-    OLDHAMITE-NININGERITE SERIES     -    MgCa7S8

This is a term of the MgS-Cas solid solution with ordered super-structure: every 8th Ca atom is replaced by 1 Mg. This ordering breaks the symmetry and makes some of the former IR modes of the ideal disordered (Mg,Ca)S solid solution Raman-acive. See article for details and experimental measurements

Crystal Structure 


Because of the translational symmetry all the calculations are performed in the primitive unit cell and not in the conventional unit cell. The following information regarding the structure is given with respect to this primitive unit cell, which sometimes can take an unintuitive shape.

Symmetry (experimental): 

Space group:  225  Fm-3m 
Lattice parameters (Å):       
Angles (°):       

Symmetry (theoretical): 

Space group:  225  Fm-3m 
Lattice parameters (Å):  0.5282  0.5282  0.5282 
Angles (°):  60  60  60 

Cell contents: 

Number of atoms:  16 
Number of atom types: 
Chemical composition: 

Atomic positions (theoretical):

Mg:  0.0000  0.0000  0.0000 
Ca:  0.5000  0.0000  0.0000 
Ca:  0.0000  0.5000  0.0000 
Ca:  0.5000  0.5000  0.0000 
Ca:  0.0000  0.0000  0.5000 
Ca:  0.5000  0.0000  0.5000 
Ca:  0.0000  0.5000  0.5000 
Ca:  0.5000  0.5000  0.5000 
S:  0.2500  0.2500  0.2500 
S:  0.7572  0.2428  0.2428 
S:  0.2428  0.7572  0.2428 
S:  0.7572  0.7572  0.2428 
S:  0.2428  0.2428  0.7572 
S:  0.7572  0.2428  0.7572 
S:  0.2428  0.7572  0.7572 
S:  0.7500  0.7500  0.7500 
Atom type 

We have listed here the reduced coordinates of all the atoms in the primitive unit cell.
It is enough to know only the position of the atoms from the assymetrical unit cell and then use the symmetry to build the whole crystal structure.

Visualization of the crystal structure: 

Size:

Nx:  Ny:  Nz: 
You can define the size of the supercell to be displayed in the jmol panel as integer translations along the three crys­tallo­gra­phic axis.
Please note that the structure is represented using the pri­mi­tive cell, and not the conventional one.
 

Parameters of the Calculation 


All the calculations have been done using the ABINIT software. This is a list of the most representative parameteres used during the Raman calculation.


Number of electronic bands: 26
k-points  
   grid: 4 4 4 
   number of shifts: 
   shifts: 0.5 0.0 0.0 
Kinetic energy cut-off: 38 Ha  [=1034.0408 eV ]
eXchange-Correlation functional: LDA pw90 

Pseudopotentials: 
Mg:  magnesium, fhi98PP : Trouiller-Martins-type, LDA Ceperley/Alder Perdew/Wang (1992), l= 2 local 
S:  sulphur, fhi98PP : Trouiller-Martins-type, LDA Ceperley/Alder Perdew/Wang (1992), l= 2 local 
Ca:  calcium, fhi98PP : Trouiller-Martins-type, LDA Ceperley/Alder Perdew/Wang (1992), l= 0 local