[Wien] change of lattice parameters while alloying
Peter Blaha
pblaha at theochem.tuwien.ac.at
Wed May 10 07:25:46 CEST 2017
So the cell is reduced from 19 19 19 to 14 14 19.
Apparently this is a sqrt(2) factor from a rotation by 45 degrees.
It clearly is a reduction of the cell to a smaller one and you have to
accept the suggestion of sgroup.
Am 09.05.2017 um 18:45 schrieb Dr. K. C. Bhamu:
>
> Thank you Prof. Peter for detailed reply,
> May be I still have some doubts on symmetries and need to read some
> literature.
> For meanwhile, from literature I came to know that the distortion in my
> doped structure "change in unit cell ("lattice parameters and angles")"
> may occur depending on size effects, deviations from the ideal
> composition and the JTE etc.
>
> So what I am doing is good and there is no way to preserve the same
> SG/unit cell.
>
> I tried to maintain the original structure and got below warnings which
> support my above understanding:
>
> 1. warning: !!! Unit cell has been reduced.
>
> ----------------------------------------------------------------------
>
> Bravais lattice: Tetragonal primitive
>
> a b c
> 13.98640455 13.98640455 19.77976300 >> the original was 19.77976300
> alpha beta gamma
> 90.00000000 90.00000000 90.00000000
>
>
> 2. SPACE GROUP CONTAINS INVERSION
>
> 3. ROTDEF - Error
> 0.0u 0.0s 0:00.00 0.0% 0+0k 0+24io 0pf+0w
>
>
> So, if I accept all warnings during initialisation, I get different SGs.
>
> Sincerely
> Bhamu
>
> On Tue, May 9, 2017 at 5:22 PM, Peter Blaha
> <pblaha at theochem.tuwien.ac.at <mailto:pblaha at theochem.tuwien.ac.at>> wrote:
>
> 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
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> --
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> P.Blaha
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