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

[shamik chakrabarti]

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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