SIMPLE HEXAGONAL AND TETRAGONAL BODY-CENTERED STRUCTURES: EFFECT OF PRESSURE AND ELECTRON CONCENTRATION

V. Degtyareva ¹ and W.B. Holzapfel ²

¹Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow district, 142432 Russia, E-mail: degtyar@ issp.ac.ru
²FB 6 Physik, Universität-GH-Paderborn, 33095 Paderborn, Germany

Keywords: structural transformations, high pressure, electron concentration

A structural transformation from a simple hexagonal (hP1) to body-centered tetragonal (tI 2) phase was observed under pressure in a Sn-Hg alloy (10 at.% Hg) by X-ray diffraction with diamond anvil cells and synchrotron radiation [1]. A comparison with previous data on the occurrence of these structures shows that they are typical for metals and alloys with nearly four valence electrons, i. e. with electron concentrations n_e~  4. Obvious features of these structures are special values of the lattice parameter ratios which depend on either composition or pressure.

The hP1 structure of the Sn-Hg alloy with an axial ratio c/a = 0.930 at ambient pressure shows a slight increasing of c/a under pressure, in agreement with previous observations on Al-Ge [2] and Cd-Sb [3] alloys. The trend of the axial ratio to increase was also noted with decreasing n_e in binary hP1 alloys of various composition [4] providing a common reason for more symmetrical atomic coordinations by either increasing pressure or decreasing electron concentration.

A tetragonal structure with small distortion from a body-centered cubic structure was observed previously under pressure on tin [5] and recently on III-V compound InBi [6]. The tI 2 structure shows in all three cases under pressure an initial increase in c/a with a saturation at a specific value of c/a depending on n_e: c/a<= 0.96 with n_e = 4 for Sn and InBi and c/a <= 0.92 with n_e = 3.8 for the Sn-Hg alloy.

The concept of Brillouin zone - Fermi sphere interaction relates the structural stability and pressure behaviour for both the hP1 and tI 2 phases to special values in n_e.

1. V.F. Degtyareva, O. Degtyareva, M. Winzenick, and W.B. Holzapfel (to be published)
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