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This chapter deals with applications of neutron diffraction to the understanding of transformation processes in geomaterials with a time-resolution from seconds to several days. The choice of neutrons rather than X-rays for such time-resolved studies is due to their specific advantages like e.g., the sensitivity to light elements, the detection of volume rather than surface phenomena or the low absorption in case of bulky sample environments. General features of diffractometers suitable for time-resolved studies are discussed and the high-flux diffractometer D20 at ILL is presented in some detail. Time-resolved processes usually are studied at non-ambient pressures and temperatures. The necessary sample environment needs to be provided and matched to the diffractometer set-up. Processes in water ices and gas hydrates are taken as examples and will be used to demonstrate the capabilities and limitations of the method.
Crystals are less static than what is usually suggested by immobile drawings of their atomic or molecular arrangements representing a time-space averaged picture of atomic positions as obtained from diffraction experiments. Rather, atoms or molecules exhibit thermal displacements and some even move inside the lattice as defects. These averaged displacements are described via atomic displacement parameters (e.g., Kuhs 2003) and are accessible also using diffraction techniques. Yet, there is even more movement in a crystal. Close to structural phase transitions the collective mobility of the constituents enables the formation of new structural arrangements to lower the free energy of the system (Redfern 2006, this volume). Such solid-solid transformations are not necessarily instantaneous, in particular when the new phase needs to be reconstructed by moving molecular and atomic constituents to a new position. Following such transitions is one of the main topics of this review.
Thermodiffractometry versus truly time-resolved studies
If a material is capable of existing in more than one polymorphic form, the process of transformation from one …