Origin of Structural Anomaly in Cuprous Halides

Cuprous halides (CuX; X = Cl, Br, or I) have been extensively
investigated
in the literature, but many of their fundamental properties are still
not very well understood. For example, debate about their crystal
stability, i.e., whether the ground-state structures of CuX are zinc-blende,
still exists. By performing rigorous first-principles calculations
for CuX using an accurate hybrid functional, we unambiguously demonstrate
that CuX are indeed stable in the zinc-blende structure, but their
accurate description requires careful treatment of the exchange interaction.
Previous calculations based on local or semilocal density functionals
underestimated the important contributions from exchange interactions
and thus underestimated the energy separation between the unoccupied
4s and occupied 3d orbitals in Cu, resulting in an overestimation
of the s–d coupling and the energy reduction of distorted CuX.
Our study clarifies a long-standing and highly debated issue with
regard to ground-state structures of CuX and advances the physics
of phase stability and the importance of s–d coupling in semiconductors.