Ab initio calculation of high pressure phases and electronic properties of CuInSe2

We used an ab initio method to calculate the high pressure phases of CuInSe2. By using the experimentally suggested phases, the enthalpy difference showed that the I4̄2d structure transforms into Fm3̄m at 12 GPa and then into Cmcm   at 42 GPa. The volume reductions at each phase transition are 13.9% and 1.9% respectively, compared with 11% and 1% from experiments. By using the sX-LDA functional, we found that the bandgap in the I4̄2d structure increases at the rate of 39.6 meV/GPa, in fair agreement with photoabsorption experiments. The band gap is closed in the Fm3̄m and Cmcm   structures. The bond lengths between Cu–Se and In–Se were investigated. We found that the bond lengths can be related to the behavior of the energy gap under high pressure. The path of transformation from Fm3̄m to Cmcm was proposed. The energy barrier between the two phases was estimated. The upper bound of the energy barrier is 17 meV which is equivalent to 198 K. This finding can explain the existence of two phases at room temperature reported by experimental study.

► We can describe the sequence of the phase transitions. ► We can describe the gap widening under pressure of the chalcopyrite phase. ► We derived the transformation path from the cubic to the Cmcm phase. ► The coexisting phases between the cubic and Cmcm phases can be explained. ► We predicted that the cubic and Cmcm phases are metallic.