Anion displacements and the band-gap anomaly in ternary $\mathrm{AB}{C}_2$ chalcopyrite semiconductors

Using a first-principles all-electron band-structure approach, we show that the anomalous (> 50%) reduction in the band gaps of the $A^{\mathrm{I}}{B}^{\mathrm{III}}{C}_2^{\mathrm{VI}}$ chalcopyrite semiconductors relative to their II-VI isoelectronic analogs results both from a pure structural effect (the anion displacements reflecting the mismatch of classical elemental radii) and from a purely electronic effect ($p-d$ repulsion in the valence band), with a small coupling between the two factors.