1. Big Molecules or Complexes

2. Flexible Molecules

2. "Nealry good" pH value in the Xstalization conditions

3. Abscence of "something more" in the condition (an osmolitique molecule for exemple)

3. "Disturbing" Cryoprotection 

4. Bad Luck !!

Thank you very much for your prompt reply

Once again may would you please  explain how can affect big Molecules or Complexes?

Crystal distortion is quite common. Perhaps you can tell your crystallographer to use squeeze technique to refine the crystal structure. Coming back to the reasons of such distortion. Big molecules or complex can have preferential interaction with the solvent molecules like MeOH, CH3CN or THF. You may have sites in your structure that can form complex with the solvent and hence it crystallizes. You can change the solvent and try Toluene or hexane  and see if you can crystallizes your compound. 

I will and I really appreciated your answer!

what molecule are you trying to crystalize ?

Thank you all of you guys for your answer and comments.

Hi Nicolas I was trying to crystalize various niobium complexes

As a rule, better crystals grow from less polar solvent (from solvents set). In your case THF should be best candidate

Cooling, evaporation, dilution with antisolvent, etc - it is only various processes to get supersaturation.

 Crystallization rate is critical parameter, and phonons are present at any temperature above OoK. By this reason some low-polar groups can rotate in solid ( -CF3, Si-O-Si) - ordering is not possible at moderate temperatures. At low temperatures sorption processes have low rate. Some molecules sorbed on wrong way can be covered with next layer and included in crystal (depend on   diffusion/sorption) .Very high crystallization rate lead to disordered solid/glass (not only from molecular compound or ceramics, but metals as well)

Your boss wants to see perfect crystalls next week...But human life is sometimes shorter than time to order molecules or ions into crystal structure...)))

Glass of your crystallization flask should be crystalline (not vitrified) as build from small, highly polar and symmetrical ions, but it need some hundreds or even thousands years. It is visible on old vitrages in cathedrals

Thank you Andrei

Entropy

Thank you all