Electronic origin of the orthorhombic Cmca structure in compressed elements and binary alloys

TL;DR

This paper investigates the electronic origins of the orthorhombic Cmca structure in compressed elements and alloys, explaining its stability through Fermi surface interactions and electron hybridization effects under high pressure.

Contribution

It introduces a nearly free-electron model analysis to explain the stabilization of the Cmca structure in various elements and alloys under high pressure, highlighting electron band overlap and hybridization.

Findings

Fermi sphere-Brillouin zone interactions stabilize the structure.

Valence electron count increases under compression, affecting properties.

Unusual electrical and superconducting behaviors linked to electron structure.

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 reduction of crystal energy and stabilization of the structure. This zone corresponds well to the 4 valence electrons in Si and Ge and leads to assume a spd-hybridization for Bi. To explain the stabilization of this structure within the same model in alkali metals, that are monovalent 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.