Structure of β-CuI: Stacking of 2D Bilayers

CuI is an important semiconductor with a transparency in the visible light range. However, the β phase structure at 643–673 K is controversial. We used density functional theory (DFT) combined with quasi-harmonic approximation (QHA) to investigate β-CuI. By analyzing the relative energy, elastic and vibrational properties, and finite-temperature free energy, we ruled out the previously proposed candidate structure (CS) with space group R3̅m and proposed a 2D bilayered stacking structure. DFT-QHA calculations showed that CSs with P3̅m1 and P3m1 space groups, which share the same 2D bilayer structure but differ in stacking, have lower free energies than γ-CuI above 310 K. Calculations of the transition pathways using the generalized solid-state nudged elastic band method indicate that the ZB to bilayered structure transformation requires an energy barrier of ∼100 meV/f.u. to overcome, while interlayer sliding needs only ∼20 meV/f.u.