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

[shamik chakrabarti]

Dear Prof. Blaha,

                        *we have also used the option 6 for optimization of
delithiated compound...but in that case also we have obtained decrements in
unit cell volume with Li ion extraction.*

We have also used GGA, and GGA+U for lattice parameter optimization. As,
usual with GGA+U approach we have obtained higher lattice parameters than
the case with GGA. However, decrements of unit cell volume with Li
extraction has been apparent for both the cases.

Yes it could be the fact that we may need to modify the script for
simultaneous optimization of coordinates with lattice parameter
optimization....whether it could lead to the solution?....whether the
decrements of unit cell volume with Li delithiation can be avoided & we
actually will get an increment by this method?.....

Most importantly, although we are getting a volume reduction with Li
de-intercalation (opposite to VASP)...we are getting very well matched
values of density of states and voltage (from total energy consideration).

Looking forward to your further discussion which may shed some light over
our problems.

with regards,


On Mon, Mar 10, 2014 at 12:16 PM, Peter Blaha
<pblaha at theochem.tuwien.ac.at>wrote:

> 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
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
> Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/
> theochem/
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-- 
Shamik Chakrabarti
Senior Research Fellow
Dept. of Physics & Meteorology
Material Processing & Solid State Ionics Lab
IIT Kharagpur
Kharagpur 721302
INDIA
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