solid solution

[张辉]

Hello everyone,

Is there any way to simulate the STEM images of the solid solution? For example, (A1-xBx)O2. Many thanks.

Regards,

Hui

[Christoph Koch]

Hello Hui,

 

yes, in the config file you simply need to define the A-atoms with an occupancy of 1-x and the B atoms with occupancy x and give them the exact same position. There are now two options how the simulation is run if you include TDS in your simulation (without TDS both options will yield equivalent results):

 

1)      If you want to simulate the average image, then  you simply generate the super cell within qstem itself (either box mode or replication of unit cells). For each TDS run, the atom positions will be distributed between A and B randomly. So each TDS run sees a different configuration, but with the same statistical occupancy of A and B.

2)      If you want to generate a single (random) configuration and only thermally displace the atoms in that, you should generate the super cell in the qstem model builder, and then load the complete super cell into qstem. Qstem will then only wobble the atoms about their positions, but will not re-evaluate the occupancy again.

 

More on this can be found in the description of the .cfg file format on qstem.org

 

Best regards,

Christoph.

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[张辉]

Dear Prof. Koch,

Thank you very much for your detailed explanation. I am a little bit confused about the second case. How the model builder describes the co-occupancy of two different atoms at one site so that the TDS run sees only one configuration? Many thanks.

Regards,

Hui

[张辉]

Hello Prof. Koch,

I am still a little bit confused about the occupancy parameters in.cfg file. In order to change the occupancy, the entry_count should be 5. The value of occupancy should be provided after the Debye-Waller factors. What if I just want to use the default Debye-Waller factors? Just put a space there? Thank you very much.

Regards,

Hui

On Wednesday, 15 November 2017 21:38:26 UTC+11, Christoph Koch wrote:

[Christoph Koch]

No, you should provide a Debye-Waller factor at the 4^th position. Leaving a space will not work. You canm compute the default Debye-Waller factor according to the equation provided at this website: https://www.physics.hu-berlin.de/en/sem/software/data-formats

 

With best regards,