Abstract
Discovery of the incommensurate structure in the element Ba under pressure 15 years ago was followed by findings of a series of similar structures in other compressed elements. Incommensurately modulated structures of the host-guest type consist of a tetragonal host structure and a guest structure. The guest structure forms chains of atoms embedded in the channels of host atoms so that the axial ratio of these subcells along the c axis is not rational. Two types of the host-guest structures have been found so far: with the host cells containing 8 atoms and 16 atoms; in these both types the guest cells contain 2 atoms. These crystal structures contain a non-integer number of atoms in their unit cell: tI11* in Bi, Sb, As, Ba, Sr, Sc and tI19* in Na, K, Rb. We consider here a close structural relationship of these host-guest structures with the binary alloy phase Au3Cd5-tI32. This phase is related to the family of the Hume-Rothery phases that is stabilized by the Fermi sphere-Brillouin zone interaction. From similar considerations for alkali and alkaline-earth elements a necessary condition for structural stability emerges in which the valence electrons band overlaps with the upper core electrons and the valence electron count increases under compression.
Highlights
Recent high pressure experiments show, structure and properties of elements change dramatically under pressure
The crystal structure of Au5Cd8 is of Cu5Zn8-cI52-type well known as a Hume-Rothery phase stabilized by a Fermi sphere – Brillouin zone interaction [9]
Complex structures observed under pressure in elements of groups I and II as well as in group V with the incommensurate host-guest cell are analyzed to define main factors of unusual asymmetric atomic arrangements
Summary
Recent high pressure experiments show, structure and properties of elements change dramatically under pressure (see review papers [1,2,3] and references therein). A different structure of the same type was in Rb and K In these elements, the host structure has the same tetragonal I4/mcm space group as in Ba, Sr, Sc and Bi, but is made up of 16 atoms. The lighter group-II element Ca transformed to similar 8-atom host structure with the commensurate axial ratio of the host and guest cells cG/cH [8]. In this paper we investigate possible causes that contribute to the formation of the complex crystal structures in elements under pressure, which have similar crystal structures and atomic volumes for the elements from the left and from the right of the Periodic table. We suggest the electronic cause for the formation of its crystal structures
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