A metal, or an alloy, can often exist in more than one crystal structure. The face-centred-cubic and body-centred-cubic forms of iron (or steel) are a familiar example of such polymorphism. When metallic materials are made in the amorphous form, is a parallel 'polyamorphism' possible? So far, polyamorphic phase transitions in the glassy state have been observed only in glasses involving directional and open (such as tetrahedral) coordination environments. Here, we report an in situ X-ray diffraction observation of a pressure-induced transition between two distinct amorphous polymorphs in a Ce(55)Al(45) metallic glass. The large density difference observed between the two polyamorphs is attributed to their different electronic and atomic structures, in particular the bond shortening revealed by ab initio modelling of the effects of f-electron delocalization. This discovery offers a new perspective of the amorphous state of metals, and has implications for understanding the structure, evolution and properties of metallic glasses and related liquids. Our work also opens a new avenue towards technologically useful amorphous alloys that are compositionally identical but with different thermodynamic, functional and rheological properties due to different bonding and structural characteristics.

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Pair distribution function analysis (PDF) of X-ray diffraction data, collected at 11-IDC and 1-ID at the Advanced Photon Source, provides the atomic structure and primary crystallite size of FeS both freshly precipitated (FeSfresh) and aged (FeSaged). The short- to medium-range structure of both FeSfresh and FeSaged are nearly identical with that of highly crystalline (bulk) mackinawite. Attenuation in the observed range of structural coherence of the PDF for FeSfresh indicates an average crystallite size on the order of ∼2 nm. This range of structural coherence increased with aging of the sample under hydrothermal conditions due to growth of the individual crystallites, although the mechanism by which this growth occurs is not clear at present. Electron microscopic imaging confirms the presence of individual nanoscale crystallites and provides some insight into their aggregation behavior as larger clusters. The initial, fresh precipitate does not exhibit long-range atomic structure because it is nanocrystalline. The so-called X-ray amorphous nature of FeSfresh is the result of the limited range of structural coherence imposed by the size of the individual crystallites rather than the result of a lack of medium- and long-range atomic order. We propose that the discrepancies in the literature over crystallite size and the atomic structure of FeSfresh are due primarily to the varying degrees of aggregation of uniformly distributed and nanocrystalline FeS particles.