[Wien] Re : Supercell problem of Al2O3

[Lubomir Smrcok]

Hi,
in your case the supercell is the best generated by hand & pencil. Draw
the atoms in the simple hexagonal cell (but now in the P1 space group)  and
then add neighbors. Calculate
new atomic coordinates by adding +-1 and renormalization to new cell.
If you manage it, you will understand the process...
Best,
Lubo



On Thu, 9 Aug 2007, swati chaudhury wrote:

> Hi Michael and all wien users,
>   Thanks for your detailed suggestions.
> I have done the conversion (rhom-hex) sucessfully by following your instruction.
> The problem now is the formation of Al2O3 supercell in Wien.
> To construct supercell in Wien I tried the foll inputs :
> (1)Selected Space group in Struct Editor :167 R-3c
> cell parameters;4.7626 4.7626 13.0032; 90.00 90.00 120.00
> positions:
> Al    0      0     0.35
> O     0.30 0     0.25
> 12-Al &12-O positions are generated.
> In case.struct file ,the lattice type is R
> For supercell calculation:
> Fatal error occured.
> Unknown lattice type:R
> Program terminated.
> (2) Selected Space group in Struct Editor : Space group:H
> cell parameters;4.7626 4.7626 13.0032; 90.00 90.00 120.00
> positions:
> Al    0      0     0.35
> O     0.30 0     0.25
> In case.struct file, the lattice type is H.
> In case.outputsgroup: cell parameters 4.7626  13.8479 8.2491;  90.00 90.00  110.1159
> Number and name of space group: 8  (C m). But my required space gp is 167 R3cH
> Although a supercell is generated it is a wrong one.
> I am not able to get the
>   What can I do now? Please help me.
> Swati
>
>   Hello again,
>
> I hope I am able to explain this correctly without causing more confusion.
> 1.) For rhombohedral lattices in WIEN2k, you need to specify the hexagonal lattice parameters, but the rhombohedral atomic positions (User's Guide, 4.3). I think you had done this correctly in your original struct-file. This is why the angles are 90 degrees (gamma should be 120 deg, but I suspect that the angles are not even read because the program "knows" that gamma = 120 deg in a hexagonal lattice). Of course, rhombohedral lattices are defined as a=b=c and alpha=beta=gamma with the angles not equal 90 deg.
> 2.) If you want to use Cryscon, you need a consistent setting, i.e. both lattice parameters and atomic positions in rhombohedral setting. If you have no information about the rhombohedral lattice parameters, you have to re-arrange the formulae given in UG, 4.3, to calculate them from the hexagonal ones.
> 3.) Once you have done this, you can start Cryscon. Here, you create a new case and select Structure axes: Cartesian (choosing "trigonal rhombohedral" delivers strange results). Than you enter your cell parameters and the two atomic positions. Only two of them because all other atoms are generated by symmetry!. Choose the space group: 167:R. Ignore error messages.
> 4.) Now you can choose transform and change the transformation matrix to Rhomb->Hex. Click "Transform" again will give you an output. From this output, you get the lattice parameters (which you already knew) and the atomic positions. Although the program displays all atoms in the cell, you only need two of them: One Al and one O. Among others, you will find coordinates similar to those that I proposed in my previous posting:
> Al (0 0 0.35)
> O (0.30 0 1/4)
> You can now enter these positions in structgen, and choose the correct space group. WIEN2k should do the rest.
>
> Hope this helped you, greetings
> Michael
> --
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