[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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>     --
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>                                           P.Blaha
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>     Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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