# Why different crystal phases have different colours?

1. ### AlKindi

19
Like quinacridones or HgS and HgS $\gamma$. I don't understand how crystal phase can act on the resonace of the electrons....Please reference! Thank!

Last edited: Jul 21, 2011
2. ### ZapperZ

30,548
Staff Emeritus
Different crystal geometry has different phonon modes and spectrum. The "colors" of such crystal is a direct function of the phonon characteristics.

Zz.

3. ### AlKindi

19
Great! Thanks!
( I've found the bibliography in Wiki's page about phonon)

So the electrons are not interest in the process, and I can't predict this sort of absorption with DFT? Or interpretate a spectra identyfing the Functional group?

Last edited: Jul 21, 2011
4. ### DrDu

4,513
I don´t think that phonon modes are of importance in case of the examples you gave. I suspect that some electronic transitions are dipole forbidden in one crystal structure but not in the other. E.g. the molecules may form dimers in the crystal. In one crystal, the dimers are lying parallel to each other, in the other structure anti-parallel. Due to the coupling of the two molecules the electronic transitions split into one of higher frequency and one of lower frequency. Which of the two is of higher intensity depends on the relative orientation of the two molecules.

5. ### AlKindi

19
Okay, maybe they act both together. I'm sure some polymorphic form of quinacridones are due to differents overlaps of the main structure that reinforce the dipole transition depending on the way in which they are set.

Thank's a lot!!!!!
P.S. Can I calculate, predict in some way this interaction?

6. ### abhi2005singh

63
Colors shown by the crystal depend upon various things and are related to the electronic structure and not on the phonon modes. Defects are one of these sources. Change in crystal structure can change the electronic structure of the material and hence the color of the crystal since it is related to the electronic transitions. If transition metal ions are present, change in crystal structure can change the splitting of the degenerate levels resulting in levels of various symmetries. The allowed electronic transitions depend on the symmetry of the ground state, the transition operator and the final state. Hence some the transition can be allowed while the others can be forbidden on change in the crystal symmetry. This can lead to change in color by changing the crystal structure.

Whatever I have mentioned above can be calculated, but I am not the correct person to say anything on as to how it is done.

7. ### AlKindi

19
Thanks, so is "simply" a change of the electronic structure. So when something can change the electronic structure? I mean, does the chemical matrix, the surrounding "molecular environment", can affect the electronic structure or must be a more strength bond like a ion bond?

8. ### abhi2005singh

63
Yes, the chemical surroundings can change the electronic structure. There are other methods as well, like, chemical substitution, change in bond length, bond strength, lattice parameter etc.

9. ### AlKindi

19
Thanks, I think a lot of work can be made in determining this sort of change!