From the literature, I know that ZrO2 have different structures at different temperatures i.e. monoclinic (m) at room temperature (RT), tetragonal (t) at 1478 K and cubic (c) at 2650 K. I want to calculate the energy of all the three phases (m, t, c) at above mentioned temperatures i.e. at RT, 1478 K and 2650 K. My expectation is that (m) phase should have the lowest energy at RT among the three, (t) phase should have the lowest energy at 1478K among the three and (c) phase should have the lowest energy at 2650 K among the three. How should I go on with such simulations in LAMMPS? Which thermostat to use and how to define temperatures that I wish?
http://kummelgroup.ucsd.edu/pubs/paper/83.pdf
https://arxiv.org/ftp/arxiv/papers/1607/1607.03141.pdf
I think such calculation is highly dependent on the potential you use. Since pure ZrO2 lattice structure has 3 phases as you mentioned I am not sure the currently used potential could cover all these three phases. As long as you have the proper potential, calculation of energy at designed temperature with selected thermostat (NVT maybe more physical) should not be the difference as you conduct the other energy calculation. So the key-point is the potential.
Please see Song and Hoyt, Acta Mater. 60 (2012) 4328-4335.
In order to find out the most stable phase, you need to find out the Gibbs energy (or practically, the Gibbs energy difference with respect to some reference phase). You can see in the above cited paper that they have used NPT (zero external pressure) ensemble with liquid as the reference phase. (The problem with NVT could be that you would be sampling the systems at different pressures and they could be under stress, at times, unless the right volume is used). But as commented by Tao Wang, it is less likely that one potential can cover all the phases in question. If if does, the transitions temperatures (obtained from the potential) may most likely be different from what you expect. So, it is better to get the free energy differences as a function of temperature with crossed fingers. Again, in the above referenced paper on Fe, you would find that the potential is not able to show the bcc-fcc-bcc transitions. So the results shouldn't be treated as surprising.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Energy