Wrong sign on piezeo d33 for barium titanate?

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Wrong sign on piezeo d33 for barium titanate?

Postby johnbrehm » Fri Apr 13, 2018 9:49 pm

I used DFPT to calculate the piezoresponse for barium titanate -- a simple 5 atom unit cell.
Calculation ran quickly and without incident.

BUT.... I get a negative value for d33 (-97). See the matrix below. The magnitude is correct. BUT the sign
is wrong: BTO is not a negative piezo mat'l.

Or is it the case that abinit reports the signs "oppositely"?

Piezoelectric d tensor (relaxed ion) (unit:pc/N)

-96.67275336 42.33753525 42.06348693
41.86155809 -97.07601604 42.22293861
41.73607024 41.79187521 -97.16557179
12.81189036 -10.81879086 -12.15171696
-10.65680069 12.87695345 -12.30689921
-10.70041572 -12.17815700 12.86743783
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Re: Wrong sign on piezeo d33 for barium titanate?

Postby ebousquet » Wed Apr 18, 2018 10:06 pm

Dear John,
I didn't pay attention to the sign before but if it looks opposite sign this might be related to bad convergence of the cell parameter, did you check how much pressure/stress you have in the reference ground state calculation?
Best wishes,
Eric
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Location: University of Liege, Belgium

Re: Wrong sign on piezeo d33 for barium titanate?

Postby johnbrehm » Thu Apr 19, 2018 9:22 pm

Thanks for your response.
Generally, I relax to a very small force, in this case :
max grad (force/stress) = 9.9119E-10 < tolmxf= 1.1000E-09 ha/bohr (free atoms)

Now, I am doing the R3mR phase of BaTiO3.
My relaxation yields the following which I carry all the way through to my phonon calc:
angdeg 8.97097924798673E+01 8.97087625785993E+01 8.97087950415756E+01
acell 7.69474233303469E+00 7.69372630864900E+00 7.69375849530342E+00
xred
-2.62152986528374E-03 -3.19046534588350E-03 -3.17144822412366E-03
4.83162648179077E-01 4.82631707831462E-01 4.82649520029828E-01
5.13368876766669E-01 5.12762392699501E-01 2.33746696455553E-02
2.40206560451921E-02 5.12743909756820E-01 5.12764100890483E-01
5.13369478890975E-01 2.33525070206667E-02 5.12783193516920E-01

Now I'm happy w/ my phonons: just 3 small neg acoustic (ie ok to assume stability and that they are == 0).
My freqs:
Phonon frequencies in cm-1 :
-3.122730E-02 -3.047196E-02 -2.813969E-02 1.540257E+02 1.540479E+02
1.572867E+02 2.587439E+02 2.591972E+02 2.649299E+02 2.884651E+02
2.884911E+02 3.072457E+02 4.568146E+02 4.568986E+02 5.329090E+02

Now, I am told my experimenters that the d33 for BTO is positive at ~100.
But I am getting a negative.
(the symmetries look ok, yes?)
Piezoelectric d tensor (relaxed ion) (unit:pc/N)

-96.67275336 42.33753525 42.06348693
41.86155809 -97.07601604 42.22293861
41.73607024 41.79187521 -97.16557179
12.81189036 -10.81879086 -12.15171696
-10.65680069 12.87695345 -12.30689921
-10.70041572 -12.17815700 12.86743783

Could the small negative inconsequential acoustic phonons be causing the issue?
I do get a warning sign right above my d tensor output:
ddb_piezo : WARNING -
Acoustic sum rule violation met : the eigenvalues of accoustic mode
are too large at Gamma point
Increase cutoff energy or k-points sampling.
The three eigenvalues are: 2.040229E-04 -6.580335E-04 -5.443626E-05


If so, how can I zero them out?

tnx,
JB
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Re: Wrong sign on piezeo d33 for barium titanate?

Postby ebousquet » Fri Apr 20, 2018 10:50 pm

Dear John,
tolmxf = 10E-9 should indeed be low enough. However, there are some subtleties that I would like to check with you.
I think your phonons are quite good, I expect more a problem related to a possible stress residual. Could you tell me more about your procedure to do all the calculations? Do you read the WFK from your relaxation run or do you run a single point energy to deeply relax the WFK before feeding it to the DFPT calculations? If this is what you have done, could check what are the values of the stress tensor at the end of the single point energy? The idea is to be sure that the low stress you have at the end of the relaxation run is well reproduced when you make the single point energy.
Let me know,
Eric
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Re: Wrong sign on piezeo d33 for barium titanate?

Postby johnbrehm » Mon Apr 23, 2018 6:42 pm

Hi.

1. I take my relaxed structure and put it into the following script to get my WFK:
iscf 17
ntime 500
nstep 500
ecutsm 0.5

tolvrs 1.0d-18

optforces 1
rfasr 1

ixc 11
ecut 25.0
diemac 9.0
diemix 0.6
dilatmx 1.2
strprecon 0.5

chkprim 0

kptopt 1
ngkpt 8 8 8
nshiftk 1
shiftk 0.5 0.5 0.5

I carry the WFK outputted from this step all the way forward.

2. Then I use the following script: (For brevity, I won't repeat the lines that remain the same from Step #1)
irdwfk 1
iscf -3 #this option is needed for ddk
kptopt 2 #use time-reversal symmetry only for k points
nqpt 1 #one wave vector will be specified
qpt 0 0 0 #need to specify gamma point
rfelfd 2 #set for ddk wf's only
rfdir 1 1 1 #full set of directions needed
tolwfr 1.0d-20 #only wf convergence can be monitored here

I carry the WFK from Step #1 forward, and the POT, DEN, and 1WF files to the next step

3. Then I use the following script:
irdwfk 1
irdddk 1
# Set 3 : response-function calculations for all needed perturbations

kptopt 2 #use time-reversal symmetry only for k points
nqpt 1
qpt 0 0 0
rfphon 1 #do atomic displacement perturbation
rfatpol 1 5 #do for all atoms
rfstrs 3 #do strain perturbation
rfdir 1 1 1 #the full set of directions is needed
tolvrs 1.0d-10 #need reasonable convergence of 1st-order quantities

I carry the DDB output file forward to the next step:

4. calculate using anaddb the piezo and elastic matrices:
elaflag 3 !the flag for the elastic constant
piezoflag 4 !the flag for the piezoelectric constant
instrflag 1 !the flag for the internal strain tensor

!the effective charge part
chneut 1 !enforce Born effective charge neutrality



Hope this helps....
And thanks for helping,
Sincerely,
JB
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