Twinning and Disorders in Crystals

Thursday, February 16, 2017


Bruno Donnadieu

Department of Chemistry

National University of Singapore

Sometimes during the growth of a crystal, or if the crystal is subjected to stress or temperature/pressure conditions different from those under which it originally formed, two or more intergrown crystals are formed in a symmetrical fashion.  These symmetrical intergrowths of crystals are called twinned crystals.  Twinning is important to recognize in crystallography, in this talk we will speak about different kinds of Twinning and the ways to recognize and handle all them.

One of the remaining challenges in applied crystallography is the proper modelling of disorder in crystal structures, especially in small and medium size molecules or even in supramolecular chemistry.  All these systems tend to have disordered solvent molecules or disordered moieties or both.  Nowadays most modern programs in crystallography such as SHELXL 1 or CRYSTALS 2 have tools and routines implemented to assist in properly handling most of the common disorders encountered.  In this discussion we will expose different cases of disorders:  

  • Substitutional disorders are indentified if a crystallographic position is occupied by more than one type of atom - in minerals and ionic crystals, for instance.
  • Positional disorders represent fact that an atom might be found in more than one position. They can be defined as rotational disorder, pseudorotational disorder and whole molecule disorder.

Finally sometimes a solvent molecule and/or small ion can be neither identified nor modeled.  In such cases the option SQUEEZE 3 procedure in PLATON 4 can be used.  In this talk we expose also the way to use SQUEEZE in solvent region modelling.  




 


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