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Crystals 2013, 3(3), 419-430; doi:10.3390/cryst3030419
Article

Electronic Origin of the Orthorhombic Cmca Structure in Compressed Elements and Binary Alloys

Institute of Solid State Physics RAS, Moscow district, Chernogolovka 142432, Russia
Received: 24 May 2013 / Revised: 21 June 2013 / Accepted: 15 July 2013 / Published: 19 July 2013
(This article belongs to the Special Issue New Trends in Intermetallics Development and Application)
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Abstract

Formation of the complex structure with 16 atoms in the orthorhombic cell, space group Cmca (Pearson symbol oC16), was experimentally found under high pressure in the alkali elements (K, Rb, Cs) and polyvalent elements of groups IV (Si, Ge) and V (Bi). Intermetallic phases with this structure form under pressure in binary Bi-based alloys (Bi-Sn, Bi-In, Bi-Pb). Stability of the Cmca-oC16 structure is analyzed within the nearly free-electron model in the frame of Fermi sphere-Brillouin zone interaction. A Brillouin-Jones zone formed by a group of strong diffraction reflections close to the Fermi sphere is the reason for the reduction of crystal energy and stabilization of the structure. This zone corresponds well to the four valence electrons in Si and Ge, and leads to assume an spd-hybridization for Bi. To explain the stabilization of this structure within the same model in alkali metals, that are monovalents at ambient conditions, a possibility of an overlap of the core, and valence band electrons at strong compression, is considered. The assumption of the increase in the number of valence electrons helps to understand sequences of complex structures in compressed alkali elements and unusual changes in their physical properties, such as electrical resistance and superconductivity.
Keywords: crystal structure; Hume-Rothery phases; structure stability crystal structure; Hume-Rothery phases; structure stability
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Degtyareva, V.F. Electronic Origin of the Orthorhombic Cmca Structure in Compressed Elements and Binary Alloys. Crystals 2013, 3, 419-430.

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