A UNIFIED PICTURE OF THE CRYSTAL-STRUCTURES OF METALS

THE crystal structures of the light actinides have intrigued physicist s and chemists for several decades(1). Simple metals and transition me tals have close-packed, high-symmetry structures, such as body-centred cubic, face-centred cubic and hexagonal close packing. In contrast, t he structures of the light actinides are very loosely packed and of lo w symmetry-tetragonal, orthorhombic and monoclinic. To understand thes e differences, we have performed total-energy calculations, as a funct ion of volume, for both high- and low-symmetry structures of a simple metal (aluminium), a non-magnetic transition metal (niobium), a ferrom agnetic transition metal (iron) and a light actinide (uranium). We fin d that the crystal structure of all of these metals is determined by t he balance between electrostatic (Madelung) interactions, which favour high symmetry, and a Peierls distortion of the crystal lattice, which favours low symmetry. We show that simple metals and transition metal s can adopt low-symmetry structures on expansion of the lattice; and r ye predict that, conversely the light actinides will undergo transitio ns to structures of higher symmetry on compression.