., etc.) and after
> replacing any number of Cl with Br should stabilise in one the SG
> belongs to perovskite structure.
>
>
>
> And remember that you do not calculate an (random) alloy, you
> calculate a periodic structure. Watch out for superstructure
> effects, perhaps by checking larger supercells with the same
> Br-concentration but different configuration.
>
>
> Sir, I am doing for bulk only like ABCl_(1-x)Br_x.
>
> Regards
>
> Bhamu
>
>
> Martin Pieper
>
>
> Am 09.05.2017 01:55, schrieb Dr. K. C. Bhamu:
>
> Dear Wien2k Users
>
> I am alloying a quaternery compound: replacing Cr with Br one by
> one.
> What I see is, the pristine compound is cubic with 225 SG. When I
> replace Cl by Br one by one then lattice parameters deviates from
> cubic to different structure.
> like a=b, c or a,b,c. SG also changes.
> During initialisation I accepted the new structure suggested by the
> w2web. Should I accept the suggested structure?
> As per my understanding from previous posts from mailing list we
> should accept the suggested structure which is with lower symmetry.
>
> Is the change of lattice parameters is normal behaviour of
> material or
> I am doing any mistake?
>
> Sincerely
>
> Bhamu
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> ---
> Dr. Martin Pieper
> Karl-Franzens University
> Institute of Physics
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> A-8010 Graz
> Austria
> Tel.: +43-(0)316-380-8564
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>
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--
P.Blaha
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