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I am doing the DFT calculations in Quantum ESPRESSO code. I have 2 different pwscf input file of the same crystal. I am confused which one is representing primitive, conventional and supercell structures. Actually I want to introduce disorder/doping in the system, so I am trying to create the supercell structure. Two input files with required information are:

4 atoms file:

 &SYSTEM
                       ibrav = 2,
                   celldm(1) = 10.703022167,
                         nat = 4,
                        ntyp = 3,
                     ecutwfc = 300 ,
                     ecutrho = 1200 ,

 /

ATOMIC_SPECIES
   Co   58.933195 Co.rel-pbe-spn-kjpaw_psl.0.3.1.UPF
   Mn   54.938    Mn.rel-pbe-spn-kjpaw_psl.0.3.1.UPF     
   Al   26.98154  Al.rel-pbe-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS alat
   Mn      0.000000000    0.000000000    0.000000000
   Al      0.500000000    0.500000000    0.500000000
   Co      0.750000000    0.750000000    0.750000000
   Co      0.250000000    0.250000000    0.250000000

For this crystal structure, I am seeing to different structures in Xcrysden. I am confused which one in primitive and conventional in it. How Xcrysden is generating the complete cubic structure (second one with 35 atoms)?

enter image description here enter image description here

16 atoms file:

 &SYSTEM
                       ibrav = 0,
                         nat = 16,
                        ntyp = 3,
                     ecutwfc = 300 ,
                     ecutrho = 1200 ,
                
 /
 
ATOMIC_SPECIES
   Co   58.933195 Co.rel-pbe-spn-kjpaw_psl.0.3.1.UPF
   Mn   54.938    Mn.rel-pbe-spn-kjpaw_psl.0.3.1.UPF     
   Al   26.98154  Al.rel-pbe-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS crystal
  Al   0.50000000  0.00000000  0.00000000  
  Al   0.50000000  0.50000000  0.50000000  
  Al   0.00000000  0.00000000  0.50000000  
  Al   0.00000000  0.50000000  0.00000000  
  Mn   0.00000000  0.00000000  0.00000000 
  Mn   0.00000000  0.50000000  0.50000000  
  Mn   0.50000000  0.00000000  0.50000000  
  Mn   0.50000000  0.50000000  0.00000000  
  Co   0.25000000  0.25000000  0.75000000  
  Co   0.25000000  0.75000000  0.75000000  
  Co   0.25000000  0.75000000  0.25000000  
  Co   0.25000000  0.25000000  0.25000000  
  Co   0.75000000  0.25000000  0.25000000  
  Co   0.75000000  0.75000000  0.25000000  
  Co   0.75000000  0.75000000  0.75000000  
  Co   0.75000000  0.25000000  0.75000000  
   
CELL_PARAMETERS angstrom
    5.6637953940    0.0000000000    0.0000000000
    0.0000000000    5.6637953940    0.0000000000
    0.0000000000    0.0000000000    5.6637953940

For 16 atoms structure following two unit cells are displayed in Xcrysden. I am confused which one is primitive and conventional? How Xcrysden is generating the complete cubic structure (second one with 35 atoms)?
enter image description here enter image description here

  • I am not able to perform doping/disorder in these structure due to less sizes? How can I create the supercell of bigger size?

  • Should I create the 1x1x2 sized supercell of 16 atoms structure which will consist of 32 atoms but will not be cubic?

  • Should I create 35 atoms cubic structure manually as displayed above by giving the coordinates of each atom in angstrom? In that case will it be primitive cell or conventional unit cell?

Please clarify my doubts.

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    $\begingroup$ Both 4 atom and 16 atom unit cell are same. One you are using ibrav=2 another ibrav=0. Both are conventional cell and same, one have periodic atoms visible,other doesn't. It is a FCC unit cell of Mn with Co at tetrahedral voids along with Al at edges and centre(octahedral void) $\endgroup$
    – Pranav kumar
    Sep 2, 2022 at 10:31

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