The “electron crystal” behavior in copper chalcogenides Cu₂X (X = Se, S)

The electrical transport properties of the emerging thermoelectric Cu₂Se are exceptionally good; this is unexpected considering the strong disordering or liquid-like structure of the Cu sublattice which normally leads to poor electrical conductivity. Via first principles calculations, however, we find that the structural variations on the Cu sites have minor effects on the band-edge electronic states and electrical transport properties. In particular, the Cu vacancies affect very little the edge states of the valence band for the low-temperature ordered Cu₂Se phase, except for shifting the Fermi levels. The feature is also applicable to the high temperature liquid-like phase with copper vacancies and disorder. The calculated Seebeck coefficient and its carrier concentration dependence are consistent with available experiments and confirm the band rigidity. The results imply that the electrical transport is predominantly determined by the face centered cubic Se sublattice in Cu₂Se. Although the lattice thermal conductivity is minimized by the disordered Cu sublattice with “Phonon Liquid” behavior, the electron transport is maintained by the ordered Se sublattice, which provides the “Electron Crystal” characteristic. The feature whereby Cu has a minor effect on the electrical transport can also be found in other copper chalcogenides such as Cu₂S, also shown in this study.