Hi, I study phase transitions of nanoporous materials using LD calculations. I calculate the energy and other properties for a system with a few unit cells (1-5, 10 nm). I found a local disorder of the studied structures under high pressure conditions. However, the use term "amorphization" is sometimes confused and experimentalist related it to the behaviour of the materials at large length scale 100 nm ). So, if I consider just a few number of unit cells, does it make sense to talk about amorphization of the structure?
Dear Jorge, in simulations using periodic boundary conditions all the properties that be computed significantly are those possessing a characteristing size smaller or equal to the simulation box. A simulation yielding an amorphous system in the simulation box in fact generates a crystal with a unit cell corresponding to the simulation box and symmetry group P1. Strictly speaking, an amorphous system does not show any translational symmetry, but the experimentalists seem to consider as amorphous a systems without symmetry in patches of about 100 nm in size, and in this sense the simulations are not fully adequate. However, if one wants to study the amorphyzation mechanism, the simulations of relatively small systems, in my opinion, can be useful, provided that the interatomic interactions are short-ranged and then the influence of the periodic images are negligible.
Strictly speaking it is not appropriate to make inference about the possible amorphous state of a solid on the basis of calculations based on finite super cells. However, since an amorphous solid is characterised by lacking crystalline long-range order, it is possible to deduce whether a solid is in reality amorphous or not through investigating the behaviour of the appropriate correlation functions as calculated on finite super cells; in particular, a careful finite-size scaling analysis of these correlation functions can provide vary valuable information about the actual state of the solid if the simulation had been carried out on an in principle infinitely large super cell. The attached two papers may serve as good starting points for orientation.
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.37.4656
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.44.11092
- Badges
- Science topic