[Wien] EFG: theory Vs experiment for a case

[Stefaan Cottenier]

In addition to what Peter said:

The small EFG you get is a supercell effect. An isolated Hf-impurity on a Sr-site (which has a cubic point group, hence zero EFG) would have zero EFG too. But by using a tetragonal supercell (3x3x2) your break that cubic symmetry. The tetragonal sublattice of Hf atoms generates a non-zero EFG at the Hf-site. If you would use a more expensive a 3x3x3 supercell, the EFG on the Hf-impurity would be zero by symmetry again.

Such a large asymmetry parameter must be due to a serious (local) distortion. Perhaps there is another defect (vacancy, interstitial,…) sitting next to Hf?

And why do you call the negative EFG a reason of no confidence? As I told before, the TDPAC experiment measures the absolute value of Vzz only. The EFG in experiment might well be negative, but your experiment is not sensitive to the sign.

Bottom line: use all indications you have from experiment to build local environments for Hf (preferably in a cubic supercell), compute their EFG, and compare the absolute values and eta with experiment. When there is a match (within 0.5 to 1.0 1e21 V/m2), that local environment is a serious candidate to represent the experimental situation.

If you want to learn more about hyperfine interactions (physical background and experimental methods), visit this online course: www.hyperfinecourse.org<http://www.hyperfinecourse.org> (selfpaced study at any time, the next supervised edition starts in February 2019).

Stefaan

From: Wien <wien-bounces at zeus.theochem.tuwien.ac.at> On Behalf Of Ashwani Kumar
Sent: Tuesday, December 18, 2018 7:39 PM
To: wien <wien at zeus.theochem.tuwien.ac.at>
Subject: Re: [Wien] EFG: theory Vs experiment for a case

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<mailto: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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