CRYSTALLOGRAPHY OF FLEXIBILITY

The crystallography of flexibility: Local structure and dynamics in framework materials
A L Goodwin
Z Krist Suppl 30, 1-11 (2009)

This paper reviews some recent diffraction-based studies of highly-flexible framework materials. Emphasis is placed on characterisation of the variations in local struc- ture and dynamical behaviour that give rise to the unusual physical properties of these mate- rials—negative thermal expansion, negative linear compressibility and pressure-induced amorphisation. The relevance of total scattering as a probe of local structure is discussed and a number of general protocols for analysing reverse Monte Carlo (RMC) atomistic refine- ments of total scattering data are described. Such protocols include both real- and reciprocal- space tools for quantifying correlated processes, including localised ferroelectric displace- ments and soft-mode lattice dynamics. These techniques are presented in the context of spe- cific case studies of a number of important framework systems, such as ZrW2O8, Zn(CN)2, Ag3[Co(CN)6] and SrTiO3. The paper concludes with a discussion of spin flexibility in mag- netic systems and their relevance to local magnetic structure determination.

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