[Wien] EFG: theory Vs experiment for a case

Wed Dec 19 12:40:58 CET 2018

As an addendum to previous comments by the experts I strongly recommend 
to always ask (and answer) yourself the question:
what symmetries does the experimental result imply, and what symmetries 
are present in my model?

You state that samples prepared in a precedure 'where synthetic 
parameters can be controlled' (whatever that means) 'shows STO with 
probe showing very pure HFI'. I take it that TDPACS finds Hf in a unique 
environment. All of the spectra is accounted for by a single value 
abs(V_zz)=1.69e21 v/m^2, eta=0.52? No (sizeable) broadening, no other 
lines from different EFG's?

In that case:

1) Whatever model you set up in case.struct, your Hf-site MUST NOT have 
a threefold (or higher) rotation symmetry in its point symmetry group 
(check outputs of structure initialzation programs).

2) Your structure (with your defect of the day) should have a 
significantly lower total energy than other possibilities - otherwise a 
superposition of the corresponding spectra is expected.

Up to now your calculations without neighboring oxygen defects seem to 
be useless because you did not take into account the first point. With 
respect to the second point I am very dubious about your calculations 
assuming oxygen defects in some neighborhood. Some Hf-vacancy pair may 
have a calculated EFG resembling the experimental one. But in an 
unrelaxed structure this is meaningless. And even if you insist to take 
something like that serious, you will have to come up with a really good 
reason why that special defect structure is the only one present in your 
sample!

Looking at your result that the artificial V_zz from your unrelaxed 
3x2x2 supercell already has the same size as the experimental one I 
strongly suspect that you will get a pretty good simulation of the 
experimental result by some very small rearrangement of atoms in the 
nearest, at most the next neighbor shell - WITHOUT oxygen vacany. Put Hf 
on a Ti-site and really DO a relaxation. Start it from a structure with 
appropriate (low) point symmetry at Hf. (shift some neighboring atom(s) 
a tiny bit out of symmetry in some plausible direction).

And remember, EFG is short range - from my experience even a 2x2x2 
supercell may give you a good idea what happens.

---
Dr. Martin Pieper
Karl-Franzens University
Institute of Physics
Universitätsplatz 5
A-8010 Graz
Austria
Tel.: +43-(0)316-380-8564

Am 19.12.2018 09:48, schrieb Ashwani Kumar:
> System under study is SrTiO3 doped with
> Hf-181 tdpac probe for TDPAC spectroscopy by thermal equillibriation
> method (1250 C for >12 hours). the doping is substitutional as Hf(IV)
> and Ti(IV) has nearby ionic radius and same oxidation state. Bulk STO
> shows no efg, no assymetry parameter as STO is cubic lattice and Hf/Ti
> present at octahedral symmetry but in another synthetic procedure
> where synthesis parameters can be controlled shows STO with probe
> showing very pure HFI. From literature, STO is known to have oxygen
> vacancies depending on synthesis route.
>                            Calculation: STO structure is optimized and
> minimized. Created supercell 3x2x2 as it allows me to use 5 processor
> with 8 gb ram (with 3x3x3, calc. restricted to only 2 processors due
> to RAM limitation. So long scf run time. i will try now) . Problem of
> breaking symmetry did not strike me at the time of calculation.
> Replaced one Ti by Hf. Then supercell calculation was done for oxygen
> vacancies in first cordination and fourth nearest neighbour
> cordination. The efg matched for the later case (oxygen vacancy at
> fourth nearest neighbour) but not assymetry parameter. Minimization of
> forces on supercell was not done ( i thought basic lattice unit was
> already optimized).
> 
>          What i understood from previous replies of this thread is
> 1.Must do 3x3x3 supercell calculation, 2. Electron electron
> correlation can also be included for improvement of calculation, 3.
> Need to check calculation with reduced RMT of Sr and Ti.
> 
> To what % agreemnet wrt experimental efg data, efg values from
> theoretical calculation can be accepted.
> Thanks Dr. Cottenier for the suggestion, i had already subscribed to
> HF course A (ID: iak). I will subscribe to Course B after finishing
> course A.
> 
> Thanks,
> 
> A. Kumar
> 
> On Wed, Dec 19, 2018 at 12:08 AM Ashwani Kumar <ashwani.ism at gmail.com>
> wrote:
> 
>> hi,
>> thanks for reply. the assymetry parameter, (Vxx-Vyy)/Vzz,  is
>> zero (wien2k calculation) whereas i got 0.52 from TDPAC (Time
>> dependent perturbed angular correlation) spectroscopy for a SrTiO3
>> (STO) defect structure. EFG component is -1.63 x10^21 V/m2 (wien2k,
>> lapw2 -efg) and i obtained 1.69 x 10^21 V/m2 (calculated from TDPAC
>> results). STO has cubic lattice so no efg and no assymetry
>> parameter (for both wien2k and TDPAC) but defect STO structure
>> showed very pure hyperfine interactions with assymetry parameter :
>> 0.52. So i am not having confidence over my wien2k calculation
>> because :
>> 1. i am getting assymetry parameter =0
>> 2. Negative EFG which i understood from previous answers that
>> negative sign indicates the rate of decrease of z-component of EF
>> wrt to distance.
>> 
>> Am i missing something?,
>> 
>> Calculation parameters are :Supercell (3x2x2) STO with doped Hf
>> atom, PBE, KGEN:200, rkmax: 7.0,
>> 
>> thanks,
>> A. Kumar
>> 
>> On Fri, Dec 14, 2018 at 11:02 PM Ashwani Kumar
>> <ashwani.ism at gmail.com> wrote:
>> 
>>> Hi,
>>> 
>>> i have calculated EFG  the defect structure of crystalline
>>> system from experimental data from PAC spectroscopy. Then using
>>> WIEN2K (crystal structure--> supercell--> defect introduced), EFG
>>> is calculated.
>>> 
>>> Exper. Calculated : 1.69 x 10^21 V/m2 whereas wien2k calculation:
>>> -1.66 x 10^21 V/m2 (crude value still have to do lapw2 -efg) on
>>> probe atom. Wien2k calculation shows negative value. is there any
>>> significance of the negative sign.
>>> 
>>> thanks,
>>> A. Kumar
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