The ABINIT Users Mailing List ( CLOSED )

[David Vanderbilt <dhv@physics.rutgers.edu>]

Dear Rongqin,

SrTiO3 is extraordinarily close to being ferroelectric at zero
temperature.  For example, its lattice dielectric constant at
zero temperature is about 20,000.  In fact, previous theory has
shown that it actually would be ferroelectric if quantum zero-point
motions of the ions did not destroy the ferroelectric state.

As a result, whether a particular DFT calculation gives a soft
(negative omega^2) mode at Gamma depends very sensitively on such
factors as the volume (experimental vs. theoretical equilibrium),
the pseudopotentials, the XC functional, etc.

For some entry points into the literature, you might go to
http://www.physics.rutgers.edu/~dhv/pub_list.html and search for
papers containing "SrTi".

Best wishes,

                                        David Vanderbilt



On Sun, 18 Jul 2004, Wu Rongqin wrote:

> Dear all,
> I use abinit to calculate dielectric properties of cubic perovskite
> SrTiO3 which is only stable at high temperature.
>
> A,The result showed negative values at gamma point.
> Phonon at Gamma, with non-analyticity in the
>   direction (cartesian coordinates)  0.00000  0.00000  1.00000
>  Phonon energies in Hartree :
>   -4.486115E-04 -4.486105E-04 -2.927791E-05 -2.869101E-05 -2.868774E-05
>    2.908060E-04  2.908060E-04  2.908060E-04  5.257838E-04  8.833161E-04
>    8.833162E-04  1.555478E-03  2.653876E-03  2.653876E-03  3.533046E-03
>  Phonon frequencies in cm-1    :
> - -9.845885E+01 -9.845862E+01 -6.425758E+00 -6.296948E+00 -6.296231E+00
> -  6.382455E+01  6.382455E+01  6.382455E+01  1.153962E+02  1.938655E+02
> -  1.938655E+02  3.413880E+02  5.824585E+02  5.824585E+02  7.754140E+02
> I have check the k point and energy cutoff. Is this result normal? Can
> softening happen at gamma point?
> 2, the effective charges seemed also unreasonable
> 1    7   1    3   -1.8723143702    0.0000000000
>    2    7   1    3    0.0000000000    0.0000000000
>    3    7   1    3    0.0000000000    0.0000000000
>
>    1    7   2    3    0.0000000000    0.0000000000
>    2    7   2    3   -4.8593631119    0.0000000000
>    3    7   2    3    0.0000000000    0.0000000000
>
>    1    7   3    3    0.0000000001    0.0000000000
>    2    7   3    3    0.0000000000    0.0000000000
>    3    7   3    3   -1.8723135534    0.0000000000
>
>    1    7   1    4   -1.8723135534    0.0000000000
>    2    7   1    4    0.0000000000    0.0000000000
>    3    7   1    4    0.0000000000    0.0000000000
>
>    1    7   2    4    0.0000000001    0.0000000000
>    2    7   2    4   -1.8723135534    0.0000000000
>    3    7   2    4    0.0000000000    0.0000000000
>
>    1    7   3    4   -0.0000000001    0.0000000000
>    2    7   3    4    0.0000000000    0.0000000000
>    3    7   3    4   -4.8593631119    0.0000000000
>
>    1    7   1    5   -4.8593631119    0.0000000000
> the -4.8593631119 is for oxygen atom. Which should be less than -2!
>
> Any comment will be appreciated!
> Rongqin Wu
>