(a) Electronic properties of Sb, SnTe, and InI (in column) in three phases α, β, and c (in row). Band gaps are filled with brown shadows. The arrow on the top shows the increasing band gap trend from Sb through SnTe to InI. The Brillouin zone with high-symmetry points for each phase is given in the third row. (b) The band gap within light absorbtion energy spectrum as a function of charge transfer between the constituents in each material. α, β and c phases are shown in black solid, blue short dash, and red dash dot lines, respectively. Sb, SnTe, and InI are represented by filled square, triangle, and circle, respectively.