Composite behavior and multidegeneracy in high-pressure phases of Cs and Rb.
The pressure-driven phases Cs III and Rb III having large unit cells are shown to be peculiar examples of commensurate modulated composites with two monatomic subsystems of striking simplicity. The two subsystems are obverse-reverse layers, symmetry related but misfitted. Modulations are smooth and describable by a few parameters within a well-defined superspace symmetry. Ab initio density-functional theory calculations show that the composite character is reflected in their physical behavior. Cs III has a low-energy mode with phason character corresponding to the relative sliding of the neighboring misfitted layers, the energy barrier being lower than 0.01 meV/atom, which is most favorable for transforming to other configurations. These phases possess a quasidegenerate energy landscape, close to the signature of incommensurate systems and quasicrystals.
Perez-Mato
J M
JM
Departamento de Física de la Materia Condensada, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain. jm.perez-mato@ehu.es
Elcoro
Luis
L
Petrícek
Václav
V
Katzke
Hannelore
H
Blaha
Peter
P