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The Mossbauer effect is the recoilless absorption and emission of γ-rays by specific nuclei in a solid (Mossbauer 1958a, 1958b), and provides a means of studying the local atomic environment around the nuclei. It is a short-range probe, and is sensitive to (at most) the first two coordination shells, but has an extremely high energy resolution that enables the detection of small changes in the atomic environment. Mossbauer spectroscopy therefore provides information on phase transformations at the microscopic level. Mossbauer spectra of materials can be recorded under a large range of conditions, including temperatures from near absolute zero to at least 1200°C, and pressures to at least 100 GPa. Spectra can also be collected under different strengths of external magnetic field, currently to at least 15 T. This enables in situ observations of changes to the atomic environment before, during and after phase transformations under varying conditions. For phase transformations that are quenchable, it is possible to characterise changes between polymorphs at conditions where spectral resolution is optimal. Over 100 different Mossbauer transitions have been observed, although unfavourable nuclear properties limit the number of commonly used nuclei. The 14.4 keV transition in 57Fe is by far the most studied, and will be the focus of this chapter since iron is the most relevant nucleus for mineralogical applications. The aim of this chapter is to provide a brief background to Mossbauer spectroscopy within the context of phase transformations. The relevant parameters are summarised and the effect of temperature and pressure are discussed, particularly with reference to identifying phase transformations and characterising the electronic and structural environment of the Mossbauer nuclei. Instrumentation is summarised, particularly as it relates to in situ measurements of phase transformations, and a brief survey of applications is given. The appendix includes a worked example that … |
