[Wien] EFG: theory Vs experiment for a case

[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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