Here we are.
I'm sorry I did not perform such simulation with Siesta.
At room temperature b-cristobalite turns to the metastable form
a-cristobalite (no longer cubic).
Silica is a very interesting mess.
Good luck.
Eric.
On 03/08/2011 06:12 PM, Prathibha Ramaprasad wrote:
Hi Eric,
Appreciate it if you can send me your cif file. Did you by any chance
use Wychoff or Barth version? Also if you have done a similar
calculation on siesta, do you mind sending me the fdf input file as
well. I am having a real puzzled time with it.
Thank you very much for your help.
Prat
On Wed, Mar 9, 2011 at 4:51 AM, Eric Germaneau <[email protected]
<mailto:[email protected]>> wrote:
Hey Prat,
b-cristobalite lattice parameter is 7.16 experimentally and it's
stable at high temperature (above ~1770K).
I can send you the cif file I usually use for this system.
Eric.
On 03/08/2011 05:28 PM, Prathibha Ramaprasad wrote:
Dear Siesters,
I am working with b-cristobalite, a silica polymorph. I am
puzzled by two different equilibrium lattice constant i obtain
using a 2-atom primitive cell (7.14A) and a 24-atom one complete
unit cell (7.57A). Maybe I doing something fundamentally wrong,
appreciate if anyone can point out the error.
Here's my fdf for 2atom version:
SystemName b-cristobalite
SystemLabel bcris_7.14A
NumberOfAtoms 2
NumberOfSpecies 2
%block ChemicalSpeciesLabel
1 14 Si.gga
2 8 O.gga
%endblock ChemicalSpeciesLabel
LatticeConstant 7.14 Ang
%block LatticeParameters
1.0 1.0 1.0 90. 90. 90.
%endblock LatticeParameters
BandLinesScale pi/a
%block BandLines
1 1.5 1.5 0.0 K # Begin at K
38 0.0 0.0 0.0 \Gamma # 38 points from K to Gamma
36 0.0 2.0 0.0 X # 36 points from Gamma to X
18 1.0 2.0 0.0 W # 18 points from X to W
26 1.0 1.0 1.0 L # 26 points from W to L
31 0.0 0.0 0.0 \Gamma # 31 points from L to Gamma
%endblock BandLines
AtomicCoordinatesFormat Fractional
%block AtomicCoordinatesAndAtomicSpecies
0.000 0.000 0.000 1 Si
0.125 0.125 0.125 2 O
%endblock AtomicCoordinatesAndAtomicSpecies
%block kgrid_Monkhorst_Pack
4 0 0 0.5
0 4 0 0.5
0 0 4 0.5
%endblock kgrid_Monkhorst_Pack
XC.functional GGA
XC.authors PBE
PAO.BasisSize DZP
PAO.EnergyShift 300 meV
MeshCutoff 100. Ry
and for the 24 atom version, I only change the add the extra
coordinates:
AtomicCoordinatesFormat Fractional
%block AtomicCoordinatesAndAtomicSpecies
0.1250.1250.1252O11
0.1250.6250.6252O22
0.6250.6250.1252O33
0.6250.1250.6252O44
0.1250.8750.8752O55
0.1250.3750.3752O66
0.6250.3750.8752O77
0.6250.8750.3752O88
0.8750.1250.8752O99
0.8750.6250.3752O1010
0.3750.6250.8752O1111
0.3750.1250.3752O1212
0.8750.8750.1252O1313
0.8750.3750.6252O1414
0.3750.3750.1252O1515
0.3750.8750.6252O1616
0001Si117
00.50.51Si218
0.50.501Si319
0.500.51Si420
0.250.250.251Si521
0.250.750.751Si622
0.750.750.251Si723
0.750.250.751Si824
%endblock AtomicCoordinatesAndAtomicSpecies
Thank you for your time,
Prat
--
/Be the change you wish to see in the world
/ — Mahatma Gandhi —
Dr. Éric Germaneau <mailto:[email protected]>
College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China
/Please consider the environment before printing this email.
Considérez svp l'environnement avant d'imprimer cet email. /
--
/Be the change you wish to see in the world
/ — Mahatma Gandhi —
Dr. Éric Germaneau <mailto:[email protected]>
College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China
/Please consider the environment before printing this email.
Considérez svp l'environnement avant d'imprimer cet email. /
#------------------------------------------------------------------------------
#$Date: 2008-01-26 13:05:32 +0000 (Sat, 26 Jan 2008) $
#$Revision: 19 $
#$URL: svn://cod.ibt.lt/cod/cif/1/1010921.cif $
#------------------------------------------------------------------------------
#
# This file is available in the Crystallography Open Database (COD),
# http://www.crystallography.net/
#
# All data on this site have been placed in the public domain by the
# contributors.
#
data_1010921
_chemical_name_systematic 'Silicon oxide'
_chemical_name_mineral 'Cristobalite high'
_chemical_formula_structural 'Si O2'
_chemical_formula_sum 'O2 Si'
_publ_section_title
;
The Cristobalite Structures. I. High-Cristobalite
;
_publ_author_name 'Barth, T F W'
_journal_name_full
;
American Journal of Science, Serie 5(1,1921-1938)
;
_journal_coden_ASTM AJSC5L
_journal_volume 23
_journal_year 1932
_journal_page_first 350
_journal_page_last 356
_cell_length_a 7.16
_cell_length_b 7.16
_cell_length_c 7.16
_cell_angle_alpha 90
_cell_angle_beta 90
_cell_angle_gamma 90
_cell_volume 367.1
_cell_formula_units_Z 8
_symmetry_space_group_name_H-M 'P 21 3'
_symmetry_Int_Tables_number 198
_symmetry_cell_setting cubic
loop_
_symmetry_equiv_pos_as_xyz
'x,y,z'
'y,z,x'
'z,x,y'
'1/2+x,1/2-y,-z'
'1/2+y,1/2-z,-x'
'1/2+z,1/2-x,-y'
'-x,1/2+y,1/2-z'
'-y,1/2+z,1/2-x'
'-z,1/2+x,1/2-y'
'1/2-x,-y,1/2+z'
'1/2-y,-z,1/2+x'
'1/2-z,-x,1/2+y'
loop_
_atom_type_symbol
_atom_type_oxidation_number
Si4+ 4.000
O2- -2.000
loop_
_atom_site_label
_atom_site_type_symbol
_atom_site_symmetry_multiplicity
_atom_site_Wyckoff_symbol
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
_atom_site_occupancy
_atom_site_attached_hydrogens
_atom_site_calc_flag
Si1 Si4+ 4 a 0.255(8) 0.255(8) 0.255(8) 1. 0 d
Si2 Si4+ 4 a -0.008(8) -0.008(8) -0.008(8) 1. 0 d
O1 O2- 4 a 0.125(15) 0.125(15) 0.125(15) 1. 0 d
O2 O2- 12 b 0.660(15) 0.660(15) 0.062(15) 1. 0 d
