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Crystal structure

As weird as they come

Nature volume 403, pages 836–837 (2000)Cite this article

Most metals have simple structures, such as face-centred cubic, which can be obtained by packing marbles into a box. But there are exceptions, notably gallium's unique structure in which each atom has seven near neighbours, and the distorted structures of mercury and tin. These are the only metals with unusual structures at normal pressures and temperatures, but there are other examples at high pressures1. Richard Nelmes and collaborators2 have now reported in Physical Review Letters what must be the weirdest known atomic structure of a metal, indeed of any pure element. Using X-ray diffraction they determined the crystal structure of the metal barium under pressures from 12 to 45 gigapascals (120,000 to 450,000 atmospheres). At high pressure most materials change the atomic arrangements that make up their crystal structures, with barium showing five structures in all, of which the latest one is number IV.

What is surprising about barium IV is that the guest and host are made from the same pure element. The host structure consists of hollow octagonal cylinders of barium atoms, inside which is the guest structure — straight chains of yet more barium atoms (Fig. 1a). Nelmes et al. find that the repeat distance along the guest chains ( cg = 3.4095 ångström, where 1 Å = 0.1 nm) is quite independent of the repeat distance along the host cylinders (ch = 4.6966 Å). Moreover, the ratio cg/ch appears to be irrational and varies smoothly with pressure, so the chains and guest cylinders do not fit well together at any of the pressures studied (Fig. 1b).

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Figure 1: The high-pressure structure of barium IV discovered by Nelmes et al.2.

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  1. Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, UK

    Volker Heine

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  1. Volker Heine
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Correspondence to Volker Heine.

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Heine, V. As weird as they come. Nature 403, 836–837 (2000). https://doi.org/10.1038/35002691

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