Figure 2.

Optical dielectric constant ε_∞ along the pseudo‐binary line between GeTe and GeSe. a) ε_∞ as a function of stoichiometry. The rhombohedral phase, found up to 70% Se, is characterized by large values of ε_∞, which exceed the value of the corresponding amorphous phases by more than 100%. Between 50% and 70% Se content the rhombohedral phase is metastable and transforms into a hexagonal phase upon further heating. This transition is accompanied by a pronounced drop in ε_∞. b) ε_∞ plotted as a function of density. The data for the amorphous, orthorhombic, and hexagonal phase follow the Clausius–Mossotti (CM) relation (dashed line), which relates ε_∞ and the mass density of the material. Only the rhombohedral phase shows an excess of the electronic polarizability and hence ε_∞, characteristic for metavalent bonding.