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A new hermetic sample holder to be used with radioactive or air‐ and moisture‐sensitive samples has been developed; it has been designed to fit most of the commercial Siemens/Bruker diffractometers (e.g. D5000 and D8 series). Thanks to the design of the sample holder and to a sample preparation process allowing two‐containment‐barrier protection, X‐ray data can be collected using a standard uncontaminated diffractometer mounted in a Bragg–Brentano geometry. The design offers very accurate and reliable sample positioning. In order to demonstrate the high quality of the data obtained, the Rietveld analysis of plutonium dioxide is presented. Good agreement between the refinement and published data demonstrates the quality of the sample preparation and the accuracy of the sample holder. High‐quality X‐ray diffraction powder patterns can be recorded for use in Rietveld refinements, even on highly absorbing radioactive materials.

Our works shows that the americium pyrochlore (241)Am(2)Zr(2)O(7) undergoes a phase transition to a defect-fluorite structure along with an unusual volume contraction when subjected to internal radiation from alpha-emitting actinides. Disorder relaxation proceeds through the simultaneous formation of cation antisites and oxygen Frenkel pairs. X-ray absorption spectroscopy at the Am-L(II) and the Zr-K edges reveals that Am-O polyhedra show an increasing disorder with increasing exposure. In contrast, the Zr-O polyhedral units remain highly ordered, while rotating along edges and corners, thereby reducing the structural strain imposed by the growing disorder around americium. We believe it is this particular property of the compound that provides the remarkable resistance to radiation (>9.4 x 10(18) alpha-decay events g(-1) or 0.80 dpa).

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