Undergrad Predicting new crystal structures

[g_mogni]

Hello,

Sorry if this is a rather basic question, but my memories of solid-state chemistry are a bit rusty. Basically I'm trying to predict new crystal structures. I understand the crystallographic aspects quite well and know that at given external temperature and pressure the most stable crystal structure will be the one that minimises the Gibbs Free Energy as well as matching internal with external pressures and relaxing all atomic forces inside the crystal (such calculations can rather easily be done with DFT techniques for instance). The question is from the point of view of the chemical composition of the repeating unit of the crystal structure (e.g. SiO2 for silica, CaTiO3 for perovskite or NaCl for salt). How does someone know when a certain chemical composition gives a stable and valid crystal? Does the repeating unit always have to obey the octet rule for example, or is it just a matter of calculating the resulting cohesive internal energy of the crystal structure and making sure it's always negative? Thanks for any input!

Gabriele

 

[Useful nucleus]

I do not fully understand the question, but I will give it a try. Given a chemical formula, and given a crystal structure that this chemical formula can fit on, there are certain stability criteria that need to be checked first before claiming that this formula can be prepared in that crystal structure. For example, no imaginary frequencies for the lattice vibrations in the Brillouin zone. Another example , the elastic constants need to adhere to certain stability criteria (inequalities). Once all these tests are passed successfully, thermodynamics can decide which of the possible candidate structures is favorable at a given T, P, chemical potential, ...