The crystallography of flexibility: Local structure and dynamics in framework materials

摘要

This paper reviews some recent diffraction-based studies of highly-flexibleframework 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-inducedamorphisation. The relevance of total scattering as a probe of local structure is discussed anda 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)8] and SrTiO3. The paper concludes with a discussion of spin flexibility in mag-netic systems and their relevance to local magnetic structure determination.