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Elemental Metals under Pressure
Author(s) -
Christensen N.E.
Publication year - 2000
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/1521-3951(200007)220:1<325::aid-pssb325>3.0.co;2-0
Subject(s) - isostructural , van der waals force , chemistry , orthorhombic crystal system , phonon , density functional theory , negative thermal expansion , phase transition , bulk modulus , thermodynamics , caesium , thermal expansion , van der waals radius , equation of state , condensed matter physics , crystal structure , crystallography , physics , computational chemistry , inorganic chemistry , molecule , organic chemistry
Parameter‐free calculations based on the density‐functional theory are used to examine high‐pressure phases of solids. For the elemental semiconductors particular attention is paid to the orthorhombic (Cmca) structure (Si‐VI). The same structure, even with very nearly the same relative atomic coordinates, is found for Cs in the high‐pressure phase Cs‐V. In the Cmca structures the atoms tend to form dimers. Ge and Rb also have high‐pressure phases with the same Cmca structure. The thermodynamic properties of the low‐pressure phases of cesium, Cs‐I (b.c.c.) and Cs‐II (f.c.c.), are examined, and the equation of state is calculated for P up to 4.5 GPa and temperatures from 0 to 300 K. The contributions to energy and entropy from the phonons are calculated within the quasi‐harmonic approximation. The thermal expansion coefficient of f.c.c.‐Cs is predicted to be negative for P above 3.5 GPa for all T . Cs‐II becomes dynamically unstable when P exceeds 4.3 GPa, where a transverse phonon mode with wavevector along (110) becomes soft. As a consequence, a Van der Waals loop does not develop in the isotherms, and an isostructural (f.c.c. → f.c.c.) transition cannot occur. In that case Cs‐III must have a structure that is not f.c.c.