We've an on-going interest in materials that show the unusual property of "Negative Thermal Expansion" and contract in volume on heating. Much of this was stimulated by ZrW2O8, which contracts isotropically and continually over a temperature range of 2 to 1050 K with a coefficient of thermal expansion of alpha = -9 x 10-6 K-1. In addition to this behaviour this and related materials show oxide ion mobility at remarkably low temperatures. Other materials such as Ag3Co(CN)6 show a remarkable "colossal" negative thermal expansion, with alpha = -130 x 10-6 K-1 in one direction. The observation of NTE behaviour is often intimately related to phase transitions in materials and in many cases these can lead to structures with a remarkable degree of complexity which present a considerable challenge for powder diffraction methods. A full understanding the synthesis and properties of such materials requires application of a range of complementary techniques such as diffraction, total scattering and multi-nuclear solid state NMR. In this presentation
I'll discuss recent advances in our understanding of these materials and highlight in particular the information available from powder methods. I'll also use the presentation to highlight methods for analysis of diffraction data such as "parametric" Rietveld refinement which can allow the extraction of "unusual" information from powder diffraction data such as temperatures or activation energies for processes in the solid state.

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