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

This paper reviews some recent diffraction-based studies of highly-flexible framework materials. Emphasis is placed on characterisation of the variations in local structure and dynamical behaviour that give rise to the unusual physical properties of these materials-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 refinements of total scattering data are described. Such protocols include both real- and reciprocal-space tools for quantifying correlated processes, including localised ferroelectric displacements and soft-mode lattice dynamics. These techniques are presented in the context of specific case studies of a number of important framework systems, such as ZrW2O8, Zn(CN)(2), Ag-3[Co(CN)(6)] and SrTiO3. The paper concludes with a discussion of spin flexibility in magnetic systems and their relevance to local magnetic structure determination.