[Wien] EFG + ETA calculation

Wed Feb 4 20:22:39 CET 2015

Cd 2+ has a full d shell and thus the contribution of the d-electrons to 
the EFG should be small. (Try an EFG anylysis).

LDA+U can be important for partially filled d-shells (Ti-Ni), but in any 
case, I'd try GGA+U, but for sure also the other position.

PS: Run   x nn   after relaxations and compare positions and distances 
before/after relaxation.
An ideal octahedra should have Vzz and eta=0. Is your Vzz very small ? 
In this case Vyy and Vxx can have any value and eta is "arbitrary", i.e. 
even very small changes in the electronic structure/geometry change eta 
from 0 to 1.
Can you relate the 3 axis of the EFG to the nearest neighbors ?

Am 04.02.2015 um 18:40 schrieb Marcelo Barbosa:
> I tried to make the supercell as cubic as possible, thus the dimensions 1x3x2.
> It is a slightly distorted near-cubic parallelepiped.
>
> I should point that the experiments were done by implanting 111In in the Ga2O3 samples, which decays to 111Cd.
> This means that the position of the probes is given by In, whose ions are 3+ as the Ga ones.
> However, the electric field gradient is measured after the decay, so it is measured in the Cd ions.
>
> Ga2O3 has two Ga sites, octahedral and tetrahedral.
> Previous studies in binary oxides using In have shown a preference for In to occupy undisturbed cation sites and in cases where an octahedral and a tetrahedral sites are present, they have a tendency to go to the octahedral site.
> In fact, InGaO3 has been reported to have the structure of Ga2O3 with the In occupying the octahedral site.
> Moreover, the octahedral Ga site in Ga2O3 has a bigger distance to the near-neighbors than the tetrahedral site.
>
> With all this information, the most likely scenario is for the In probes to go to the octahedral Ga site in Ga2O3 and then they decay to Cd, thus the need to simulate the electric field gradient of Cd in the octahedral Ga site.
>
> I haven’t calculated the electronic bands, so I don’t know if there is an “impurity” band, but I will look for it.
>
> Isn’t GGA+U or hybrid functionals supposed to change the electric field gradient values, or you think the differences are not important in this case?
>
> Once again, thanks for your help,
> Marcelo
>
>> On 03/02/2015, at 17:04, Peter Blaha <pblaha at theochem.tuwien.ac.at> wrote:
>>
>> The size should be big enough, at least for a first calculation.
>>
>> Is the supercell nearly "cubic", i.e. similar length in all dimensions ?
>>
>> Is there only one Ga site ?
>>
>> How are the distances after relaxation ?
>> Would it correspond to a more or less axial symmetry or is x,y,z very different and eta=1 is to be expected ??
>>
>> Cd at an interstitial site ? I'm not sure if a Cd2+ ion really wants to substitute Ga3+.
>> Is your cell metallic ?
>> Do you find an "inpurity" band ?
>>
>>
>> Am 03.02.2015 um 11:49 schrieb Marcelo Barbosa:
>>> Dear Prof Blaha,
>>>
>>> The biggest supercell I used for Ga2O3 was 1x3x2, which has 120 atoms.
>>> Then I changed one of the Ga atoms to Cd and relaxed the atomic positions.
>>>
>>> Since GGA+U or hybrids won’t affect the EFG and ETA in this case, do you think I should try bigger supercells?
>>>
>>> Thank you very much for your help,
>>> Marcelo
>>>
>>>> On 03/02/2015, at 06:43, Peter Blaha <pblaha at theochem.tuwien.ac.at> wrote:
>>>>
>>>> No, I don't think that for this system GGA+U or hybrids are appropriate.
>>>>
>>>> Is your supercell large enough (~100 atoms) ??
>>>> Did you relax the atomic positions ??
>>>> Maybe the Cd sits in a different position ??
>>>>
>>>> Am 02.02.2015 um 20:09 schrieb Marcelo Barbosa:
>>>>> Dear all,
>>>>>
>>>>> I need to calculate the EFG and ETA in a Cd impurity in Ga2O3 but using the usual GGA method gives me an ETA near 1 when experimentally it is known to be between 0.0 and 0.1.
>>>>> I would like then to ask what is the best method to get good EFG and ETA values in such a case:
>>>>> - use GGA+U for the 3d orbitals of Ga and 4d orbitals of Cd?
>>>>> - use onsite-exact-exchange / hybrid functionals for those orbits?
>>>>> - use hybrid functionals for all electrons?
>>>>> or do you think these different options won’t change the ETA?
>>>>>
>>>>> Best regards,
>>>>> Marcelo Barbosa
>>>>> _______________________________________________
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>>>>>
>>>>
>>>> --
>>>> Peter Blaha
>>>> Inst.Materials Chemistry
>>>> TU Vienna
>>>> Getreidemarkt 9
>>>> A-1060 Vienna
>>>> Austria
>>>> +43-1-5880115671
>>>> _______________________________________________
>>>> Wien mailing list
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>>>
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>>
>> --
>> Peter Blaha
>> Inst.Materials Chemistry
>> TU Vienna
>> Getreidemarkt 9
>> A-1060 Vienna
>> Austria
>> +43-1-5880115671
>> _______________________________________________
>> Wien mailing list
>> Wien at zeus.theochem.tuwien.ac.at
>> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
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>
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-- 
Peter Blaha
Inst.Materials Chemistry
TU Vienna
Getreidemarkt 9
A-1060 Vienna
Austria
+43-1-5880115671