If you talk about crystal do you mean chemical composition or substance? Can you tell us, which polymorphs you are interested in?

Dear Dr. Gert Nolze,

I mean 3D periodic structure like TiO2 has 3 polymorphs (1) rutile (tetragonal), (2) anatase (tetragonal) and (3) brookite (orthorhombic) and others are MnO2 (4 polymorphs), etc. Except metal dioxide others are WO3, BiVO4, LaVO4 etc.

I want to know why few system have less and few have more polymorphs. What is the reason behind occurrence of these polymorphs. Which phenomena decides number of polymorphs ?

Is there any relation between oxidation state of cation (like Ti and Mn) and polymorphs ? Ti has 4 oxidation state and Mn has 2, 4 and 7 oxidation state and both have different number of polymorphs.

Thank You

Shilendra

My very personal impression is that our understanding of nature is quite limited. We try to find correlations however a correlation does not prove that this is the reason for a certain effect observed (especially since this is also related to our experimental technology applied). Therefore, I don't think that there is a currently understood correlation. You can definitely find special systems where physical approaches describe an existing material system in an acceptable way, however, if this would be a general approach it should work for any material. Thus, we can use it where it works, but we do not have any problem to say that there are numerous exceptions since it is obviously rather a rule than a law. I assume that it will still take some years or decades until a general approach will be available explaining everything from the atom until the components we are creating. Presently, we are not that much better than alchemists (although we do not like to hear this). Simulations are nowadays done in order to test them. Nobody would seriously trust a material designed in PCs without any experimental test (in order to test the simulations). But...we are moving... :-)