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["D. R. Hamann" <drhamann@mat-simresearch.com>]

Dear Andrey,

The basic physics problem here (as opposed to the Abinit problem) is that this is not the structure of Cristobalite, either alpha or beta.  This simple structure with 180-degree Si-O-Si bonds was published long ago, and unfortunately referenced in Wyckoff.  For a recent discussion, see
Sinisa Coh and David Vanderbilt, "Structural stability and lattice dynamics of SiO2 cristobalite," Phys. Rev. B 78, 054117 (2008).

Don Hamann

Andrey Chibisov wrote:

595511224651854@webmail40.yandex.ru" type="cite">

Can you help me with unit cell of Cristobalite?
Data for Cristobalite from site http://rruff.geo.arizona.edu/AMS/amcsd.php (American Mineralogist Crystal Structure Database) is:

Title               Cristobalite
Lattice type        F
Space group name    Fd-3m
Space group number  227
Setting number      1

Lattice parameters
   a        b        c       alpha    beta     gamma
 7.16600  7.16600  7.16600  90.0000  90.0000  90.0000
Unit-cell volume = 367.985229
Structure parameters
                       x          y          z          g          B
   1 Si  Si          0.12500    0.12500    0.12500    1.000    3.335
   2 O   O           0.00000    0.00000    0.00000    1.000    7.146

In the input abinit file I write:
spgroup 227
For the primitive vectors of the lattice form a FCC lattice:
  rprim 
        0.0 0.5 0.5
        0.5 0.0 0.5
        0.5 0.5 0.0
However the abinit gives out:
Unit cell is not primitive?
In what is my error?
     
  

-- 
D. R. Hamann
Mat-Sim Research LLC    | Deptartment of Physics
P.O. Box 742            |  and Astronomy
Murray Hill, NJ 07974   | Rutgers University
phone: 908-370-8079     | 732-445-4381

email: drhamann@mat-simresearch.com