Alpha-AlF3

Exercise : Determine the ²⁷Al C_q value and the Al and F shieldings.

α-AlF₃ crystallizes in the trigonal R -3c space group.

a = b = 4.9305 Å; c = 12.4462 Å

α = β = 90°; γ = 120°

The unit cell contains two independent atoms (1 Al and 1 F) with 6 formula units (AlF₃) per unit cell (Z=6). AlF₆ octahedron units are linked together by corner sharing. As the conventional unit cell is non-primitive, the primitive rhombohedral one is used for the calculation. It saves a lot of computational time !

We suggest you to use vesta for generation the POSCAR file from the AlF3.cif file. In the standard export procedure, the POSCAR file is generated with the conventional unit cell (non primitive R-cell with 24 atoms inside). Ask VESTA to reduce to unit cell to the primitive one. You will then have only 8 atoms in the POSCAR file.

In this exercise one wants first to calculate the EFG tensor components of ²⁷Al. This is very fast task calculated at the end of the first SCF calculation (ground state property). The experimental values for the C_q is 0.21 MHz. The nuclear quadrupolar momentum used to transform EFG in C_q is Q = 14.66 10^-30 m² (see the paper of Sadoc et al. (http://www.sciencedirect.com/science/article/pii/S0926204014000022) (Flurine has a 1/2 nuclear spin, so Q is zero)

In a second step one wants to calculate the shielding parameters for Al and F. This is done using the linear response using the GIPAW formalism. As the calculation is quite time consuming, only very few k-points and small ENCUT are used with standard PAW data sets. The calculated shielding tensors components can be compared to the ones obtained by Sadoc et al.

INCAR

 SYSTEM       =  Al F3
 GGA          = PE
 ISTART       = 1
 ICHARG       = 0
 INIWAV       = 1
 LREAL        =  AUTO
 ISYM         = 2
 ISPIN        = 1

Ionic minimisation

 NSW          = 0
 ISIF         = 2
 IBRION       = 2

EDIFFG = -2E-2

 POTIM        = 0.1

Electronic minimisation

 IALGO        = 38
 LWAVE        = .TRUE.
 EMIN         =   -20.0
 EMAX         =   10.0
 NEDOS        = 1601

EFG Calculation

 LEFG         = .TRUE.
 QUAD_EFG     = 146.6 0.0

Chemical Shift

 PREC         = Normal    # nice
 ENCUT        = 400.0      # typically higher cutoffs than usual are needed
 ISMEAR = 0; SIGMA= 0.1 # no fancy smearings, SIGMA sufficiently small
 EDIFF        = 1E-9      # you'd need much smaller EDIFFs.
 LCHIMAG      = .TRUE.   # to switch on linear response for chemical shifts
 DQ           = 0.001         # often the default is sufficient
 ICHIBARE     = 1       # often the default is sufficient
 LNMR_SYM_RED = .TRUE. # be on the safe side
 NLSPLINE     = .TRUE.  # only needed if LREAL is NOT set.
 LREAL        = A          # helps for speed for large systems, not needed
 NBANDS       = 25       # to safe memory, ??? = NELECT/2

KPOINTS

automatic mesh
0
Auto
20

POSCAR

Al1 F3
1.0
        4.9305000305         0.0000000000         0.0000000000
        2.4652500153         4.2699382798         0.0000000000
        2.4652650832         1.4233214594         4.1486879977
   Al    F
    2    6
Direct
     0.000000000         0.500000000         0.000000000
     0.500000000         0.000000000         0.500000000
     0.177499995         0.250000000         0.750000000
     0.822499990         0.750000000         0.250000000
     0.677500010         0.322499990         0.250000000
     0.322499990         0.677500010         0.750000000
     0.250000000         0.177499995         0.250000000
     0.750000000         0.822499990         0.750000000

Download

AlF3_NMR.tgz