[Pw_forum] Negative value of polarization for doped BaTiO3

[Mojtaba Mirseraji]

Dear Saraf

I think this is not an error because negative or plus values of
polarization point out the direction of polarization vector !!!!!

your result shows that your polarization is decreased from 28.22 to 22.18

Best

Mojtaba Mirseraji
Ph.D. Candidate
Condensed Matter Physics
Arak Univ.
IRAN


On 10/24/15, Deepashri Saraf <deepashri.saraf at gmail.com> wrote:
> Dear Users,
>
> I am trying to perform berry phase calculations on pristine as well as
> doped BaTiO3 (tetragonal). I get reasonable results for the value of
> polarization for pristine BaTiO3.
>
> ------------------------------------------------------------------------------------------------------------------------
>                               SUMMARY OF PHASES
>                                ~~~~~~~~~~~~~~~~~
>
>                           Ionic Phase: -0.12600 (mod 2)
>                      Electronic Phase:  0.40671 (mod 2)
>                           TOTAL PHASE:  0.28071 (mod 2)
>
>
>                              VALUES OF POLARIZATION
>                              ~~~~~~~~~~~~~~~~~~~~~~
>
>         The calculation of phases done along the direction of vector 3
>         of the reciprocal lattice gives the following contribution to
>         the polarization vector (in different units, and being Omega
>         the volume of the unit cell):
>
>            P =   2.1352035  (mod  15.2130988)  (e/Omega).bohr
>
>            P =   0.0049373  (mod   0.0351775)  e/bohr^2
>
>            P =   0.2822719  (mod   2.0111573)  C/m^2
>
>         The polarization direction is:  ( 0.00000 , 0.00000 , 1.00000 )
>
> ------------------------------------------------------------------------------------------------------------------------
>
> But in case of doped BaTiO3, I get negative value of polarization. I used a
> 2x2x3 supercell for these calculations.
>
> -------------------------------------------------------------------------------------------------------------------------
>
>                               SUMMARY OF PHASES
>                                ~~~~~~~~~~~~~~~~~
>
>                           Ionic Phase: -0.50398 (mod 2)
>                       Electronic Phase: -0.37438 (mod 2)
>                           TOTAL PHASE: -0.87836 (mod 2)
>
>
>                              VALUES OF POLARIZATION
>                              ~~~~~~~~~~~~~~~~~~~~~~
>
>         The calculation of phases done along the direction of vector 3
>         of the reciprocal lattice gives the following contribution to
>         the polarization vector (in different units, and being Omega
>         the volume of the unit cell):
>
>            P = -19.9950561  (mod  45.5280337)  (e/Omega).bohr
>
>            P =  -0.0038804  (mod   0.0088355)  e/bohr^2
>
>            P =  -0.2218486  (mod   0.5051413)  C/m^2
>
>         The polarization direction is:  ( 0.00000 , 0.00000 , 1.00000 )
> --------------------------------------------------------------------------------------------------------------------------
>
> I also tried using a finer k-mesh, but landed up with the same value of
> polarization. Does the negative polarization have any physical meaning or
> is it a numerical error?
>
> Any help appreciated. Thanks in advance.
>
> Best,
> DS.
>
> --
> Deepashri Saraf
> PhD Student
> Department of Physics, SPPU
> Pune (India)
>