[Wien] Question regarding volume expansion-contraction after extraction of Li ions

[Peter Blaha]

The options "optimize vol, c/a or b/a" are "special" options for certain 
properties, but for sure a sequential optimization along these lines 
does NOT give the minimum a,b,c.

In your case it seems you should use option [6], vary a,b,c.

Furthermore: it is vital, that you optimize internal coordinates (MSR1a 
or min_lapw) for each of these steps (modify the resulting script).

When you are talking about Fe2+,3+; you have to think whether GGA+U is 
necessary ??

On 03/10/2014 06:32 AM, shamik chakrabarti wrote:
> Dear wien2k users,
>
>       We have simulated structural, elcetronic properties and cathode
> characteristics (in terms of voltage) of a Li based material. When we
> extract some Li ions from the parent compounds and optimize its volume
> and lattice parameters, we have obtained a decrements in volume. This
> decrements has been followed from the concern that Fe+2 ion get oxidized
> in Fe+3 ions in the delithiated compounds & hence there is an decrements
> in Fe-O bond length in comparison to the parent (lithiated) one.
>
> However, simulation report on the same compound, computed in VASP, has
> shown that there is actually an expansion of volume after Li extraction.
> This expansion occurs as when Li ions are taken out from the parent
> materials there is a decrements of attraction between two layers
> connected with Li ions previously. At least this is the explanation
> given by earlier researchers.
>
> we optimize the material and its Li extracted counterpart by following
> the methods:
> (1) volume optimization by keeping a:b:c constant, (2) optimizing c/a
> ratio and (3) optimizing b/a ratio
>
> we have taken lattice parameters after (3) rd operation (optimization)
> and calculated  DOS and voltage.
>
> we have also seen....that after 3rd optimization
>
> (1) there is an increment in b/a ratio in the delithiated part than in
> the lithiated part in agreement with VASP
> (2) However, unit cell volume of the de-lithiated part is less than the
> lithiated compound due to more dominant reduction of Fe-O bond lengths
> in comparison to the increment of layer-layer separation due to Li ion
> extractions in our calculation.
>
> Now my queries are:
>
> (1) Is the reduction of Fe-O bond length after Li extraction is more
> than the decrements of layer-layer separation in FPLAPW calculation?...&
> that's why we have achieved decrements of volume instead of increments
> as obtained in VASP?
>
> (2) Whether this difference came out from some inherent fact of FPLAPW
> and PAW (VASP) approach?
>
> (3) Whether this is the limitation of the wien2k code itself?
>
> (4) Or, whether our methodology of calculation by first volume
> optimization & then further optimizing c/a and b/a ration....is actually
> a wrong path...& we should choose some other way for lattice parameter
> optimization after Li extraction.
>
> Sorry for such a long email. However, the answer of these queries are
> very necessary for our further research and I believe, this discussion
> will help the community as a whole.
>
> Thanks in advance,
>
> with regards,
>
> --
> Shamik Chakrabarti
> Senior Research Fellow
> Dept. of Physics & Meteorology
> Material Processing & Solid State Ionics Lab
> IIT Kharagpur
> Kharagpur 721302
> INDIA
>
>
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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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Email: blaha at theochem.tuwien.ac.at    WWW: 
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