Using VASP I calculated the activation barrier of vacancy diffusion on the surface of ZnO for a 2x2 3 layered cell, where i had removed 1 oxygen atom out of 4 on the surface, and it came out to be 1.35eV and then I calculated the migration barrier for a 3x3 3 layered cell, where I had removed 1 out of the 9 oxygen atoms on the surface, the barrier came out to be 1.67 eV.
The thing which is bugging me is, why is the barrier higher at a lower vacancy concentration. Is there an explanation for this? I was wondering if i should use a similar sized cell for both the calculations ( a 6x6 cell)? The only thing that is different in the two calculations are the KPOINTS. For the 2x2 cell I have used a 2x3x1 mesh and for the 3x3 cell I have used a 1x2x1 mesh.
Thanks for the reply.
Yes I am using NEB to calculate the activation barrier. I too had doubts regarding the bigger cell relaxing more.
Do you think that using a similar sized cell for both the calculations will help? I could use a 6x6 cell for both. But I am a little worried regarding convergence, the 3x3 cell took 941 steps to converge. I am using the simple NEB method, I have not tried the climbing image method yet.
Thanks again.
In principle activation barrier energy for the vacancy hoping motion is independent from its concentration since this an individual phenomena unless one has another vacant side in the immediate neighbor of the vacancy in question. But the probability to find another vacancy next to it is proportional with square its concentration given by atomic fraction, which means X^2 that is extremely small even at the melting point it is in the range of 10^-8.
I am not familiar with the computational procedure of VAPS. But what I know sure is that if you wish to compute activation energy associated with a single vacancy one should not use a model which assumes that vacancy distribution inherently injected to the system, whether it is random or not. That is absurdity otherwise you have to increase the size of the computational cell till the computed values should be stabilized, which is very costly of course. BEST REGARDS
Secondly, İf you know how to manipulate distribution function parameters, which may be the width and peak position of Gaussian probability function, and you can easily concentrate the most probable position of the vacancy and its occurrence frequency at any given position of the selected computation cell.
In our department Prof. Kadri Aydinol and his Ph.D. student have made an extensive simulation studies on the subject related to electromigration, which also appeared in RG.
Atomistic simulation of self-diffusion in Al and Al alloys under electromigration conditions
F. G. Sen · M. K. Aydinol
[Show abstract]
Journal of Applied Physics 11/2008; 104(7-104):073510 - 073510-4. DOI:10.1063/1.2988140 · 2.19 Impact Factor
Download Follow
Hi Salman,
I will be calculating the same property in SiO2 using vasp. Since I am new in this field, can you please detail me how do you calculate vacancy migration barrier?
Thanks very much
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Physical Phenomena
- Luminescence
- Fluorescence