Flexible framework materials – compounds whose underlying crystal structures allow local distortions at very low energy costs – often exhibit a range of interesting physical properties: examples include negative thermal expansion, amorphisation under pressure, and the existence of nano-sized polar domains in relaxor ferroelectrics. Understanding these effects involves understanding the local environment of atoms, how this differs from the time-average environment, and how the atoms move in the process. These are concepts that are heavily disguised, if not inaccessible, in “conventional” average structure analysis, and yet key to a number of important problems in materials science.

This talk will discuss some recent developments in the use of powder diffraction techniques (principally neutron and x-ray “total” scattering) to probe local structure and dynamics in framework materials. The emphasis is twofold: (i) on producing large and realistic supercell descriptions of atomic structure, and (ii) on developing methods of extracting useful information from these atomistic configurations, such as domain size and structure, vibrational energies, and displacement correlation functions. Reference will be made to a number of recent case-studies, including colossal positive and negative thermal expansion in Ag₃[Co(CN)₆] [1], dynamic instabilities and polar nano regions in SrSnO₃ [2], and the determination of vibrational energies in MgO [3] and SrTiO₃ [4].

[1] A L Goodwin, M Calleja, M J Conterio, M T Dove, J S O Evans, D A Keen, L Peters and M G Tucker, Science 319, 794 (2008).
[2] A L Goodwin, S A T Redfern, M T Dove, D A Keen and M G Tucker, Phys. Rev. B 76, 174114 (2007).
[3] A L Goodwin, M G Tucker, M T Dove and D A Keen, Phys. Rev. Lett. 93, 075502 (2004).
[4] A L Goodwin, M G Tucker, E R Cope, M T Dove and D A Keen, Phys. Rev. B 72, 214304 (2005).

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