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.
Highlights
Experimental studies of elements under high pressure reveal a great variety of structural transformations, summarized in review papers
The crystal structure oC16-Cmca belongs to the family of structures described by Pearson [12] as a planar square-triangle nets of atoms, assigned as 32434 where ―3‖ specifies a triangle, ―4‖ a square, and the superscript shows a multiplicity
In this paper the oC16-Cmca structure is analyzed for the purpose of understanding the formation and stability of this structure in a number of elements from both sides of the Periodic Table, as well as in some binary alloys, for instance, in Bi alloys with In, Pb, and Sn
Summary
Experimental studies of elements under high pressure reveal a great variety of structural transformations, summarized in review papers (see [1,2,3] and literature cited therein). Elements on the right-hand side of the Periodic table—main group IV-VII semi-metals and non-metals—crystallize in open-packed structures following the 8-N coordination rule. These elements tend to transform under pressure to a metallic state with closely packed structures. On the left-hand side of the Periodic table, the group 1 and II elements (the alkali and alkali-earth elements) are good metals at ambient pressure that crystallize in the closely packed structures. Under pressure, they show an anomalous behavior, transforming to open-packed structures
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
