[Wien] Question regarding volume expansion-contraction after extraction of Li ions
shamik chakrabarti
shamikphy at gmail.com
Mon Mar 10 08:29:17 CET 2014
Sir,
The forces in our fully optimized structure is less than 3mRy/a.u. per
atom...also we have done coordinate optimization by taking fully optimized
lattice parameters......
Yes...we have not done the simultaneous optimization of lattice parameters
and atomic coordinates...while we have done all the optimizations in steps:
volume, c/a, b/a, atomic coordinates.
Also, there are several peer reviewed papers on calculation on the same
material with VASP and they have all achieved volume expansion.......
Sir...is this mean ...from this discussion..it is coming out that we should
try with simultaneous optimization of lattice parameters and atomic
coordinates for final verification....?
but as the energy is mainly depends on volume...& not on the lattice
parameters...I really have doubt whether this method would change the trend
of volume contraction towards expansion.
On Mon, Mar 10, 2014 at 12:46 PM, Peter Blaha
<pblaha at theochem.tuwien.ac.at>wrote:
> You cannot expect that a,b,c is correct when you fix atomic positions
> (which are allowed to vary according to symmetry). But without doing these
> calculations (or at least checking how large are the forces in your "best"
> structure) one cannot say anything.
>
> On the other hand: I do not know if the VASP calculation is correct either
> ...
>
>
>
> On 03/10/2014 08:06 AM, shamik chakrabarti wrote:
>
>> 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 <mailto: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
>> <mailto:blaha at theochem.tuwien.ac.at> WWW:
>> http://info.tuwien.ac.at/__theochem/
>> <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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>>
> --
>
> 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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